Gtr0 5/10/2018 2:47 AM
Tubes that can handle 500VDC on screens, or tame the power supply??
Hi All.

I have a high gain amp that puts out 516 VDC on the plates and 511 at the screens. It is an EL34 based amp so screen resistors are 1K 5W. I have popped a few tubes in the last year, never the same position (tubes are losing vacuum, turning white) and someone mentioned that running the bias at 70%, with current producton tubes and with these voltages, very well may be too much. I play a lot - probably some 30+ hours per week.

So let us assume this is true for the moment, the only tubes I can even find that are close to accepting 510VDC on the screens are the TAD EL34B-STR (claiming 500 VDC on screens max). Shuguang I guess? I have a set of these in a plexi and I find that they sound fine.

The transformer is custom made and similar to PT for a SLO100, so it's a 360-0-360.

The question is should I have a new tranny made, something that will yield something a bit less like 340-0-340??? Or should I put a dropping resistor after the rectifier circuit? Or is there a set of tubes out there that will accept a bit of abuse in this area (by your experience)? Or even perhaps raising the screen resistors??

I have used JJ E34L for the last decade and a half... But after popping two of them some months back, I bought a set of Tung-Sol EL34s... it was about a month going before I lost one about a week ago. Again I assume they are losing vacuum because they are turning white above the getters.

My temperature probe says the tube's surface runs at about 280 degrees F. But damn does it get HOT in the room.

Thanks for any input you may be able to offer.

Thanks.
 
Zozobra 5/10/2018 4:09 AM
I know folks with vintage amps that run high plate/screen voltages that have been using the Shuguang's. You can buy them cheap from RS :
https://uk.rs-online.com/web/c/semic...triode-valves/

I've used them in the past in a Trace Elliot Speedtwin and that runs about 500-510. It also has a standard Marshall 100W replacement OT as the tech who repaired it is a hack. It really should have a 2k-2.2k primary for that kind voltage.
Screen voltage rating do get vastly abused in most amps. Said speedtwin also runs 6L6's and they are only rated for 450V on the screens and they see 500! Same is true of the SLO.
 
pdf64 5/10/2018 5:01 AM
What (loaded) Vac is the PT putting on the heaters? If there's some leaway there, a bucker may be a good option.
Are there any higher voltage taps on the PT primary?
 
alexradium 5/10/2018 5:26 AM
for the best reliability,i would regulate the screen voltage to 400V max,with a mosfet and zener diodes.
EL34 tubes are very unpredictable as far as lifespan,Winged C were good but expensive,today the most reliable seems to be the EHx,but over 500V?
 
Pedro Vecino 5/10/2018 7:42 AM
Quote Originally Posted by Gtr0 View Post
I have popped a few tubes in the last year, never the same position (tubes are losing vacuum, turning white)
...before I lost one about a week ago. Again I assume they are losing vacuum because they are turning white above the getters.
So much repetition is suspicious. Do you use any type of spring retainer on them? Sometimes they do more damage than other things. When a tube turns white it means there is a crack
 
Chuck H 5/10/2018 7:49 AM
I built a custom order a few years ago that was high-ish voltage, but not as high as your rig. I blew up a bunch of tubes of different brands. Shuggies were the only ones to consistently survive and that's what the amp is using now. Unfortunately it's a combo amp and the Shuguang EL34's tend to be microphonic. I ended up buying three pair to get one that was good enough. If I had known I would have bought a sextet and selected the best two. I have the other four tubes in my stash marked "head only". They sound as good as any EL34 IMO.
 
nsubulysses 5/10/2018 10:26 AM
JJ 6CA7 is rugged on even higher voltages than 510V plate and 500V screen.

maybe the choke needs to be replaced with a 300-1000 ohm resistor ?? (tame the power supply idea)

Also though, it seems like the tubes are not dying because they would short and blow a fuse, or just turn on but not work at all . if the tube is losing vacuum the tube is not failing because of high voltage IMO. It would be failing from........being banged around???
 
J M Fahey 5/10/2018 10:27 AM
Rather than looking for Terminator approved tubes , which to boot are used beyond any manufacturer approved rating (they *should* know ) Iīd lower screen voltages to sensible levels.

The simplest and cheapest solution is to downgrade the screen supply: drop the choke and use a plain resistor instead.
Anything from 1k 5W or 10W and up, whateverīs needed so at idle they sit at no more than 450V, dropping more when amp is driven full blast ... which is good.

Or you can go Hi Tech and use a MosFet regulator set to fixed 400V.
 
nsubulysses 5/10/2018 10:35 AM
It is good to drop the screen supply as juan says but it will make it sound more mushy and undefined in pick attack. Which I would guess this player wants since he is playing an SLO clone.

What would you do to re-tighten the tone? Adjust NFB resistor?

I usually use a 20W 1K resistor for 140W amp.
This is a nice one -- https://www.mouser.com/ProductDetail...NE5tU87GTlE%3d

I thought I could be a kinda lazy and use 500 ohm 10W in series with 470 ohm 6.5W in series (no 20w resistors around to use). By 3rd practice the 6.5W almost fell out because it was getting so hot the solder was melting and the resistor almost becaame loose in chassis! This is with amp absolutely cranked so it's an extreme scenario but I like 20W just to be safe.

if you look at 5150 and a bunch of other amps of similar 100W power rating that don't want to buy chokes, they usually use 400 or 500 ohm 10W resistor. Ampeg V4 is about 540V plate and 530V screen and they use 470 ohm 7W, which occasionally falls out of circuit from getting too hot if you crank the shit out of it.
 
Jazz P Bass 5/10/2018 10:37 AM
The failed tubes turning 'white' is not indicative of overvoltage or a screen failure.

'White' means loss of vacuum, plain & simple.

You may very well have a mechanical/ shock issue.

30 hours a week play time, yes.
But is the amplifier being transported?
 
Chuck H 5/10/2018 12:19 PM
Quote Originally Posted by Jazz P Bass View Post
But is the amplifier being transported?
As in loaded onto a rolling cart while still piping hot and wheeled out into the cold night air through the back door of a bar, grating along over bumpy, busted up asphalt past sleeping bums and broken Thunderbird bottles then heaved into the back of a van sitting on the metal floor for the drive home on surface roads.?. I did that all the time when I was gigging. I never cracked a tube but I did get circuit faults on the boards of my Marshalls three times.
 
dstrat 5/10/2018 12:28 PM
biasing a high gain type amp 70% might be a problem? Why everyone does this is beyond me.

many a burn , melted 6550 in my marshall over the years.
 
g1 5/10/2018 1:59 PM
Agree with dstrat, 70% at higher plate voltages is often a problem. Try lowering it until you have too much degradation of the tone. Bias as low (%) as you can get away with. The voltages will increase somewhat, but that is not as much a problem as higher dissipation.
Now it seems your problem is a mechanical one (cracked glass/vacuum leak), but the excess heat may be a contributor.
Nevetslab has posted here about needing to insulate the metal type 'top hat' retainers which were causing cracks in their SVT rental inventory. Could be a similar issue if you are using those type of retainers, like Pedro mentioned above.
 
Gregg 5/10/2018 2:14 PM
The easiest way to lower the screen voltage is to add 5W zeners in series with each tube screen.
 
J M Fahey 5/10/2018 3:04 PM
Quote Originally Posted by Chuck H View Post
broken Thunderbird bottles
Had to Google that.
Wow!!!!
 
Gtr0 5/11/2018 1:40 AM
Thank you for the suggestions. Much appreciated.

I had prepared a long reply to most comments and somehow the forum asked me to sign back in and only the quotes remained in my reply.... [breathe]. So I will try to just address the overall comments.

Let me take a step back and give a better overall story of what's going on. I first started learning and buying parts from Metro amp back in 2004 to upgrade my then JMP mater volume. As time went I started replacing more and more, then started building whole amps. But this is where I have done the bulk of my learning. Perhaps some of the info from the old Metro forums wouldn't apply to high gain amp techniques. Such as biasing to 70%. And I admittedly say I have always pushed those boundaries until two days ago. Now I get the idea that it is not such a good idea. Especially with higher voltages such as what I am seeing here.

Having been building amps on and off for 14 years, you do learn some things, but learning this way leaves HUGE holes in one's education. I am trying like hell to fill those gaps in but there is still some mystery to things that I am desperately trying to get a grip on (like impedance). The internet is so flooded with bad info (and good), it's tough to weed out. I have turned to books where possible... started with Merlin's books and next (?) who knows... I will cross that bridge when I get to it.

This amp initially set out to be close to a SLO. Looking at every schematic I can get my hands on though, I took some ideas from the dual rectifier (LOVE the 680pF on the treble control for example). The idea was that I would build one out, tune it to my taste, and use that as a template for what in my mind is the ultimate amp. I have been working on this amp for the vast majority of the last year and this is the third time that the amp has been rebuilt, from the ground up. So it is not this particular iteration of the amp that is blowing all of the tubes. But this iteration was involved with the latest tube blunder. Having never ever blown a tube in 30 years until last year, I had to look at what is different in this amp from the previous amps I have had in the past.

Of course there could be an error somewhere in my wiring, however I have been over it, literally, at least a dozen times, tracing through schematics and my own drawn up plans. Perhaps I could post the schem and someone could take a look to aviod any "user" errors.

In addressing comments;
I do not have any retainer clips or other mounting hardware for the tubes. Since I built this more or less as some kind of prototype I just left them out. And yes, I do gig the amp. Not a good idea to cart this thing around, but I had no other suitable (as in high gain) amp at the time for this particular brand of music. I have since finished another prototype of the amplifier while the circuit evolves... now I have two to cart around... I know it is a somewhat asinine thing to do. I am working toward correcting that, but for the moment it is what it is.

I am interested in the whole choke/resistor thing. I thought I read, or saw, in one of Merlin's books where he had a dropping resistor placed before the choke. At least I think it was one of his books and I will have to reference that. BUT is that a possibility? To receive the best of both worlds? As far as I understand the choke is far better at smoothing than a resistor. So if I could drop the supply something like 40 volts (I know that's a lot), that would help tremendously, would it not? I can make up for it with lower resistors between filter stages.

Until now, that my second amp is finished, I did NOT have a fuse in the DC side of the rectifiers in the latest version of the amp (I did in previous versions). I did this for two reasons, one was I began to model the filtering/power section from the Dual Rec where there are NO fuses after the main fuse... bad idea, I know. The other reason was that I planned to fuse up the ac secondary as per ValveWizard recommendations (I just hadn't gotten there until yesterday). My latest build I have secondary fusing on the AC side. Today I will add fuses on the DC side of the rectifiers AND a fuse on the heaters.

Do I abuse the tubes in general? I suppose so. As I said I am pushing the boundary of 70% dissipation. Until a few days ago when I was coming to the conclusion that I should not do that, I was pushing my tubes to the 30 to 35 mA area. I suppose the 25 to 28 mA area would be better?

I have three other amp builds - two marshall 50 watt style and one AB165'ish (or at least used to be) build. Beside that I have a 100 Marshall plexi re-issue and a JTM45 reissue. Oh yea, and a Fender Hot Rod Deluxe also. None of them (though I do not remember the plexi's readings) put out 515VDC on the plates of the tubes, or 510 on the screens. SO these are the reasons I arrived to the original post. In general I am worried about such high readings.

Thank you, everyone, if I can learn one thing from posting then I am happy. I have definitly learned and drew better conclusions from your comments!!
 
J M Fahey 5/11/2018 2:27 AM
Blanket statements should have little disclaimer labels attached

A choke is "better than a resistor" because for any given impedance it has less DC resistance than an equivalent resistor, so it has lower voltage drop.

Since in this particular case we do need significant voltage drop, the opposite is truth .
 
Mick Bailey 5/11/2018 2:54 AM
I've also been using Shuguang EL34s in high voltage amps without any problems. My previous high-voltage tubes were late-90s Svetlana stock that's now too expensive to use. Shuguang tubes have thin envelopes compared with TADs, but in my post-mortem on both tubes I was unable to discern any difference in the spacers, electrode assembly, finish or construction. The TADs weigh a little more (I think about 6g heavier) due to the thicker envelope. TADS are less microphonic than factory-branded Shuguangs, possibly down to the glass.

As a sidestep the JJ KT77 spec sheet rates it for high plate and screen voltage, though I have not personally used these.

I agree with other observations that a white tube is a mechanical failure of the vacuum seal and is not usually symptomatic of high voltages. A tube that fails due to voltage issues will often short, arc internally or if you're lucky the screen will go open.

For me, I use a MOSFET voltage reducer to tame high amp voltages. Just a few components that are hard-wired onto the MOSFET pins and bolted to the chassis. It's dead easy, super-reliable and easy to change the voltage if needed. I reduce the overall voltage rather than regulate the screens. See R.G.'s 'MOSFET follies' for details. Here's how it looks (not my picture). There's no free ride - the MOSFET still produces as much heat as a resistor or Zener. Just cheaper, easier and neat.

[ATTACH=CONFIG]48813[/ATTACH]
 
Pedro Vecino 5/11/2018 3:11 AM
Thanks, Mick. Two questions: What is the reference of the mosfet? Does it need an insulating mica or can it be screwed directly onto the chassis?
 
dstrat 5/11/2018 3:54 AM
Have a look at the bottom of the page.

http://www.geofex.com/Article_Folder...osfetfolly.htm

Although not sure have never tried one, but it should be isolated from the chassis and large enough heat sink to keep the smoke in.
 
Gtr0 5/11/2018 4:51 AM
Quote Originally Posted by J M Fahey View Post
Blanket statements should have little disclaimer labels attached

A choke is "better than a resistor" because for any given impedance it has less DC resistance than an equivalent resistor, so it has lower voltage drop.

Since in this particular case we do need significant voltage drop, the opposite is truth .
And right after rectifier output is the right place? or in exact replacement of the choke? (between B+1 and B+2) - Thanks
 
dstrat 5/11/2018 5:03 AM
that would be the place.
you asked "I suppose the 25 to 28 mA area would be better?"
Yes it would , Tubes dont so much care what the voltage is , its the current thru the tube that makes it hot.
with that kind of plate volts 60-65% bias would not be a bad idea.
 
Zozobra 5/11/2018 5:12 AM
If you're after a tight metal/high gain sound then I'd try out the mosfet regulated screen supply as was mentioned earlier. I'm going to try this in my next high power build. It's a pretty simple circuit that you can squeeze on a bit of tag board quite easily.
Check this out: Amplified music : Guitar valve amp service

regulated screens deets are towards the bottom of the page.
 
Gregg 5/11/2018 6:52 AM
Although not sure have never tried one, but it should be isolated from the chassis and large enough heat sink to keep the smoke in.
Unless the metal back of your MOSFET is other than the drain then you can bolt it to directly to the chassis which will be both your ground connection and heatsink.
 
J M Fahey 5/11/2018 7:35 AM
Quote Originally Posted by Gtr0 View Post
And right after rectifier output is the right place? or in exact replacement of the choke? (between B+1 and B+2) - Thanks
In original choke place.
I am dropping only screen voltage, which to boot is a way lighter and colder duty (just a few mA) than reducing the *full* power supply voltage, including power tubes, which means *a lot* more power dissipation.

Just what was asked for in the original question; itīs assumed that plates by themselves are quite more robust and can take higher voltage without big trouble.

Of course, if you want to reduce full supply voltage, be my guest

EDIT:
Tubes dont so much care what the voltage is , its the current thru the tube that makes it hot.
Well, both do, to be more precise their product
W=V*A

EDIT:
Unless the metal back of your MOSFET is other than the drain then you can bolt it to directly to the chassis which will be both your ground connection and heatsink.
Please look at the picture shown above, that drain is hovering at around +400/500V above ground; I would very carefully insulate it from chassis/heatsink.
 
Chuck H 5/11/2018 8:07 AM
A note about dropping the screens with a resistor instead of the choke. This will drop voltage on the PI and preamp tubes as well and THAT will likely change the tone of the amp more than the difference on the screens. The following HV rail resistor feeding the PI and preamp in the SLO schematic is a 10k/2W. You should be able to reduce this value to bring the downstream voltage up again. Just measure voltage on the downstream side of that 10k resistor before changing the choke to a resistor so that you have a target voltage.
 
Mick Bailey 5/11/2018 9:26 AM
Quote Originally Posted by Pedro Vecino View Post
Thanks, Mick. Two questions: What is the reference of the mosfet? Does it need an insulating mica or can it be screwed directly onto the chassis?
The last batch I had were STW9NK90Z - 8A/900v/TO247. I've stuck with that rating for quite a few years, though various different types, depending on what's cheapest or easily available. I've never had an amp back with a failed circuit and it's robust and reliable - fit and forget. I just use mica and heatsink grease. The rating is way over what's needed and generally a 75W device such as the IRF830 is fine (4.5A/500v). I like the larger packages because the greater surface area means the device runs much cooler on a steel chassis as a heatsink and there's no problem in finding space. The legs are also more robust and the spacing is ideal for mounting the other components.
 
J M Fahey 5/11/2018 12:31 PM
Quote Originally Posted by Chuck H View Post
A note about dropping the screens with a resistor instead of the choke. This will drop voltage on the PI and preamp tubes as well and THAT will likely change the tone of the amp more than the difference on the screens.
Well. *that* is the point.
If original preamp expected to be fed from around 400V and in this particular amp itīs fed from around 500V , in both cases from the screen supplying node, taming that extra high voltage to the expected one is the reasonable thing to do.
The following HV rail resistor feeding the PI and preamp in the SLO schematic is a 10k/2W. You should be able to reduce this value to bring the downstream voltage up again.
Why?
You are now feeding that 10k resistor the proper voltage, why would you want to overvolt the preamp?
Just measure voltage on the downstream side of that 10k resistor before changing the choke to a resistor so that you have a target voltage.
I suggest you use the original SLO voltages as "targets".
Why correct a wrong one (in this case a too high one) at the screens and then "uncorrect" it?
 
Gregg 5/11/2018 1:42 PM
Please look at the picture shown above, that drain is hovering at around +400/500V above ground; I would very carefully insulate it from chassis/heatsink.
It looks like it's not used as suggested in R.G.'s article (to CT or to bridge negative) but in series with the B+ supply.
 
J M Fahey 5/11/2018 1:48 PM
Quote Originally Posted by Mick Bailey View Post
The last batch I had were STW9NK90Z - 8A/900v/TO247. I've stuck with that rating for quite a few years, though various different types, depending on what's cheapest or easily available. I've never had an amp back with a failed circuit and it's robust and reliable - fit and forget. I just use mica and heatsink grease. The rating is way over what's needed and generally a 75W device such as the IRF830 is fine (4.5A/500v). I like the larger packages because the greater surface area means the device runs much cooler on a steel chassis as a heatsink and there's no problem in finding space. The legs are also more robust and the spacing is ideal for mounting the other components.
It may be fine, but in general the datasheet rated dissipation is the most misleading rating of them all.
None of your fault of course, but "creative" Datasheet writing.

In the Real World no semiconductor will ever be able to dissipate that much, and will sweat a lot to even approach *half* that, if at all.

Practical example, and straight from IRF830 datasheet:
Maximum Power Dissipation TC = 25 °C PD 74 W
Looks good, huh? 74W!!!!!

read again: to be able to dissipate that, the *case* , not the heatsink, needs to be held at steady 25C .... good luck with that.

In the beginning I corrected my mental image to transistor, this one or any other one (as in *all* others), being rigidly mounted to, say, a 1 cubic meter block of Aluminum held at steady 25C , only guarantee for it to *stay* there at least for some time.

Hardly a realistic heatsink but whatīs implied by the rating.

Only later I noticed an even bigger flaw in that rating: holding that "infinite heatsink" (their words, not mine) at steady 25C is not enough, since we are ignoring the thermal resistance between case and heatsink .... conveniently ignored in most modern datasheets (it was present in old ones, such as old 2N3055 datasheets, those which came, ugh!! , *printed*!!!! and using, forgive my French: ink and paper !!!
Ok Mom, Iīll wash my mouth with Laundry soap now
Maybe that spoiled me because to this day I use that value to design.

I am talking Rcs here:
[IMG]https://electronics-cooling.com/wp-content/uploads/1995/06/a1f1.jpg[/IMG]

Searching a lot (since itīs not even mentioned any more) I found "typical" minimum values of 0.3 to 0.5 C/W , no mica, greased case to sink junction.

Assuming 0.5C/W as a safe value (since datasheet shows none), 75W through that means a 35C thermal drop.

Which also means that if case is to be held at 25C, and itīs mounted to a "best heatsink in the World" 2.5 Ton aluminum cube, said cube still needs to be held 35C below that, or minus 10C ... 10C below ice temperature.

Donīt know you but I find that dissipation rating "slightly" unrealistic

Ok, ok, what is realistic then?

Just a practical example, using same IRF820.

Suppose itīs in an SS environment (nearby HOT tubes will definitely worsen the situation) , inside a cabinet, inside temperature may reach 40C , maximum design junction temperature is 120C (considering absolute maximum is 150C and we want a little safety margin) and we use a reasonable 2C/W heatsink such as this one:
[IMG]https://www.accelthermal.com/assets/images/116-500.jpg[/IMG]
then final junction to air thermal resistance is: 1.69 + 0.5 + 2 = 4.19 C/W
(the 1.69 Rjc value is "shown but hidden" in the datasheet, as its inverse: "linear derating" parameter )

So maximum *actual* dissipation is (120-40)/4.19=about 20W .... a far cry from big bold lettering "74 W"

And that in a very conservative design: mild ambient temperature, no mica, large heat sink, etc.

For poor MosFets inside a HOT Tube amp chassis, bolted to not-too-thick steel sheet and of course using mica or Silpad, using a larger TO247 one such as suggested by Mick Bailey is the way to go.

And in any case I would search (by Finger-O-Meter at least) for the relatively coolest spot on that chassis to mount that MosFet.
 
Mick Bailey 5/11/2018 2:44 PM
Agreed, the power rating is a paper maximum. When you think though we're generally dropping say 30v to 40v at maybe 500mA peak, then 20W is about what the dissipation will be at full volume when the tubes are drawing maximum current. That's not factoring RMS. A TO220 gets hot in this application and needs careful consideration about mounting. It does actually work OK in practice, given that most amps aren't run to maximum output. It's not a device selection I personally like to use because of the issues discussed and when I've experimented with TO220 packages with an amp into a dummy load they get way hotter than a TO247 when pushed to maximum output. That's how I settled on that particular package even though people often use TO220s. I just don't like the hotspot they create.

In reply to Gregg, The circuit works in the B+ line or CT as per R.G.s original article. I use either method depending on the PS configuration and convenience of access.
 
bob p 5/11/2018 5:51 PM
I might be in the minority on this, but I don't think that clamping the screen sounds good, especially if you aren't also clamping B+. IMO you need to regulate both of them or neither of them to get the best sonic results. Clamping one while letting the other sag just doesn't work well.
 
bob p 5/11/2018 7:30 PM
Quote Originally Posted by J M Fahey View Post
Had to Google that.
Some of the cultural references may not be recognizable outside of an urban slum in the USA, which is the marketing niche where Tbird became famous. It was a cheap screw-cap fortified wine. And when I say it was cheap, I mean really cheap. Like 60-cents for a bottle. It had a high alcohol content (like a Port) and a cheap price, which made it popular with skid row bums.

I grew up in an urban area where Tbird was popular. When I was riding my bike as a kid I had to be careful about taking shortcuts through the alley as it almost always meant a flat tire, caused by the shattered remains of a Tbird bottle. Tbird was one of the famous "bum wines", which is probably why Chuck mentioned it in his post. It was so famous in the 'hood that people even wrote songs about it:



Tbird came around when the wine purity laws in the USA were relaxed in the 50s to allow "flavored wines" and alcohol-fortified products to be sold as "wine." That allowed the cheap vineyards to dump their low grade products by blending them, and adding flavorings and distilled liquor to bring the alcohol content up to about 20%, like a Port. The combination of low price and high alcohol content made Tbird popular in urban areas. The bums liked it because it was a cheap drunk, and the poor people liked it because it was a cheap mixer. I remember going to the grocery store as a kid and seeing Tbird on the shelf with packages of lemon Kool-Aid next to it. And there was the catchy jingle on the radio:

What's the Word? Thunderbird!
How’s it sold? Good and cold.
What’s the jive? Bird’s alive.
What’s the price? Thirty twice.

Mixing tbird with lemon Kool-Aid was pretty common in the neighborhood. It was always regarded as a cheap mixer. I have to confess, as a kid we used to sneak Tbird and Kool-Aid at a friend's house. We did the same thing with Mogen David and 7-up at another friend's house. Even if you cut them in half, you still had 10% alcohol content, and one glass was enough.

Tbird was always regarded as a cheap wine, but in the 70s they tried to make the brand more upscale with the grapefruit juice "Shake 'em Up!" campaign in the commercial that Juan linked. I confess ... I've also had Tbird and grapefruit juice "shakers", but that was something like 50 years ago when I lived in the 'hood ...

Gallo was one of the most successful companies at selling cheap wine. Some of their most successful brands from the 50s-70s were Thunderbird, Ripple and Boone's Farm. They all had a screw cap. When the market got tired of these products, the same low grade wines ended up being marketed in a more upscale fashion as carbonated "wine coolers" in the 80s. Same juice, different blend.
 
Chuck H 5/11/2018 8:02 PM
Juan, If he likes the way the amp sounds now, but just wants it to stop eating power tubes then maintaining the existing preamp voltages might suit the end result of the effort. Submitted for consideration only.
 
Gtr0 5/12/2018 3:24 AM
Pardon my ignorance on the topic... But what is the nature of the relationship between the screens and plate anyway? I *thought* the screens purpose is to better control the flow of electrons from the cathode?

How is it then does an amp like the silver jubilee cut out the screen resistor connection.. effective turning the tube into?? A triode?

Also how are amp manufacturers getting away with such his voltages on the screens as well? Obviously the SLO and similar amp the Dual Rectifier??
 
J M Fahey 5/12/2018 6:25 AM
In no particular order:
* thanks for the Sociologycal/Anthropological data, something which always interests me.

Among tons of other things, I have read quite a lot about Skid Row which is a unique phenomenon with its own (unwritten) laws, didnīt know the brand Thunderbird, but would have instantly recognized the generic "muskie" , as well as its users: "winos" and even their "starting a Corporation" (pooling coins and spare change so as to buy one or two bottles and then share them).
Plus the infamous "flophouses" where just *imagining* the smell makes me retch.

Oh well, poor people, Iīm certain nobody drifts there on his own will

EDIT: almost forgot: my own simplified explanation is this:

* the basic tube is the triode.
Plate attracts electrons with a strength proportional to its voltage.
That makes electron flow (current) VERY dependant on plate voltage.
Datasheets show that, curves are inclined, with current increasing/decreasing a lot depending on plate voltage.

* personal explanation, others may differ: screen which is always at a certain high voltage, strongly attracts a lot of electrons.
Most of them miss it because being a fine wire grid it occupies very little physical space ... so most electrons miss it and overshoot.
Then itīs easy peasy for plate to capture them, net result is that current is much higher than in a similar triode, and quite independent of plate voltage, so lines are *almost* horizontal.

* tying screen to plate kills its unique effects and turns it into little more than a plate extension.

Under same conditions, a triode strapped pentode will provide less power because maximum available current is greatly reduced.
 
pdf64 5/12/2018 6:36 AM
Quote Originally Posted by Gtr0 View Post
... But what is the nature of the relationship between the screens and plate anyway? I *thought* the screens purpose is to better control the flow of electrons from the cathode?...
The effect of the screen grid g2 is to 'de-couple' plate voltage from plate current.
See chart at top of p6 http://www.mif.pg.gda.pl/homepages/f...93/6/6L6GC.pdf
So, all else being equal, plate voltage can vary over a wide range without plate current being affected.

Quote Originally Posted by Gtr0 View Post
... How is it then does an amp like the silver jubilee cut out the screen resistor connection.. effective turning the tube into?? A triode?
...
Yes, the low power switch re-configures the power tubes as triodes; the current limiting resistors, eg R55 & R56 https://drtube.com/schematics/marshall/2554amp.gif are still in circuit.
 
Gtr0 5/15/2018 1:11 AM
Most of them miss it because being a fine wire grid it occupies very little physical space ... so most electrons miss it and overshoot.
Then itīs easy peasy for plate to capture them, net result is that current is much higher than in a similar triode, and quite independent of plate voltage, so lines are *almost* horizontal.
Hmmm... to better clarify what I meant (I was on the road during the last post, so it was by phone) and you may have answered is what is their relationship voltage wise as well as physically. So why is it that the screens would come from the next B+ node and see only a very tiny (usually only a handful of vdc at most) v drop. So in relation to the plates they should only be slightly less in vdc as to appear somewhat negative in relation to the plates and highly positive to the cathode?

But to be completely honest, I can't see the picture here as to why that would make the amp so much louder... only unless the high voltage of plates AND screens would attract that much more electrons...


Then, meanwhile back at the ranch...
As if on cue, I had a screen resistor go up in flames some days ago. Bastard still read 1K after removing it though.

Once I replaced it, and the tubes for well roundedness, it has been fine. But that voltage is still high, though I have yet to order a 10 watt resistor in the 180 ohm zone to bump the voltage down. My EL34 tube supply has been stretched beyond limits. Time to stock up again

BUT I did order a new custom tranny, this time 345-0-345 instead of 360-0-360 - so that's at least a start.
 
Jazz P Bass 5/15/2018 8:52 AM
I think in reality, the screen grid, being between the cathode and the plate, is the anode of the circuit.
The electrons reach the plate because of the wide open spacing of the screen grid.
By lowering the screen voltage you are throttling back the dissipation of the tube.
 
eschertron 5/15/2018 9:07 AM
I like that explanation ^^^

Think of the tube as a setting for a slasher movie.
The electrons (having been boiled off the cathode) are simply milling about (space charge) as if they were teenagers with nothing to do. The high positive potential of the screen grid is calling "psst. Hey! C'mon over here!" so the potential exists for the electrons to travel to the screen. BUT WAIT! IT'S A TRAP! The wide spaces between the screen grid elements allow the electrons to whizz past and be collected by the "monster", a.k.a. the plate electrode.

The reason it's a trap is because for large portions of the signal cycle, the plate is actually at a relatively low voltage compared to the screen. The plate voltage varies with the rate of electron collision, but the screen collects fewer electrons and doesn't vary significantly from the B+ voltage at its node. So the electrons are still highly attracted (you know how teenagers can be) to the screen/plate despite the - on average - lower plate voltage. They "see" the continuously high screen voltage.

Triodes (and triode-strapped pentodes) do not exert the continuous higher attractive force, and so do not deliver the higher currents, because the electrons do "see" the - on average - lower plate voltage.
 
J M Fahey 5/15/2018 2:13 PM
So do-nothing teenagers are attracted to Movie Theater by a bloodcurling Bela Lugosi movie ... and when there they are attracted by the *real* Monster.
Yes, I think I get it

 
bob p 5/15/2018 3:18 PM
The Blob is a great analogy.

But that trailer has been colorized. Oh, how I hate it when they do that to B&W movies.
 
Gtr0 5/16/2018 10:51 PM
Quote Originally Posted by eschertron View Post
The plate voltage varies with the rate of electron collision, but the screen collects fewer electrons and doesn't vary significantly from the B+ voltage at its node.
BINGO - there's the line that triggered the lightbulb.

Thank you!!
 
cerrem 5/30/2018 9:28 PM
The Screen current is more closely resembles a hyperbolic Cotangent function ..... ie, the screen current will climb as the plate load swings more plate positive... and decreases to it's idle current of a few mA as the load line swing back down towards the idle state...then into cut-off....when the other side of the Push-Pull is already conducting within the overlap angle...
Marshall amps from the early 70's run about 500V plate and 498V screen at idle .... 1K screen dropping resistors... choke is roughly 110 Ohms..
With the 1.7K plate load and running the amp at full tilt with Siemens EL34's..... the power supply will sag to roughly 430 Plate and 400V on the screens... of course your mileage mat vary depending on the PT's regulation..
The screen current for each EL34 at idle will be about 6mA ...the screen during clipping is up about 32mA RMS ...this includes the Duty Cycle since the screen does not conduct the full degrees....roughly 225 give or take degrees in Class AB1...
The typical NOS EL34 had 8W MAX of Screen Dissipation... What size screen resistors are you using ??
One time I had seen a Laney Super Group amp have similar issues where some of the El34's were blowing up with white internals... It wound up being internal arcing of the tube that damaged the tube.... It only happened on a certain speaker cabinet... So I measured the fly-back voltages up to 1.1kV ...this arced internal to the tube between pins 3 and pin 2 heater....since heaters were at ground potential ....
 
Gtr0 6/2/2018 12:09 PM
Quote Originally Posted by cerrem View Post
The Screen current is more closely resembles a hyperbolic Cotangent function ..... ie, the screen current will climb as the plate load swings more plate positive... and decreases to it's idle current of a few mA as the load line swing back down towards the idle state...then into cut-off....when the other side of the Push-Pull is already conducting within the overlap angle...
Marshall amps from the early 70's run about 500V plate and 498V screen at idle .... 1K screen dropping resistors... choke is roughly 110 Ohms..
With the 1.7K plate load and running the amp at full tilt with Siemens EL34's..... the power supply will sag to roughly 430 Plate and 400V on the screens... of course your mileage mat vary depending on the PT's regulation..
The screen current for each EL34 at idle will be about 6mA ...the screen during clipping is up about 32mA RMS ...this includes the Duty Cycle since the screen does not conduct the full degrees....roughly 225 give or take degrees in Class AB1...
The typical NOS EL34 had 8W MAX of Screen Dissipation... What size screen resistors are you using ??
One time I had seen a Laney Super Group amp have similar issues where some of the El34's were blowing up with white internals... It wound up being internal arcing of the tube that damaged the tube.... It only happened on a certain speaker cabinet... So I measured the fly-back voltages up to 1.1kV ...this arced internal to the tube between pins 3 and pin 2 heater....since heaters were at ground potential ....
For all intents and purposes, it is basically a plexi/2203 output section, with one exception. 1k-5W dropping resistors, also choke is in the same area, 115 ohms if I recall correctly. However, the exception here is that I have a 2k plate load, not 1.7 like a typical 100 watt marshall.
 
cerrem 6/2/2018 4:14 PM
Quote Originally Posted by Gtr0 View Post
For all intents and purposes, it is basically a plexi/2203 output section, with one exception. 1k-5W dropping resistors, also choke is in the same area, 115 ohms if I recall correctly. However, the exception here is that I have a 2k plate load, not 1.7 like a typical 100 watt marshall.
Having 2k for plate load is perfectly fine that should not be an issue...
I would suggest confirming the transformer color codes and make sure the solder terminals on the ohm selector line up with what the selector is indicating....
When the EL34 tubes fail, where is the crack located ??
 
Gtr0 6/4/2018 4:56 AM
Quote Originally Posted by cerrem View Post
Having 2k for plate load is perfectly fine that should not be an issue...
I would suggest confirming the transformer color codes and make sure the solder terminals on the ohm selector line up with what the selector is indicating....
When the EL34 tubes fail, where is the crack located ??
I have not ever been able to find a crack, see the following image...
[ATTACH=CONFIG]49093[/ATTACH]

As you can see the tube has whited out, and there is a burn at the base... this is near pins 7 and 8. Heater to Cathode violation?? In another case, the tube was white and I was unable to find any cracks or burns... but when they go down there is usually an accompanying flashing inside the tube with it. So I am at a bit of a loss on this subject. I am trying to make sense of it, but taking into account the reputation of "modern production tubes" and the fact that my PT is grossly abusing some of those voltage maximums... and while I can not rule out the possibility of a mis-wiring, I have gone over wiring several times not to mention used the exact same scheme in several amps. However one boo-boo I have found is that I was using metal oxide screen droppers... I will be switching those to wirewound.
 
Leo_Gnardo 6/4/2018 5:48 AM
Quote Originally Posted by Gtr0 View Post
As you can see the tube has whited out
It's kaput, broke, busted. Doesn't need lightning flashes inside to prove it. Glass need not be cracked in any place you can see it. A popular leakage site is the "header" where metal pins protrude through the glass tube base. Each metal/glass seal must be perfect, and stay that way, or else pfffft (sound of a vacuum leak.) It happens, even to the best of 'em.
 
Justin Thomas 6/4/2018 7:07 AM
I thought the getter was for trapping residual gas inside the tube. Any possibility that an an internal tube element is melting down or overheating and releasing gases inside the tube? I've always heard that overheated plates release gas... why not other elements? There may not BE a crack in the tube...

Either way, as Leo said, it's dead. If they all are failing the same way, either there's a run of bad tubes, or...
Justin
 
Chuck H 6/6/2018 7:31 AM
Quote Originally Posted by Gtr0 View Post
My EL34 tube supply has been stretched beyond limits. Time to stock up again
Quote Originally Posted by Justin Thomas View Post
either there's a run of bad tubes, or...
Justin may be onto something. If you've been experiencing tube failure on a regular basis and pulling from a "stash" of tubes I have two questions...

Is this tube stash a collection of all the same brand/type/batch? In which case those tubes may be faulty or simply not be up to the conditions in your amp. And,..

When you have a tube failure are you simply replacing THAT tube from your "stash"? This could also be significant because in a hundy W if the tubes at one or the other side of the push/pull aren't at least reasonably matched then the higher current tube will be doing most of the work. So if you had a tube failure, then just plugged in a tube and THAT tube draws less current than the other one on that side then that other tube may be the next to fail. Making the problem seem random in nature.

I'm not saying "matched" tubes are all that important for most guitar amp circumstances. But when you start pushing higher voltage and have tubes sharing larger wattage duty, well then yes, you really should have tubes of reasonably similar current draw.

I don't think you need to replace the PT or add a series resistor or otherwise mitigate high voltage. Plenty of old Marshalls and others are notorious for plate voltages over 500 and are working just fine. If you like the sound of the amp you should leave the circuit alone. If you're not married to the tone it has then sure, go ahead and change it for lower voltage and enjoy longer tube life and reliability. But I think the real problem may just be mediocre tubes, unmatched and in an amp with somewhat demanding operating conditions. This is actually the notion you opened the thread with and I think you had it right then.

Ruby selected Shuguang EL34B tubes. Buy them in matched quads. It may seem expensive at first, but it costs less than new transformers and the time it takes to do other modifications and in the long run it's likely that the life of the tubes will more than prove to be a value.
 
Gtr0 6/6/2018 12:35 PM
Quote Originally Posted by Chuck H View Post
Justin may be onto something. If you've been experiencing tube failure on a regular basis and pulling from a "stash" of tubes I have two questions...

Is this tube stash a collection of all the same brand/type/batch? In which case those tubes may be faulty or simply not be up to the conditions in your amp. And,..

When you have a tube failure are you simply replacing THAT tube from your "stash"? This could also be significant because in a hundy W if the tubes at one or the other side of the push/pull aren't at least reasonably matched then the higher current tube will be doing most of the work. So if you had a tube failure, then just plugged in a tube and THAT tube draws less current than the other one on that side then that other tube may be the next to fail. Making the problem seem random in nature.

I'm not saying "matched" tubes are all that important for most guitar amp circumstances. But when you start pushing higher voltage and have tubes sharing larger wattage duty, well then yes, you really should have tubes of reasonably similar current draw.

I don't think you need to replace the PT or add a series resistor or otherwise mitigate high voltage. Plenty of old Marshalls and others are notorious for plate voltages over 500 and are working just fine. If you like the sound of the amp you should leave the circuit alone. If you're not married to the tone it has then sure, go ahead and change it for lower voltage and enjoy longer tube life and reliability. But I think the real problem may just be mediocre tubes, unmatched and in an amp with somewhat demanding operating conditions. This is actually the notion you opened the thread with and I think you had it right then.

Ruby selected Shuguang EL34B tubes. Buy them in matched quads. It may seem expensive at first, but it costs less than new transformers and the time it takes to do other modifications and in the long run it's likely that the life of the tubes will more than prove to be a value.
No problem...I am used to paying 100 - 130 euros for a matched quad.... I have a quad of TAD EL34B-STR (Shuguang), and so far so good - They have been in a Marshal Plexi for the better part of a year now. So those are also great options.

As for the current problems and replacing them... I was convinced it was a batch problem. So much so that I emailed my retailer and complained. Of two tubes that blew... rather fantastically.. they were within 30 days, so I got replacements that were matched for my previous orders... so they were drop in replaceable. Always checking the bias then as well if they fall within 4mA, then I do not worry any further and set bias according to the highest tube. However, having emailed my retailer and asking about this particular brand, they claimed that they were one of the more solid tubes - these were JJ E34L's - I have used them for well over a decade, religiously, without a single failure, until last year. Since the two sets were purchased a month or two apart, I am going to continue under the bad batch / pushing them toward their limits assumption.

My latest tube fiasco (mentioned above), a Tung Sol EL34 that - after about of month of awesome performance - began red plating - I caught it and got the amp turned off within a few seconds... and the amp fired up the next day and ran fine for at least 30 minutes. On a whim I swapped in another set... fired up the amp and the same tube (socket) flashed... This is a recent revelation, but doesn't change the fact of getting bad tubes these days.

So I have not tested any further with the Tung Sols... In fact I ripped the amp apart down to the chassis because it is a roughed out prototype anyway - rebuilding it now.

Of all of the tubes I have... The Tung Sol EL34 was a "to die for" tone - for me. Certainly haven't tried all, or nearly enough, brands
Tung Sol EL34
JJ E34L
Svetlana EL34 (came in my plexi, originals from the late 1990's)
TAD EL34B-STR

That's my list, in preferred order based on sound. The difference between TAD and Svets were small... then E34L has some kind of lower mid juicyness, then the Tung Sol was midrange madness - in a great and musical way. I was seriously disappointed to see the one side red plating.

Of course It is allllll subjective... but even in 6L6GC/5881 area, I found the Tung Sols were way nicer sounding than other brands... again subjective.

Anyway, again, thank you! I am going to continue on bad batches... see if I can get a super solid build together and further experiment from there.

Cheers!
 
Justin Thomas 6/6/2018 2:28 PM
Quote Originally Posted by Gtr0 View Post
... but even in 6L6GC/5881 area, I found the Tung Sols were way nicer sounding than other brands... again subjective.
Slightly off topic, but...
I really hope everyone knows that the TS "reissue" 5881s are NOT equivalent to a 6L6GC, but are a 19W tube equivalent to a 6L6GB... It's the Sovtek 5881WXT that is the "super tube" that's equivalent or better than a 6L6GC.

Also, did TS ever make an EL34? Hard to "reissue" something that was never manufactured in the first place. Instead, now we have Mullard 6L6GCs and TS KT66s. Maybe I'm just being petty, but this kind of crap on the part of New Sensor (yeah,I'm calling them out) just really pisses me off, because of the confusion and muddying it causes.

Anyway, carry on.

Justin
 
Leo_Gnardo 6/6/2018 2:52 PM
Quote Originally Posted by Justin Thomas View Post
Also, did TS ever make an EL34? Hard to "reissue" something that was never manufactured in the first place. Instead, now we have Mullard 6L6GCs and TS KT66s. Maybe I'm just being petty, but this kind of crap on the part of New Sensor (yeah,I'm calling them out) just really pisses me off, because of the confusion and muddying it causes.
Spot on Justin. To the best of my knowledge there was never a real TungSol EL34. It's a marketing gimmick by New Sensor to sell the s.o.s. for an extra couple of bucks. You can call 'em out but they don't care, har har we get the money, ka-CHING!
 
cerrem 6/6/2018 3:00 PM
Now see more detail into the tube damage...
Very convinced this is Fly-Back arc-over voltage...
Seen this quite a number of times....
On at least several of these situations..I first asked the customer, who are DIY, to double check the impedance selector...I re-ask a thousand more times and they always come back saying it's fine....
Well the amp gets on my bench and I find the impedance sector wired up backwards or off by a few solder tabs..... main reason is that many times some people assume that the solder tab right behind the OHM number is the correct solder tab , when it is actually the opposite end .... or even worse are the Marshall selectors of the 70's with those "Window Selectors" which are more deceiving....
Getting back on point, the amp in many cases has the 4 Ohm wire inadvertently going to the 16 Ohm tab on the selector, thus making for HUGE plate load which made for crazy flyback voltage ratio ....
On strong transients with tight compliance speakers....the back EMF can by significant...this back emf generated from the speaker acting as a generator will get stepped up by the OT ratio... then this fly-back voltage is additive to the amps B+ voltage and any IN-Phase signals on the primary winding... Now you can get some pretty high voltage spikes in the kV range....
There are several paths this high voltage spike could take to arc-over..... Tube sockets can break-down, Air between PINS 3 and 2 can break -down.... or tube internals can break down at the bass pins...
One method I used to prevent the arc-over was to eliminate the ground potential all together.... Since the arcing was from pin 3 to 2 ,,,since PIN 2 is at Ground Potential from the Heater Center-Tap being grounded... I lifted the Heater Center-Tap off ground and floated it.... Used a LARGE film cap to provide the AC imbalance current to flow with a little impedance as possible...I might have paralleled several caps to get the HUM to acceptable level...
Then never blew a tube again
 
Leo_Gnardo 6/6/2018 4:12 PM
Quote Originally Posted by cerrem View Post
One method I used to prevent the arc-over was to eliminate the ground potential all together.... Since the arcing was from pin 3 to 2 ,,,since PIN 2 is at Ground Potential from the Heater Center-Tap being grounded... I lifted the Heater Center-Tap off ground and floated it.... Used a LARGE film cap to provide the AC imbalance current to flow with a little impedance as possible...I might have paralleled several caps to get the HUM to acceptable level...
Then never blew a tube again
Please forgive if I've mentioned previously in this thread, I know I have somewhere within the recent past, Ampeg's ground float method they used in many 60's guitar amps. Just like you, Ampeg placed a cap between filament winding center tap and ground. They usually employed a 0.1 uF 200V film cap. No harm in upping the voltage say to 600 or 630V to withstand almost any B+, and changing cap value to whatever minimizes hum. I have occasionally been able to use this technique to good effect. Having the filament floating on the cap eliminates it as a ground potential source for arcs from the nearby plate terminal as you said.

If there's been any arcing, tube bases and sockets should be closely examined for carbonized tracks or traces and dealt with accordingly. Replacement is always a prime option. In some cases I've been able to "save" bases or sockets by grinding away the carbonized areas with a Dremel tool.
 
cerrem 6/6/2018 9:33 PM
When the arc occurs over the air space between tube socket pins... I have used black RTV between the pins with some success..
 
Gtr0 6/7/2018 1:32 AM
Quote Originally Posted by Justin Thomas View Post
Slightly off topic, but...
I really hope everyone knows that the TS "reissue" 5881s are NOT equivalent to a 6L6GC, but are a 19W tube equivalent to a 6L6GB... It's the Sovtek 5881WXT that is the "super tube" that's equivalent or better than a 6L6GC.

Also, did TS ever make an EL34? Hard to "reissue" something that was never manufactured in the first place. Instead, now we have Mullard 6L6GCs and TS KT66s. Maybe I'm just being petty, but this kind of crap on the part of New Sensor (yeah,I'm calling them out) just really pisses me off, because of the confusion and muddying it causes.

Anyway, carry on.

Justin
Quote Originally Posted by Leo_Gnardo View Post
Spot on Justin. To the best of my knowledge there was never a real TungSol EL34. It's a marketing gimmick by New Sensor to sell the s.o.s. for an extra couple of bucks. You can call 'em out but they don't care, har har we get the money, ka-CHING!
Not knowing if the US Tung Sol ever made an EL34, it doesn't really matter to me personally... there is no reason that the Russian Tung Sol (New Sensor) can't make a variety of tubes, new or reissue. But saying this I realize there is a HUGE layer of BS to what tube manufacturers - especially New Sensor, do. One thing that disturbs me is that in one thread, maybe in this one or another thread at a different forum, someone said that Tung Sols are basically made in DIY basements. Then of course I have to wonder how this person knew that and if it is a directly known fact or the result of hear-say. Well, either way....

Quote Originally Posted by cerrem View Post
Now see more detail into the tube damage...
Very convinced this is Fly-Back arc-over voltage...
Seen this quite a number of times....
On at least several of these situations..I first asked the customer, who are DIY, to double check the impedance selector...I re-ask a thousand more times and they always come back saying it's fine....
Well the amp gets on my bench and I find the impedance sector wired up backwards or off by a few solder tabs..... main reason is that many times some people assume that the solder tab right behind the OHM number is the correct solder tab , when it is actually the opposite end .... or even worse are the Marshall selectors of the 70's with those "Window Selectors" which are more deceiving....
Getting back on point, the amp in many cases has the 4 Ohm wire inadvertently going to the 16 Ohm tab on the selector, thus making for HUGE plate load which made for crazy flyback voltage ratio ....
On strong transients with tight compliance speakers....the back EMF can by significant...this back emf generated from the speaker acting as a generator will get stepped up by the OT ratio... then this fly-back voltage is additive to the amps B+ voltage and any IN-Phase signals on the primary winding... Now you can get some pretty high voltage spikes in the kV range....
There are several paths this high voltage spike could take to arc-over..... Tube sockets can break-down, Air between PINS 3 and 2 can break -down.... or tube internals can break down at the bass pins...
One method I used to prevent the arc-over was to eliminate the ground potential all together.... Since the arcing was from pin 3 to 2 ,,,since PIN 2 is at Ground Potential from the Heater Center-Tap being grounded... I lifted the Heater Center-Tap off ground and floated it.... Used a LARGE film cap to provide the AC imbalance current to flow with a little impedance as possible...I might have paralleled several caps to get the HUM to acceptable level...
Then never blew a tube again
Interesting... never heard of floating the CT. How would one go about calculating the required value of this capacitor? I started another thread trying to resolve if I picked up some legit R3000 diodes... this should help to a degree as well right? At least in terms of fly back voltage.

Quote Originally Posted by Leo_Gnardo View Post
Please forgive if I've mentioned previously in this thread, I know I have somewhere within the recent past, Ampeg's ground float method they used in many 60's guitar amps. Just like you, Ampeg placed a cap between filament winding center tap and ground. They usually employed a 0.1 uF 200V film cap. No harm in upping the voltage say to 600 or 630V to withstand almost any B+, and changing cap value to whatever minimizes hum. I have occasionally been able to use this technique to good effect. Having the filament floating on the cap eliminates it as a ground potential source for arcs from the nearby plate terminal as you said.

If there's been any arcing, tube bases and sockets should be closely examined for carbonized tracks or traces and dealt with accordingly. Replacement is always a prime option. In some cases I've been able to "save" bases or sockets by grinding away the carbonized areas with a Dremel tool.
As far as I can tell, there has not been any carbon scoring. BUT... as said I ripped the amp down to the chassis... rebuilding it now including new sockets. AND (@cerrem) while of course it is entirely possible that I had wired the ohm selector backwards, though I do not think I did, I have no way of knowing now that I tore it apart (though perhaps I have a photo and in such a case I am extremely curious)... in addition I am wiring impedance taps direct to their own/own pair of switchcraft jacks this time to eliminate the switcher.

EDIT: Okay, I did confirm via photos that I did wire it correctly. HOWEVER... there was a time that I had one of my cabinets wired wrong and the amp was set for 16ohm and cab was lower. Its hard to remember if these things occurred at the same time... I wonder.....

Of course I realize, as usual, 99% of the problem is the person sitting at the amp, user error.
 
Gtr0 6/7/2018 1:32 AM
Quote Originally Posted by Justin Thomas View Post
Slightly off topic, but...
I really hope everyone knows that the TS "reissue" 5881s are NOT equivalent to a 6L6GC, but are a 19W tube equivalent to a 6L6GB... It's the Sovtek 5881WXT that is the "super tube" that's equivalent or better than a 6L6GC.

Also, did TS ever make an EL34? Hard to "reissue" something that was never manufactured in the first place. Instead, now we have Mullard 6L6GCs and TS KT66s. Maybe I'm just being petty, but this kind of crap on the part of New Sensor (yeah,I'm calling them out) just really pisses me off, because of the confusion and muddying it causes.

Anyway, carry on.

Justin
Quote Originally Posted by Leo_Gnardo View Post
Spot on Justin. To the best of my knowledge there was never a real TungSol EL34. It's a marketing gimmick by New Sensor to sell the s.o.s. for an extra couple of bucks. You can call 'em out but they don't care, har har we get the money, ka-CHING!
Not knowing if the US Tung Sol ever made an EL34, it doesn't really matter to me personally... there is no reason that the Russian Tung Sol (New Sensor) can't make a variety of tubes, new or reissue. But saying this I realize there is a HUGE layer of BS to what tube manufacturers - especially New Sensor, do. One thing that disturbs me is that in one thread, maybe in this one or another thread at a different forum, someone said that Tung Sols are basically made in DIY basements. Then of course I have to wonder how this person knew that and if it is a directly known fact or the result of hear-say. Well, either way....

Quote Originally Posted by cerrem View Post
Now see more detail into the tube damage...
Very convinced this is Fly-Back arc-over voltage...
Seen this quite a number of times....
On at least several of these situations..I first asked the customer, who are DIY, to double check the impedance selector...I re-ask a thousand more times and they always come back saying it's fine....
Well the amp gets on my bench and I find the impedance sector wired up backwards or off by a few solder tabs..... main reason is that many times some people assume that the solder tab right behind the OHM number is the correct solder tab , when it is actually the opposite end .... or even worse are the Marshall selectors of the 70's with those "Window Selectors" which are more deceiving....
Getting back on point, the amp in many cases has the 4 Ohm wire inadvertently going to the 16 Ohm tab on the selector, thus making for HUGE plate load which made for crazy flyback voltage ratio ....
On strong transients with tight compliance speakers....the back EMF can by significant...this back emf generated from the speaker acting as a generator will get stepped up by the OT ratio... then this fly-back voltage is additive to the amps B+ voltage and any IN-Phase signals on the primary winding... Now you can get some pretty high voltage spikes in the kV range....
There are several paths this high voltage spike could take to arc-over..... Tube sockets can break-down, Air between PINS 3 and 2 can break -down.... or tube internals can break down at the bass pins...
One method I used to prevent the arc-over was to eliminate the ground potential all together.... Since the arcing was from pin 3 to 2 ,,,since PIN 2 is at Ground Potential from the Heater Center-Tap being grounded... I lifted the Heater Center-Tap off ground and floated it.... Used a LARGE film cap to provide the AC imbalance current to flow with a little impedance as possible...I might have paralleled several caps to get the HUM to acceptable level...
Then never blew a tube again
Interesting... never heard of floating the CT. How would one go about calculating the required value of this capacitor? I started another thread trying to resolve if I picked up some legit R3000 diodes... this should help to a degree as well right? At least in terms of fly back voltage.

Quote Originally Posted by Leo_Gnardo View Post
Please forgive if I've mentioned previously in this thread, I know I have somewhere within the recent past, Ampeg's ground float method they used in many 60's guitar amps. Just like you, Ampeg placed a cap between filament winding center tap and ground. They usually employed a 0.1 uF 200V film cap. No harm in upping the voltage say to 600 or 630V to withstand almost any B+, and changing cap value to whatever minimizes hum. I have occasionally been able to use this technique to good effect. Having the filament floating on the cap eliminates it as a ground potential source for arcs from the nearby plate terminal as you said.

If there's been any arcing, tube bases and sockets should be closely examined for carbonized tracks or traces and dealt with accordingly. Replacement is always a prime option. In some cases I've been able to "save" bases or sockets by grinding away the carbonized areas with a Dremel tool.
As far as I can tell, there has not been any carbon scoring. BUT... as said I ripped the amp down to the chassis... rebuilding it now including new sockets. AND (@cerrem) while of course it is entirely possible that I had wired the ohm selector backwards, though I do not think I did, I have no way of knowing now that I tore it apart (though perhaps I have a photo and in such a case I am extremely curious)... in addition I am wiring impedance taps direct to their own/own pair of switchcraft jacks this time to eliminate the switcher.

EDIT: Okay, I did confirm via photos that I did wire it correctly. HOWEVER... there was a time that I had one of my cabinets wired wrong and the amp was set for 16ohm and cab was lower. Its hard to remember if these things occurred at the same time... I wonder.....

Of course I realize, as usual, 99% of the problem is the person sitting at the amp, user error.
 
Justin Thomas 6/7/2018 2:18 AM
I don't deny New Sensor the right to make tons of different tube types. But you can't reissue something that never existed in the first place.

My bigger point was to NOT simply think of the TS "reissue"5881s as equivalent to a 6L6GC. They never were. The 5881 is a completely different tube. Sure, it might work in the same sockets, but it still only dissipates 19W to the GC' s 30. Except for the Sovtek 5881WXT, which is as tough as they come.

Justin
 
Mick Bailey 6/7/2018 2:40 AM
Quote Originally Posted by Leo_Gnardo View Post
Having the filament floating on the cap eliminates it as a ground potential source for arcs from the nearby plate terminal as you said.
I'm not convinced. An arc will still develop if the voltage is sufficient and then the capacitor actually intensifies the arc by causing an oscillatory circuit and can reach a much higher, self-sustaining voltage that will short out the cap unless it's one designed to stand tens of Kv. I've used 30Kv series capacitors in a successful ignition system for agricultural engines to intensify the spark during startup for low-flashpoint fuels. The capacitor increases the arc energy enormously by storing and releasing energy.
 
Leo_Gnardo 6/7/2018 6:56 AM
Quote Originally Posted by Gtr0 View Post
Not knowing if the US Tung Sol ever made an EL34, it doesn't really matter to me personally... there is no reason that the Russian Tung Sol (New Sensor) can't make a variety of tubes, new or reissue. But saying this I realize there is a HUGE layer of BS to what tube manufacturers - especially New Sensor, do. One thing that disturbs me is that in one thread, maybe in this one or another thread at a different forum, someone said that Tung Sols are basically made in DIY basements. Then of course I have to wonder how this person knew that and if it is a directly known fact or the result of hear-say. Well, either way....
Much of New Sensor's so-called "variety" comes from applying different labels to the same tubes, making different claims for them and charging different prices. Some of them are OK tubes, others - well you better use them in low voltage applications or else. What "brands" are currently in the New Sensor family? Sovtek, Electro-Harmonix, Mullard, TungSol, GEC Gold Lion. Sound off readers if I left any out. Don't worry, NS's TungSols aren't hand made in some comrade's basement or garage, rather the same factory, same assembly line as all the rest in beautiful suburban Saratov out there in the steppes north of Georgia.

Interesting... never heard of floating the CT. How would one go about calculating the required value of this capacitor
I don't know of any formula but you could start with Ampeg's 0.1uF value, 600/630V. Add extra caps & see whether there's a reduction of hum.
 
Chuck H 6/7/2018 8:35 PM
Perhaps worth noting...

IMHE (Not: In my "honest" experience as some think, but, In my "humble" experience", as it should be.) the problem is likely occurring at high frequency. Since the typical load for a guitar amp is VERY high impedance at higher frequencies (just look at the plots for this!) and the choke to the screen grids is always limited in henrys it's possible to have very high voltage spikes (and kick backs, thank you cerrem) that find their way through the screen circuit. At these higher frequency impedances it's often possible that the screen circuit represents a lower load than the plates!!! Bumping henries on the choke or adding resistance (in the form of a resistor) after the choke can work. As can bumping the individual screen grid resistor value. Other than that you'd need to mitigate grid drive to reduce spiking. I've dealt with this in builds a couple of times and this is the scenario I interpreted. All of this will change the tone of the amp. for better or worse I suppose since tone is subjective. But I think it's possible that this may be a circumstance to consider.
 
Gtr0 6/8/2018 12:23 AM
Quote Originally Posted by Justin Thomas View Post
I don't deny New Sensor the right to make tons of different tube types. But you can't reissue something that never existed in the first place.
Justin
No doubt - I didn't mean at all for it to sound snarky.. in fact I appreciate any and all info on these types of things.

AND AND AND... I JUST looked at my supplier and in fact they list it as a reissue - so yes, you are totally correct. I had paid no attention to that when I bought them.
 
Chuck H 6/9/2018 6:32 AM
And... Even if the "reissue" tube never was, that's not important to the matter at hand. If you like the tube and it's up to the challenge that's more important. Is it up to the challenge? Well the New Sensor data sheet for the el34b (Tung Sol "reissue") is vague, but seems to indicate that the tube is equal to other el34's. That doesn't mean it's safe to exceed the screen voltage as is typical of older amp designs. I don't even know that it's fair to say it's implied by the equivalent specs to older tubes. So there's THAT. My point is, exceeding the screen grid voltage on older el34 amps was common, but that may not mean that new el34's are up to the challenge. New Sensor is notorious for regurgitating info from old tube data sheets. In fact it appears their Tung sol 5881data sheet IS the Tung Sol data sheet with the vintage dates removed. It's also common to find some arbitrary criteria, like voltages for "typical operation", along with all associated performance data at those voltages, etc. that are spot on with vintage data sheets such it's clearly just copied info. It's for this reason that I don't trust modern data sheets that are even a little suspect of not being data acquired by testing the actual tube in question. Because WHY would a tube manufacturer publish copied data if they have actual info on the product they're selling?!? I think these designs are based on the tube models in question and then tossed at the public wall to see if they stick and it's only assumed, by virtue of design copy, that the old data sufficiently matches the new products performance. But are the materials, tolerances and expertise of the new manufacturers equal to that of the old? I do not think so. That basically makes the data sheets cartoons. That is, they look vaguely like real life, but you probably can't safely do some of that stuff.
 
cerrem 6/10/2018 11:19 PM
Quote Originally Posted by Chuck H View Post
And... Even if the "reissue" tube never was, that's not important to the matter at hand. If you like the tube and it's up to the challenge that's more important. Is it up to the challenge? Well the New Sensor data sheet for the el34b (Tung Sol "reissue") is vague, but seems to indicate that the tube is equal to other el34's. That doesn't mean it's safe to exceed the screen voltage as is typical of older amp designs. I don't even know that it's fair to say it's implied by the equivalent specs to older tubes. So there's THAT. My point is, exceeding the screen grid voltage on older el34 amps was common, but that may not mean that new el34's are up to the challenge. New Sensor is notorious for regurgitating info from old tube data sheets. In fact it appears their Tung sol 5881data sheet IS the Tung Sol data sheet with the vintage dates removed. It's also common to find some arbitrary criteria, like voltages for "typical operation", along with all associated performance data at those voltages, etc. that are spot on with vintage data sheets such it's clearly just copied info. It's for this reason that I don't trust modern data sheets that are even a little suspect of not being data acquired by testing the actual tube in question. Because WHY would a tube manufacturer publish copied data if they have actual info on the product they're selling?!? I think these designs are based on the tube models in question and then tossed at the public wall to see if they stick and it's only assumed, by virtue of design copy, that the old data sufficiently matches the new products performance. But are the materials, tolerances and expertise of the new manufacturers equal to that of the old? I do not think so. That basically makes the data sheets cartoons. That is, they look vaguely like real life, but you probably can't safely do some of that stuff.
I have done consulting for New Sensor over the years.... mostly with pedals and transformer designs for their amps... I happen to own two Tektronix 570 curve tracers that I fully restored and calibrated .... I offered New Sensor a great deal ...I would draw up proper data sheets for their product line of vacuum tubes... providing I-V curves , Dynamic transfer curves, Grid 1 current and screen current curves, Ultra-Linear curves....plate curves at all various screen voltages, ie usable data sheets..... I have thermal imaging camera that can give accurate bulb temperatures at various dissipation, to ensure proper tube capabilities.... Well I got a big NO !!!! They are not even remotely interested in having this done....they like their situation just as is ...they don't want to spend a penny...
Whatever... I thinks it pathetic that they want to be a big player in tube distribution but show ZERO interest in providing proper data sheets.... I have seen their "In house" tube tester...looks like a 5th graders science project ....
 
cerrem 6/10/2018 11:50 PM
Getting back to the original subject at hand.... Last night I blew a power tube in my Ampeg SVT while playing a show..... First time in many many years this happened to me...
The reason is because I put the SVT head on top of the speaker cabinet and I was playing loud and VERY DEEP frequencies !!!
You can see from photo the crack on the tube ..most likely mechanical vibration caused the fracture of the glass, which compromised the vacuum which then provided the lightening show...
I normally stack 2 or 3 SVT heads on a separate dolly and keep it to the side, so not to direct contact with the cabs....
Last night I made exception due to not much room on the stage due to a drummer that takes up 80% of the stage area...
I am thinking maybe the original poster to this thread maybe if he has has amplifier in contact to the speaker cabinet ???
[ATTACH=CONFIG]49237[/ATTACH] [ATTACH=CONFIG]49238[/ATTACH]
 
Leo_Gnardo 6/11/2018 3:31 AM
Quote Originally Posted by cerrem View Post
They are not even remotely interested in having this done....they like their situation just as is ...they don't want to spend a penny...
Whatever... I thinks it pathetic that they want to be a big player in tube distribution but show ZERO interest in providing proper data sheets.... I have seen their "In house" tube tester...looks like a 5th graders science project ....
Agreed, that is revealing. And pathetic. I will continue to mostly avoid New Sensor's tube offerings.
 
Justin Thomas 6/11/2018 4:36 AM
I'll still buy their tubes, but only if they're branded "Sovtek." Cuz despite the "reissues" being unable to hang, that ol' 5881WXT keeps chugging away; anybody know the actual max voltage & plate dissipation ratings on those yet?

Justin
 
Gtr0 6/14/2018 12:02 AM
Quote Originally Posted by Chuck H View Post
And... Even if the "reissue" tube never was, that's not important to the matter at hand. If you like the tube and it's up to the challenge that's more important. Is it up to the challenge? Well the New Sensor data sheet for the el34b (Tung Sol "reissue") is vague, but seems to indicate that the tube is equal to other el34's. That doesn't mean it's safe to exceed the screen voltage as is typical of older amp designs. I don't even know that it's fair to say it's implied by the equivalent specs to older tubes. So there's THAT. My point is, exceeding the screen grid voltage on older el34 amps was common, but that may not mean that new el34's are up to the challenge. New Sensor is notorious for regurgitating info from old tube data sheets. In fact it appears their Tung sol 5881data sheet IS the Tung Sol data sheet with the vintage dates removed. It's also common to find some arbitrary criteria, like voltages for "typical operation", along with all associated performance data at those voltages, etc. that are spot on with vintage data sheets such it's clearly just copied info. It's for this reason that I don't trust modern data sheets that are even a little suspect of not being data acquired by testing the actual tube in question. Because WHY would a tube manufacturer publish copied data if they have actual info on the product they're selling?!? I think these designs are based on the tube models in question and then tossed at the public wall to see if they stick and it's only assumed, by virtue of design copy, that the old data sufficiently matches the new products performance. But are the materials, tolerances and expertise of the new manufacturers equal to that of the old? I do not think so. That basically makes the data sheets cartoons. That is, they look vaguely like real life, but you probably can't safely do some of that stuff.
I have noticed their deceptive datasheets. As if to imply that the new tube *is* the old tube. Perhaps it relays a psychological message to some - one of comfort and security - to know your using solid old school tech, which in this case is a desirable thing.

Quote Originally Posted by cerrem View Post
I have done consulting for New Sensor over the years.... mostly with pedals and transformer designs for their amps... I happen to own two Tektronix 570 curve tracers that I fully restored and calibrated .... I offered New Sensor a great deal ...I would draw up proper data sheets for their product line of vacuum tubes... providing I-V curves , Dynamic transfer curves, Grid 1 current and screen current curves, Ultra-Linear curves....plate curves at all various screen voltages, ie usable data sheets..... I have thermal imaging camera that can give accurate bulb temperatures at various dissipation, to ensure proper tube capabilities.... Well I got a big NO !!!! They are not even remotely interested in having this done....they like their situation just as is ...they don't want to spend a penny...
Whatever... I thinks it pathetic that they want to be a big player in tube distribution but show ZERO interest in providing proper data sheets.... I have seen their "In house" tube tester...looks like a 5th graders science project ....
Wow. As I was reading I was hoping to see it develop the other way... Just reinforces my thoughts above.

It's too bad because it seems EL34s, in general, are generally not reliable. I used EL34 in various amps for almost 2 decades and didn't have a single problem until the last few years. So either they are getting worse or I am pushing the boundries too much... likely a combination.

A few days ago, I saw Dave Friedman say similar things (checked out Tone Talk on YouTube), that it is difficult to find a reliable EL34. But at the current his amps come with EH, but that could be old news by now.
 
J M Fahey 6/14/2018 4:41 AM
Painting with a VERY broad brush, I trust 6L6 because they have been made since FOREVER (think WW2 ) by Russians and designed/tested/produced for *Military* equipment (donīt think Uncle Vania and his Magic Accordionīs amp were the main focus) ; and a very distant second by their disciples: the Communist Chinese, who were very close friends up to 1965 so must have gotten at least some Technology transfer.
Of course Chinese worry more about cost and huge production than quality control but even so, they have been playing the game non only for long but continuously.

Now EL34 went plain out of production by old game players, Czech/Slovak/Yugo makers were always "second source" at best, and Chinese/Russians never ever made them in the old times to begin with, so the EL34 pond is quite murky.
 
Pedro Vecino 6/14/2018 6:02 AM
As I understand it for my experience buying tubes in the last 30 years and in relation to the European market: EL34 Tesla (Tesla, not JJ) had similar quality to Philips, Telefunken and RFT. These four are the best. When production moved to JJ they got worse. Same with ECC83 family.
In Russia there was no EL34 (at least in the western market I never saw it). The firsts Russian EL34 I found in the first 90īs (G and G +) were poorly made copies of the EL34 RFT (East Germany).
At that time (a little before) Chinese EL34 were horrible. Marshall used them for a time experimentally in a few JCM800 and as spare for a while for the JCM900 series with a lot of problems. The first JCM900 came equipped with Tesla and when Tesla closed the company, seeing the impossibility of using the Chinese EL34 Marshall redesigned the JCM900 amps for the 6Pi3cE.
The two Yugoslav EL34 (EL34 and 6CA7) were never good and did not have good distribution in Western Europe. The ECC83, however, were distributed in large quantities by National, Edicron, Ruby, Groove Tubes, etc.
The best Russian EL34 was Svetlana, from St. Petersburg. It came to the international market later (I guess it was a new design because I never saw it before) parallel to the launch of the JCM2000 Marshall series.
Thirty years later, the offer has multiplied, both in Russia and China, raising the quality enormously. Today, for amplifiers with 500 or more volts I still use E34L JJ. I think they have the best connection to the original model, which was always very robust. Bogner uses them in Ecstasy (500/520V) with conservative bias settings.
However, EL34 EH, Tung Sol and Mullard (they are different) sound more similar to the old classics.
 
Gtr0 6/18/2018 1:17 AM
I decided to buy a set of 5881's. So I ordered a quad of Harma from Watford. At best one or two component change in the bias, if any. But I think it will help to use a much more conservative bias as well.
 
Roberto 6/18/2018 6:02 AM
Quote Originally Posted by Chuck H View Post
A note about dropping the screens with a resistor instead of the choke. This will drop voltage on the PI and preamp tubes as well and THAT will likely change the tone of the amp more than the difference on the screens. The following HV rail resistor feeding the PI and preamp in the SLO schematic is a 10k/2W. You should be able to reduce this value to bring the downstream voltage up again. Just measure voltage on the downstream side of that 10k resistor before changing the choke to a resistor so that you have a target voltage.
Another option is to keep the main power supply unchanged, while adding a derivation to supply the screens: from the supply cap after the choke you go both to the resistor that supplies the PI and to another resistor that goes to a cap and to the screen resistor.
This way you can choose the drop across that resistor and, by controlling the two RC values due to screen resistors * cap and "new supply node resistor" * cap, you can control the attack/release of the compression of the pentodes by controlling the dynamics of their voltage.
 
Gregg 6/18/2018 2:16 PM
In Russia there was no EL34 (at least in the western market I never saw it). The firsts Russian EL34 I found in the first 90īs (G and G +) were poorly made copies of the EL34 RFT (East Germany).
Actually there was a Russian EL34 and it's designation was 6П27Б (6P27S) but even in the Soviet union they were unobtainium because only 5000 experimental pieces were ever released. Obviously they were not able to nail them from the beginning.
 
Pedro Vecino 6/19/2018 4:16 AM
Quote Originally Posted by Gregg View Post
Actually there was a Russian EL34 and it's designation was 6П27Б (6P27S) but even in the Soviet union they were unobtainium because only 5000 experimental pieces were ever released. Obviously they were not able to nail them from the beginning.
I have never seen it. But in the data sheets it says that itīs a output beam tetrode.
https://rudatasheet.ru/tubes/6p27s/
 
Gregg 6/19/2018 6:26 AM
Yes, it's a tetrode but according to that table all other data is identical.
 
dai h. 6/20/2018 9:27 AM
Gregg, thanks for the info on the USSR tube. I vaguely recalled that there was some EL34 equivalent info in old datasheets or books (and wasn't aware that they actually made one) so I didn't wonder much beyond that. I found some pics!

https://olxkz-ringkz04.akamaized.net...4-temirtau.jpg

https://www.olx.kz/kk/obyavlenie/rad...4-ID6ImxU.html

https://ic.pics.livejournal.com/trio...7/3177_900.jpg

https://triodnyvacuum.livejournal.co...um/312/?page=1


re: EL34s,

Matsushita (Panasonic) made some (they were part of the Philips family), as well as Ten (Fujitsu), Toshiba, and NEC (if memory serves). I think a beam type 6CA7 was made in S.Korea. GEC (England) made the KT77 beam tube (think they had a bit higher plate dissipation). USA had Sylvania and GE beam 6CA7s. W.Germany also made (a pentode) one (Telefunken--these are not the same as the E.German RFT which are sometimes branded Telefunken).