mikeboone 4/19/2010 2:10 PM
Cathode Bias EL34 - Output Wattage?
Guys,

I have an amp that I put together that is a 2xEL34 PP with the volume/tone configuration of an 18W Marshall.

The amp sounds pretty good and is very stable, but nothing is ever good enough right?

I want to change this to cathode bias so I looked at the HotCat 30 and Matchless Chieftain to get an idea of how to reconfigure my output section. Seems straight forward.

My question is: Using this cathode biased configuration and a 50W OT from a JCM800, what will my estimated output power be? Shouldnt it be somewhat lower than the fixed biased config? How will the 50W OT affect that?

The reason I ask is I am looking different flavors of Alnico speakers and one of my favorites is the Em. Red Fang that is rated at 50W. Will I be cutting it too close with a single 50W speaker?

Thanks...
Mike
 
fyl 4/19/2010 2:29 PM
From the Mullard datasheet:

EL34 PP @ 375 plate, pentode mode, 3K5 OT
Cathode bias app. 30W / 270 Rk per tube
Fixed bias app. 48W / -33V bias

The OT is rated for max power/current, it'll be fine in both modes.

A single Red Fang is OK, but IMO an EL34 PP sounds better with two.
 
mikeboone 4/19/2010 3:07 PM
Thanks for the input...

The amp is in a 1x12 combo cab that currently has a Celestion G12K100. I gutted an old Blues Deluxe to build this. I use it on top of a 2x12 open back with a 1 Red Fang and 1 Wizard.

I want to change the G12K100 and wanted to ensure that the speaker I put in the 1x12 will be OK when i use it by itself.

FYI... The Red Fang and Wizard pretty much drown the Celestion out.
 
Wilder Amplification 4/19/2010 7:05 PM
Kind of impossible to calculate your output power if we don't know your B+ voltage or the rated plate-plate load of your OT. If we knew these figures, we could easily calculate it assuming the power transformer can source the current and has zero sag.
 
Chuck H 4/19/2010 7:27 PM
It's worth noting that in fyl's tube data example circuit all things are not created equal. You have to take the cathode voltage differential into account. The cathode voltage wasn't indicated but it has to be around 40 or 50, so that's a plate to cathode voltage of only 325 to 335. It's also worth noting that even if you upped the voltage to 420 (for a plate to cathode voltage of about 375) you probably still wouldn't get quite as much output, it depends of a few things , but it would be a lot closer and more of an apples to apples comparison as output power relative to bias method goes.

Chuck
 
mikeboone 4/21/2010 3:11 PM
Thanks again guys...

I found some info in another thread that lead me to another etc, etc... I eventually found some info on how to calculate it myself.

The main reason I asked is that I have read (but do not know) that operating the EL34's cathode biased would yield around 30 watts or so. Right now in fixed bias, they are somewhere around 50 according to the votages and bias settings I am currently running.

I wanted to be sure that it will be around 30 W so I dont blow my $265 50 W alnico speaker running it too close to its rated max power.

I played through a friends Hot Cat 30 (which is cathode biased EL34) on the clean channel and I really liked the feel of the cathode bias over the fixed. They sound similar until you start to crank them. I find it is "spongy" and lends itself nicely to blues. The fixed bais is a bit stiffer and breaks up later.
 
mikeboone 7/7/2010 8:29 AM
Hi again...

I am revisiting this amp project. I noticed that in the schematics for Matchless Chieftain and Hot Cat 30 (bothe cath. biased EL34) that the dopping resistors in the power supply are desginated as 10-12W resistors instead of 1-2W resistors that I am accustomed to seeing.

Does a cath biased amp need to dissipate more heat in these resistors than their fixed bias cousins? I have looked at 5e3 schematic and seen that they are not using this high W value resistors.

Any thoughts???
 
Albert Kreuzer 7/7/2010 10:39 AM
Quote Originally Posted by mikeboone View Post
I noticed that in the schematics for Matchless Chieftain and Hot Cat 30 (bothe cath. biased EL34) that the dopping resistors in the power supply are desginated as 10-12W resistors instead of 1-2W resistors that I am accustomed to seeing.

Does a cath biased amp need to dissipate more heat in these resistors than their fixed bias cousins?
No. IMO it's just over-engineering to be on the safe side. Or to impress the customer
For the cathode R I'd use a 25W wirewound and keep it away from the bypass cap.
 
mikeboone 7/7/2010 10:55 AM
 
mikeboone 7/7/2010 1:20 PM
Quote Originally Posted by Albert Kreuzer View Post
No. IMO it's just over-engineering to be on the safe side. Or to impress the customer
For the cathode R I'd use a 25W wirewound and keep it away from the bypass cap.
Are you suggesting separating the resistor and cap due to noise related issue or heat related issue? How far apart should i put them?
 
Albert Kreuzer 7/7/2010 1:27 PM
Quote Originally Posted by mikeboone View Post
How far apart do they need to be? Is this for noise or to keep the cap from drying up from the heat from the resistor?
I'd keep them at least an inch apart, more if possible. It's to keep the cap from drying up. The resistor will dissipate 4 or 5W at idle, probably 8W or more at full bore. A 25W resistor will get less hot than a 10W. Noise isn't an issue here.
 
mooreamps 7/8/2010 9:43 PM
Quote Originally Posted by mikeboone View Post
Hi again...



Does a cath biased amp need to dissipate more heat in these resistors than their fixed bias cousins? I have looked at 5e3 schematic and seen that they are not using this high W value resistors.

Any thoughts???
Well, it's not so much if you're using self bias or fixed bias. It's mostly the combination of cathode current squared times the resistance. So typically what I have seen, builders who go with fixed bias only put in a 1 ohm cathode resistor, for measuring and balancing the power tube current. This can be only a 1/4 watt since the I_squared_R is not very high.

But, the above comment is correct. A higher wattage part will not run as hot as a lower wattage will.



-g
 
R.G. 7/8/2010 10:27 PM
Quote Originally Posted by mikeboone View Post
Hi again...

I am revisiting this amp project. I noticed that in the schematics for Matchless Chieftain and Hot Cat 30 (bothe cath. biased EL34) that the dopping resistors in the power supply are desginated as 10-12W resistors instead of 1-2W resistors that I am accustomed to seeing.

Does a cath biased amp need to dissipate more heat in these resistors than their fixed bias cousins? I have looked at 5e3 schematic and seen that they are not using this high W value resistors.
Cathode biased amps use their power supply less efficiently than fixed-bias amps.

For the same power supply (i.e. same available voltage and current out of the transformer/rectifier/capacitors) a cathode biased amp will put out less power. This is because the bias voltage is generated in series with the output stage. The bias voltage is significant, being 20-60V depending on which tubes and how much "on" bias is used. The voltage used in generating bias is not available for making voltage swing on the output tube plates, so cathode biased amps are, for the same power supply, able to put out less power.

The current which generates the bias voltage in cathode resistors also heats the cathode resistors. The power they generate is equal to the voltage across them squared divided by the value of the resistor. Doubling the cathode bias voltage quadruples the cathode resistor heat generated.

The current which the output tubes need to flow is dependent on the operating class. Most cathode biased amps are either deeply toward Class A, or completely to Class A. That means they let a lot of cathode current flow compared to Class AB biased amps. Note that cathode biased does not automatically mean Class A, nor does fixed bias mean Class AB. That's just the most common situation.

The actual power a resistor generates is easily calculable. It's equal to the voltage across it times the current through it - P= V*I. If you know the voltage across a resistor and the resistance, you know how much current flows in it, and therefore how much heat it generates.

Once you know the heat it generates, you get to pick how hot the resistor's surface gets by picking the wattage rating. Power dissipated and temperature are NOT the same thing. Temperature depends on how fast the heat can get out, not the amount of heat per se. A small grain of wheat bulb in an appliance may run on 12V at 50ma, or P = 12*0.05 = 0.6W. But the filament gets so hot it glows yellow-white. A 1W power resistor dissipating 0.6W will burn your finger, but will certainly not glow in the visible spectrum.

Resistors are often designed to run with a surface temp under about 200C. Some more, some less, but often, about 200C. This is the outcome of a number of factors. It is good engineering practice to de-rate them, specifying a resistor power rating twice the actual maximum power it will dissipate. This keeps the maximum surfact temp on the resistor down, and also gives it lower thermal stresses and longer life in general. Specifying a resistor derating of even greater amounts lowers the temperature, thermal stress and wear-out mechanisms even more. Some people go huge with power resistors. Some use 2:1.

Back at the question: Cathode biased amps *tend to be* more heavily toward Class A operation than fixed bias amps. This means they pull more idle current from the power supply, and get hotter inside from the power dissipated in the tubes and the cathode resistors. Dropping resistors from power amp stages to preamp stages don't necessarily need more current in a cathode biased amp - they're already class A - but a conservative designer might go way overboard with dropping resistor power ratings if the inside of his box is already hot.

So do cathode biased amps inherently need more wattage in the dropping string? No, not as a consequence of the cathode-biased-ness. But it's often very hot in there, and the designer may have gone way conservative on the power rating.

Look at the resistance and voltages dropped. You can calculate the power dissipated from that, and make your own judgements on how overboard 10W dropping resistors are.
 
Enzo 7/9/2010 12:19 AM
But isn't mikeboone's question about the resistors in the B+ string, rather than the cathode resistor?

If I read him right, he was asking about the B+ dropping resistors and wondering if they needed to be larger for some reason relating to the fact of cathode bias in the power tubes, not the cathode resistor itself.

Which leads me to agree with Albert - they want to imprss the customer.

Of course we also do want to keep hot cathode resistors away from bypass caps who prefer cooler environs.


AM I wrong here? A larger power resistor - of same resistance - wil itself run cooler than a small one, but the amount of heat dissipated into the chassis will remain the same. The larger resistor will make it less of an intense hot spot - lower temperature, but a larger surface radiating. Same net watts heating the space. Or am I missing something?

Point being the cooler resistor will indeed be more friendly to the cap next door, but for overall temperature under the chassis, not much difference. Or so I wants to think.
 
R.G. 7/9/2010 6:49 AM
Quote Originally Posted by Enzo View Post
But isn't mikeboone's question about the resistors in the B+ string, rather than the cathode resistor?

If I read him right, he was asking about the B+ dropping resistors and wondering if they needed to be larger for some reason relating to the fact of cathode bias in the power tubes, not the cathode resistor itself.
Yes, that's right. I probably answered more completely than he needed. And I also threw in an answer to the original poster about amp wattage.

I read it as about the B+ dropping resistors too. There's no inherent difference in the dropping resistors string in a cathode bias vs fixed bias amp. Amps is amps, volts is volts, and watts is watts.

Which leads me to agree with Albert - they want to imprss the customer.
...
AM I wrong here? A larger power resistor - of same resistance - wil itself run cooler than a small one, but the amount of heat dissipated into the chassis will remain the same. The larger resistor will make it less of an intense hot spot - lower temperature, but a larger surface radiating. Same net watts heating the space. Or am I missing something?
Nope, you're right.

Point being the cooler resistor will indeed be more friendly to the cap next door, but for overall temperature under the chassis, not much difference. Or so I wants to think.
Mother Nature thinks that too. It's right.
 
mikeboone 7/19/2010 11:29 AM
Well, guys...

I think I owe everyone a bit of thanks for the input. I put this guy to the test this weekend and it sounds really nice. The end design is the input (vol/tone) of a 18Watt coupled with the output section of a Chieftain... Oh, and I added the MV across the back of the PI.

I split the difference. I used 5W on my screens and the 2W dropping R on the supply that were already there from when it was fixed bias.

All in all it seems to be a success and another round of experience gained.

Thanks!!!
 
isaac 12/14/2012 9:15 AM
Hello,
I am building a Marshall 1974x preamp into a el34 PP power section. How did you hook up your power tubes? I just started a week or so ago, and ran across this thread, funny we are doing basically the same thing? Are there any big things needed? Can I use the 1974x power supplies and everything, so far I've been ok, but just figured out a rectifier, because i hadnt been successful in getting enough voltage but I should be fine now. Is there a layout of anything that you used? Any help would be very helpful.
-Thanks,
-Isaac
 
mikeboone 12/14/2012 9:46 AM
Issac... What I ended up doing was copying the Power Supply architecture from a JCM800 to operate the EL34's. I am not an engineer so I stick to tried and true methods for most stuff and tinker with values to get were I need to be.

I dont think the PT for an 18W will source enough current and may not provide adequate plate voltage for the EL34's. The heater supply also has to be ale to source 5A or your PT will get really hot. I tried a PT from a Bassman Reissue and had the heat issues. Triode Electronics sells transformer sets for Marshall style builds at very reasonable prices. I built a Plexi kit from there and it is AMAZING!!! I Digress...

To configure your power tube section, you can copy the architecture of the Matchless Chieftain or Bad Cat Hot Cat 30.

For what its worth, here is a clip of the final product... PPIMV - YouTube
 
isaac 12/14/2012 11:04 AM
How did you do the biasing. in the jcm 800 it looks like it has no bias trim pot or anything. Did you need all the extra capcans. I have a transformer which should provide correct voltages. My dad builds amps, so eventually once he gets time he and I should be able to sit down and figure out everything. Was your soundclip only guitar and amp or did it have pedals, that was quite a bit of drive for bedroom levels.
Thanks.
 
mikeboone 12/14/2012 2:27 PM
I only adopted the power supply from the JCM 800 as I knew it would support the EL34's from the perspective of sourcing the current. The output tube section is cathode biased so there is no trim pot. The cathodes of the power tubes are not grounded as in a fixed biased amp. There is a resistor and capacitor between each cathode and ground. Its "self" biasing so to speak. I used the same values as Matchless.

This amp of mine is a mutt. It has the power supply of an 800, the preamp and PI of a Blackface Fender (with tweaked values to kill low end flub), a post PI master volume, a cut control similar to a Vox AC30 and the power tube section from Matchless Chieftain.

I screwed around with it and somehow backed into a really cool sounding amp thats not quite like anything particular, it just kinda kicks. Dumb luck!!!

The PPIMV is how I get the high gain at low volumes. However, the PPIMV (LarMar I think they call it) is thinner at low volumes, not bad but not as fat. The best feature of this amp is that I can get into a sweet spot in the power tubes at nearly the perfect volume for playing a club.

No pedals on the clip. Just the guitar straight in. The changes were pickups and guitar volume control. The guitar is an Ernie Ball Axis Super Sport.
 
eschertron 12/14/2012 8:35 PM
Nice tones on the audio clip. Is that recording through a 1x12" open cab, red fang?
 
Gingertube 12/16/2012 3:54 PM
On the weekend just gone, I finished rebuilding my "Junkbox Trainwreck".

Output is cathode biased 6CA7/EL34 as per:
- individual 270 Ohm with 200uF bypass on each cathode
- shared 470 Ohm % Watt screen resistor
- 3K4 Output Transformer (copy of an early Marshall 50W O.T. made by a local winder)
- B+ = 380V

For calculating bias resistor power, assume that cathod ecurrent (= anode + screen currents) will be 120mA per tube at maximum signal. For the 270 Ohm indivicual resistors that means 3.9 Watts, That was a bit too close to 5W for my liking so I used 10W resistors.

Screen current at max signal is 19.5 mA per tube, that has to be managed. That high screen current (compared to some other output tubes) is why EL34 have a reputation of having fragile screens. At the B+ of 380V which I use then the common 470 Ohm 5 Watt screen resistor is fine. The common screen resistor is an old Mullard trick, in a push pull amp as the screen current goes up on one side of teh oush pull it goes down on the other side (and vv) resulting in a toatl screen current which is reasonably constant. Using a common screen resistor results in a stable screen voltage.
If running higher B+ voltages then you neeed to start using individual screen resistors starting at 2 x that 470 Ohm value and gouing higher as you use higher B+

Just below the onset of clipping I measured 32 Watts RMS into 4 Ohm dummy load, datasheet says 35 Watts.

Cheers,
Ian

P.S.
Why did I rebuild the amp? It was a quick an dirty conversion of an old PA Amp., seriously bad hum (from random grounds) and hiss AND when volume control was turned passed about 7 of 10 it broke into oscillation. During the re-build I decided that may have been due to bad lead dress (the reason I decided to rebuild it) but was probably due to a lack of any power supply decoupling between the power amp phase splitter and the final gain stage in the preamp (added during the rebuild). Also identified a microphonic tube in the input stage and replaced that.

The amp is tight and punchy and when you pull harmonics on the guitar they just hang in there for ever - I love it. Testing the power amp by itself showed that it was VERY HiFi'ish -3dB at abot 25Hz and somewhere around 45kHz.
 
woodyc 12/16/2012 5:32 PM
Quote Originally Posted by mikeboone View Post
...Will I be cutting it too close with a single 50W speaker?...
Yes. Unless you play really conservatively, use two...

- With a distortion pedal, your output can be close to double the amp power rating.

- The speaker will fail some time after you reach the low frequency excursion limit, which occurs at WAY LESS power than the "power rating" manufacturers provide (the voice coil heating limit.)

- Besides distortion, other risk factors for speakers include open or vented cabs, drop tunings, pentode output stages, tubes with little or no feedback, and transistors with current feedback.
 
kleuck 12/17/2012 4:16 AM
Well, AC30 with blues are 33 watts amps equipped with a pair of 15 watts speakers....
 
woodyc 12/17/2012 6:03 AM
Quote Originally Posted by kleuck View Post
Well, AC30 with blues are 33 watts amps equipped with a pair of 15 watts speakers....
The recone kit is currently available from CEDist for $78.12.

Actually, for speakers with paper voice coils the excursion limit and heat limit are not as different as they are for kapton, but the power rating is still bogus for the other reasons I mentioned. In any event, if you think AC15s and AC30s don't blow speakers, you are mistaken.
 
mikeboone 12/17/2012 11:37 AM
Quote Originally Posted by eschertron View Post
Nice tones on the audio clip. Is that recording through a 1x12" open cab, red fang?
Thanks... That amp is built in a 1x12 combo. Its built in the gutted chassis of a ragged out Fender Blues Deluxe. The speaker is not the Red Fang I was going to use. First the Red Fang wont fit in that cabinet, too deep. Second the Red Fang has way too much high end respnse for a high gain amp. In the recording is actually a 150W Black Shadow 12 that come out of my Mesa Mark II C+ just to test it. The high gain brings about some unwanted high end artifacts that seem to be filtered best though this speaker. I tried a few others but this one works best. Its a pretty dark speaker.
 
jbefumo 5/17/2017 2:23 AM
[ATTACH=CONFIG]43543[/ATTACH]

Can anyone comment on the configuration described in the above attachment? It claims 40W, but those current figures seem kind of high to me.

Thanks
 
Chuck H 5/17/2017 8:21 AM
The end of a necrotic thread probably isn't a good place to put this post. Probably better to start a new thread. I'll bite though...

According to the numbers the total dissipation for each tube in the 40W output model is 24W. So class A, pretty much. The tubes would likely operate in class A until very near their maximum output. Note how the screen current jumps a lot more in the 40W model than it does in the 35W model. A compromise had to me made at 450Vp, nudging to the cooler side of class A to avoid over stressing the tubes. 60mA doesn't seem unusual for class A with a big bottle. The 40W example does seem a little odd because of the high-ish distortion figure compared to the 35W model. Still lower than one could expect for AB1 though and that is probably why the 40W class A model was offered. It gives designers the opportunity to choose their preferences for power and/or distortion at 450V.
 
jbefumo 5/17/2017 9:13 AM
Thanks SO much. I've reached the point where I'm pretty comfortable with everything up to the phase inverter, but from there back, kind resort to a bit of hand waving.

Haven't found any similar published configurations for the 6L6GC, but (guessing again) that substituting would offer a bit more of a safety margin?

It's going into a minuscule 1X8" combo, so the power figure is mainly a bragging-rights thing for punching above it's weight class. (The prototype turns out 24W using a pair of 5881s, and is remarkably capable.)

Thanks again.




Quote Originally Posted by Chuck H View Post
The end of a necrotic thread probably isn't a good place to put this post. Probably better to start a new thread. I'll bite though...

According to the numbers the total dissipation for each tube in the 40W output model is 24W. So class A, pretty much. The tubes would likely operate in class A until very near their maximum output. Note how the screen current jumps a lot more in the 40W model than it does in the 35W model. A compromise had to me made at 450Vp, nudging to the cooler side of class A to avoid over stressing the tubes. 60mA doesn't seem unusual for class A with a big bottle. The 40W example does seem a little odd because of the high-ish distortion figure compared to the 35W model. Still lower than one could expect for AB1 though and that is probably why the 40W class A model was offered. It gives designers the opportunity to choose their preferences for power and/or distortion at 450V.
 
Chuck H 5/17/2017 9:34 AM
Since you're building a very small combo, keep in mind that class AB1 will create a lot less heat at idle. Which is where the tubes will be most of the time. Since you want disproportionate power for the amps size I would skip class A models and go AB1 for sure. Though the example above doesn't expressly say "class A", maybe because it's not quite strictly, it pretty much is. 24W from a pair of 5881's may also indicate close to class A depending on you're plate voltage. I might consider that a pair of 6V6's in AB1 at 400+ volts would give 20W (which is pretty much the same volume) with less idle current (heat) and give you a little more room in the cabinet (to make room for the speaker) because of their smaller size.
 
jbefumo 5/17/2017 11:04 AM
D'OH! Thermodynamics 101. I knew that .... made a mental note to consider it early on, then forgot amidst all the sexy electronic stuff! Well, was getting ready to stress-test it into a dummy load, so will have to include some temperature probing as part of that ... THANKS!
 
pdf64 5/17/2017 11:09 AM
Quote Originally Posted by jbefumo View Post
[ATTACH=CONFIG]43543[/ATTACH]

Can anyone comment on the configuration described in the above attachment? It claims 40W, but those current figures seem kind of high to me.

Thanks
It happens that I've been trying a few months ago to get an old hifi amp up and running that looks to have used this as its model.
The big problem is that unless a very well regulated HT supply is used, to achieve 450Vdc at full load, HT sag will cause it to rise above 450 at idle, thereby pushing the idle dissipation up above 100% of plate rating. Barely get 30W output with good Winged Cs even then.
I've put it to one side for a while, with the plan to implement fixed bias when I go back to it.
The upside is that it's got an RS Deluxe OT
 
Chuck H 5/17/2017 10:09 PM
I thought the cathode resistance looked a little high for the current figure. But I didn't question it because I don't have anything like it on the bench to check. I do know that rapid rising screen current only gets worse with greater grid drive and can be a real problem. In fact you were part of my revelation on that matter, Pete. Increasing screen supply impedance/resistance and reducing grid drive to the sub meltdown level was the answer. That amp was a single ended, class A el34 in fact. I didn't see a voltage rise at idle though. That would be indicative of push/pull class bias of some kind for sure.?. Current decreases and voltage goes up.

jbefumo,

I wasn't necessarily saying you CAN'T run a hot bias in a small combo. If that's the tone you like and a small, uber powered combo is what you want then yes, the heat factor needs to be considered. That doesn't mean a hot bias or even true class A to 24 watts can't be done. But you may need to be creative with venting, a fan or something. Sans that, a cooler operating class helps mitigate the problem already. And if you've ever tried to fit a speaker into a crowded combo (as I have many times) you'll appreciate any room you can make.
 
pdf64 5/18/2017 4:56 AM
Yes, those Mullard operating conditions are definitely AB1.
PC case type fans, eg powered via the heater supply (when rectified & smoothed), can help in keeping combo temperatures reasonable.
A bit of air flow around the PT etc seems to go a long way.
 
jbefumo 5/19/2017 6:10 AM
Yeah, I considered a fan or two -- I have several on hand and they're amazingly quiet.

It took a bit of time but it finally dawned on me (I think) why cathode biasing should result in additional heat .... It's because they're typically biased way closer to the maximum dissipation, right? So if one were to bias them at 70% or so, like a typical fixed bias, then it would be a wash, but the peak power figure would be lower because of the negative feedback introduced by the cathode resistor ... ?
 
jbefumo 5/19/2017 6:23 AM
If my hypothesis (above) is correct, that would explain why the mighty 6550, in cathode bias, is making the same power as the EL34 figure I posted initially -- here's it's biased far more conservatively, and is consequently developing way less distortion, right?

That really fits my objective a lot better -- beyond just uber-power in small package, I'm trying to come up with something that old farts like myself can easily grab in one hand, with guitar case in the other, stow in a modern compact vehicle, and show up somewhere and be able to play along with a drummer without driving into extreme distortion (because there's no such thing as a 'clean pedal').
 
Zozobra 5/19/2017 7:07 AM
We've got to be careful to make sure we're comparing apples and apples here. The 6550 datasheet you posted states a 5k primary. I'd guess the EL34 example will be a lower primary (~3k5) as posted earlier?
 
jbefumo 5/19/2017 7:14 AM
correct. Trying to get this all correct before buying the transformer and then trying to make everything else work with it .... as usual ... ;^)
 
Chuck H 5/20/2017 6:37 AM
I used an Edcor XSE10 with a 4k primary for an el34 silver Champ conversion. Even at 4k I had to increase screen resistance and decrease grid drive to keep the screens from over dissipating when fully clipping. I might sacrifice a watt (or less) of efficiency and go with a 5k primary just to mitigate this problem. These are guitar amps after all so their operation has a slightly different set of requirements than what the data sheets account for. JM2C on that.

FWIW the Edcor XSE sounded fantastic. Obviously it would be better than the cheesy stock OT, but the guy I built this amp for calls it his little *umble. And that IS what it sounds like. The amp also got a new 10" speaker and a little re voicing, but the XSE OT was part of a winning package. The XSE is available in several primary and secondary options and I would definitely use it again.
 
jbefumo 5/20/2017 6:55 AM
I REALLY like Edcor iron. Quality is excellent and prices are hard to beat. I was just playing around in Multisim and thinking that 5k a-a would be the way to go.

Spent the past few days beating on my 5881 prototype (8k primary), and the sound is great, and no overheating problems so far. Attached is the full schematic with all the measured values included.[ATTACH]43570[/ATTACH]



Quote Originally Posted by Chuck H View Post
I used an Edcor XSE10 with a 4k primary for an el34 silver Champ conversion. Even at 4k I had to increase screen resistance and decrease grid drive to keep the screens from over dissipating when fully clipping. I might sacrifice a watt (or less) of efficiency and go with a 5k primary just to mitigate this problem. These are guitar amps after all so their operation has a slightly different set of requirements than what the data sheets account for. JM2C on that.

FWIW the Edcor XSE sounded fantastic. Obviously it would be better than the cheesy stock OT, but the guy I built this amp for calls it his little *umble. And that IS what it sounds like. The amp also got a new 10" speaker and a little re voicing, but the XSE OT was part of a winning package. The XSE is available in several primary and secondary options and I would definitely use it again.
 
Chuck H 5/20/2017 7:03 AM
Yep, the XSE is a single ended OT, so not what you need for this project, but I'm looking forward to trying their other OT's in the future. The 16xx off the shelf Hammonds (that I love) have become a bit pricey. FWIW I read on another audio forum where a guy tried the XSE clipped in side by side with two other OT's with the same specs, including another of Edcor's more expensive offerings and the XSE produced an extra watt over the others. That was for a 6W application so an extra watt would be a lot. Very efficient I suppose. That's what you want. An efficient transformer and an efficient speaker. It can make a lot of difference when you want to get louder.
 
jbefumo 5/20/2017 7:11 AM
Actually, I just double checked and the configuration I am looking out for EL34s uses 6.5k load:[ATTACH=CONFIG]43571[/ATTACH]
 
jbefumo 5/20/2017 7:35 AM
I built a single-ended KT120 amp a while back using an Edcor OT (CXE series -- way heavy!) -- Worked out very well. Current owner absolutely loves it.