eschertron 6/1/2018 7:12 PM
I never thought this would happen to me! (PT failure)
Two days ago I was taking this last winter's project out for a test run at full volume, as the missus was not at home. This is a 2x 6V6 amp, kind of a mashup of a BF princeton or Tweed Deluxe power section, with switchable NFB. Had a dirt box in front, driving the amp into heavy grind territory. When suddenly there was a drop in volume for a few seconds, then the mains fuse blew (1.0A time delay). A sniff check seemed OK, and shooting the tubes with an IR thermometer revealed about 200F on the plates in the few minutes that had elapsed since the event.
I had convinced myself that a screen collapse and g2-cathode fault could have drawn enough current to blow the primary fuse. Examination of the circuit board did not show any charred components, and what I tested - screen resistors, cathode resistor, rectifier diodes, power supply caps (testing no short to ground), power supply node resistors - all OK. Power up, all tubes out, on a light bulb limiter, lamp shone brightly. Rats.
measuring PT secondary winding resistance - first in circuit and then out - revealed 43R red-red/yel (CT) on one side, and 163R on the other. Oddly enough, red-red measures about 120R, less than the one side to CT. The nominal winding resistance is about 170R each side.

Hammond 270CX PT spec sheet here

The PT is rated for 550v @ 75ma = 41.25VA
2 x6V6 may pull 12W each diss plus 18W (yeah, I'm optimistic!) audio power for 42W
Seems like the right PT for the application

Is this kind of failure attributable to specific causes? or is this a simple infant mortality? I have contacted Hammond with a nice email, asking what their tech dept thinks. I'm more concerned with spec'ing the right PT for this amp than anything else. Comments?

edit: a quick check of the output tubes (LBL, then a floating guitar cable "noise" test - nice and buzzy, even on the LBL) in a champ suggests no problems with the tubes. Also the OT resistance check seems OK at about 200R each side.

Hammond 1760H OT spec sheet here

So I'm thinking the HV winding short was the problem, not a symptom. Reality check?
 
dstrat 6/1/2018 7:42 PM
with a 100ma transformer you would be pushing it. ( from 6v6-gt tube data ab1 )
mabe a 120ma would do better.
 
dstrat 6/1/2018 7:51 PM
I have seen fender tweed princeton? is it that uses a 70ma (champ style) for 2x 6v6 but bet it wasn't foreseen someone would drive it like that.
 
eschertron 6/1/2018 8:03 PM
Quote Originally Posted by dstrat View Post
I have seen fender tweed princeton? is it that uses a 70ma (champ style) for 2x 6v6 but bet it wasn't foreseen someone would drive it like that.
I suspect that this would NOT have happened without crushing the input the way I did. And I don't think I intended the amp to be driven like that. But hey, musicians... what can you do?
But of course 'what I can do' is to understand the design requirements for a robust amp. Over-rate the PT by 30%?
 
The Dude 6/1/2018 8:07 PM
I'm sure a PITA and expensive, but it was at least fun running the hell out of it?
 
eschertron 6/1/2018 8:31 PM
Folks seem to pop up with amp problems, and the always start with "I think it's my PT" or "I think it's my OT", right? And the answer is always the same. 'It's not the tranny', 'not likely to fail', 'last thing I'd suspect', etc.

So I was surprised at my findings, and I'm thinking that if I challenged the 'least likely to fail' statement I'd meet with the response "that holds true in a properly designed amp". It leaves me wanting to know what I missed, other than a fudge factor.
 
nosaj 6/1/2018 8:49 PM
Quote Originally Posted by eschertron View Post
Folks seem to pop up with amp problems, and the always start with "I think it's my PT" or "I think it's my OT", right? And the answer is always the same. 'It's not the tranny', 'not likely to fail', 'last thing I'd suspect', etc.

So I was surprised at my findings, and I'm thinking that if I challenged the 'least likely to fail' statement I'd meet with the response "that holds true in a properly designed amp". It leaves me wanting to know what I missed, other than a fudge factor.
That's why you start at the power supply secondaries first right?

nosaj
 
Chuck H 6/1/2018 8:51 PM
Not being tech savvy I can't explain the reasons, but I've read that peaks can be EXTREAM with tube guitar amp designs when clipping hard. Further, it can't be discounted that your design isn't driving too much signal into the power tube grids and causing even worse behavior. I've learned to mitigate that stuff in a rather rote fashion in my travels. I can see where a pair of fairly robust tubes, driven too hard, could take out a marginally rated component. And I agree with dstrat that a minimum rating of 100mA is in order with 6V6's clipping near their dissipation rating for a guitar amp. I've done some stupid things and made some mistakes with my 2xel84 prototypes and I'm pretty sure the 144mA rating on the Hammond 270EX has saved my bacon on more than one occasion An expensive lesson, but not horribly so. Bump the PT and grind away
 
tedmich 6/1/2018 9:19 PM
Ahh the amp designers worst enemy...

HIMSELF!

[ATTACH=CONFIG]49049[/ATTACH]

Seriously though, I hope a rebeefed PT does the trick!
 
drewl 6/1/2018 9:19 PM
Had a Weber 18w kit PT die at a gig a week after I built it.
Nothing wrong with the amp, PT just failed.
Had a backup amp and they sent me a replacement PT free.
Sometimes these things happen.
 
Chuck H 6/1/2018 11:08 PM
Quote Originally Posted by drewl View Post
Sometimes these things happen.
^^^^^^^^^^^^^^^^^^ That too.
 
nsubulysses 6/1/2018 11:30 PM
If it's a weber PT maybe the saying should be "most of the time these things happen" har har har

I recently got a Science Mother amp in with a shorted OT primary winding. It is a 230W 4xKT88 amp. The owner of Science said it is the first transformer that has ever failed in one of his amp builds. Not sure how many amps he's made but it's about 7 years worth, so I'd think it's a couple hundred or more. Heyboer is replacing it for free since they're pretty kind
 
pdf64 6/2/2018 3:43 AM
Quote Originally Posted by eschertron View Post
Two days ago I was taking this last winter's project out for a test run at full volume, as the missus was not at home. This is a 2x 6V6 amp, kind of a mashup of a BF princeton or Tweed Deluxe power section...
Those amps have rather different power amps and power supplies
Could you provide more detail about your amp?
 
eschertron 6/2/2018 7:48 AM
Quote Originally Posted by pdf64 View Post
Those amps have rather different power amps and power supplies
Could you provide more detail about your amp?
I didn't mean to infer that this amp is clone of either, but simply that it borrows the topology (2x 6V6, cathodyne PI, common values for cathode & plate resistors, etc.)
I'll try to attach the schem here:
[ATTACH]49054[/ATTACH]
There's not much too it, and I can't see how the PT can take a dump without being part of a cascade failure. Like I said in the first post, nothing smelled hot or burnt right after the event. I admit I didn't touch the PT or read the surface temp, so I could have missed a clue there.

Maybe I'm asking the wrong thing, or the wrong way - I spec'd the HV secondaries based on the modeled power consumption of the tubes, adding dissipation and audio out power. I've ignored power lost in the rest of the HV circuit, assuming it will be negligible. I may be not looking at the right resources, as I don't have a good number in my head for expected current through the tubes, other than at idle.

edit: Looking at Hammond's replacement offerings, I see the 290AX can provide 550v @ 100ma on the HV windings. The nerd in me still is asking 'why?' How do I know to spec that current rating when I 'm designing from the ground up?
 
dstrat 6/2/2018 8:22 AM
Most 6v6-gt / gta tubes seem to have max signal plate of 92ma for 2 tubes (ab1) and only give 285vdc for plate and screens in the datasheets.
the screens at max signal is 13.5ma.
what it seems to me 105.5ma at max signal. I am not sure how that would differ for cathode bias?
classictone has a 50s deluxe (5e3) at 100ma and the BF, SF deluxe at 120ma.
knew I had seen that but had to look again.
 
Steve A. 6/2/2018 9:01 AM
Can you explain the 27v zener and 470R cathode resistor for the pair of 6V6's?

Thanks!

Steve A.
 
Chuck H 6/2/2018 9:33 AM
It "fixes" the bias once the cathode voltage rises to 27V with current and prevents further cooling of the bias. I didn't invent it and it was actually suggested to me a long time ago by Ken Gilbert (SN=kg). But I sort of championed it's use with the Paul Ruby mod in a sort of calculated pairing of the two circuits for 2xel84 amps that are prone to buzzing with crossover distortion. Some here and on other forums have taken to calling the Zener across the cathode resistor the "Chuck H" mod anyway. But alas, it can't be found on the Blue Guitar site
 
dstrat 6/2/2018 9:44 AM
Chuck I have never tried that , how would you describe the sound if different?
 
Chuck H 6/2/2018 10:07 AM
Well, looked at strictly as a circuit it can be done with different results. You could, for example, measure your cathode voltage and then choose a Zener value at that voltage. This would effectively make the amp fixed bias with the operating point for the power tubes floating at the cathode voltage instead of ground and I would expect it to behave like a fixed bias amp.

The way I do it is to scope the amp to the point of obvious clipping. Not just compressed tops, but visible flats starting to show. I measure the voltage on the cathode at that point and that would be my Zener value. The result is that the amp behaves like a cathode biased amp for clean tones and tightens up for distortion tones rather than continuing to get colder in bias and lending more compression and crossover distortion.

The typical instructions for the Paul Ruby mod (this is the mod where Zeners are placed across the grid leaks) from his site use to suggest measuring cathode voltage and making THAT the Zener voltage. The problem, as I saw it, is with AB amps the cathode voltage rises significantly with current. That should mean that the diode clipping is then in the signal. I didn't want this so I start by choosing the cathode Zener value as described and then choose a PR mod Zener voltage 1 volt higher than the cathode Zener. That way the tubes will always be in cutoff before the PR Zeners clip, effectively keeping the diode clipping out of the signal.
 
dstrat 6/2/2018 10:14 AM
thanks for that!..
 
mikepukmel 6/2/2018 4:07 PM
Quote Originally Posted by eschertron View Post
I suspect that this would NOT have happened without crushing the input the way I did. And I don't think I intended the amp to be driven like that. But hey, musicians... what can you do?
But of course 'what I can do' is to understand the design requirements for a robust amp. Over-rate the PT by 30%?
Oh man, when we were kids and got these amps cheap used, we'd crank them up to 9. And this one guy had another amp (some odd name solid state amp, I still remember the name "Mike Matthews Freedom Amp), he'd out the OUTPUT of the freedom amp into the old princeton so he could get that little added extra 'zip'. I do remember he made a few amps go 'pouf'.
 
mikepukmel 6/2/2018 4:15 PM
Quote Originally Posted by eschertron View Post
Two days ago I was taking this last winter's project out for a test run at full volume, as the missus was not at home. This is a 2x 6V6 amp, kind of a mashup of a BF princeton or Tweed Deluxe power section, with switchable NFB. Had a dirt box in front, driving the amp into heavy grind territory. When suddenly there was a drop in volume for a few seconds, then the mains fuse blew (1.0A time delay). A sniff check seemed OK, and shooting the tubes with an IR thermometer revealed about 200F on the plates in the few minutes that had elapsed since the event.
I had convinced myself that a screen collapse and g2-cathode fault could have drawn enough current to blow the primary fuse. Examination of the circuit board did not show any charred components, and what I tested - screen resistors, cathode resistor, rectifier diodes, power supply caps (testing no short to ground), power supply node resistors - all OK. Power up, all tubes out, on a light bulb limiter, lamp shone brightly. Rats.
measuring PT secondary winding resistance - first in circuit and then out - revealed 43R red-red/yel (CT) on one side, and 163R on the other. Oddly enough, red-red measures about 120R, less than the one side to CT. The nominal winding resistance is about 170R each side.

Hammond 270CX PT spec sheet here

The PT is rated for 550v @ 75ma = 41.25VA
2 x6V6 may pull 12W each diss plus 18W (yeah, I'm optimistic!) audio power for 42W
Seems like the right PT for the application

Is this kind of failure attributable to specific causes? or is this a simple infant mortality? I have contacted Hammond with a nice email, asking what their tech dept thinks. I'm more concerned with spec'ing the right PT for this amp than anything else. Comments?

edit: a quick check of the output tubes (LBL, then a floating guitar cable "noise" test - nice and buzzy, even on the LBL) in a champ suggests no problems with the tubes. Also the OT resistance check seems OK at about 200R each side.

Hammond 1760H OT spec sheet here

So I'm thinking the HV winding short was the problem, not a symptom. Reality check?
Oh boy can't click "like" on this one. About how long were you running the amp loud before the PT went? I haven't worked up the guts to crank deluxe "amp build 0" yet, similar to your setup (at least same output tube set) My brother played it a bit, ran it into clipping, at 8 on vol or so, no overdrive pedals, but only a couple of minutes. its got a 138ma rated PT, very similar to yours, hammond 291bx.
 
eschertron 6/2/2018 6:29 PM
Quote Originally Posted by dstrat View Post
Most 6v6-gt / gta tubes seem to have max signal plate of 92ma for 2 tubes (ab1) and only give 285vdc for plate and screens in the datasheets.
the screens at max signal is 13.5ma.
what it seems to me 105.5ma at max signal. I am not sure how that would differ for cathode bias?
classictone has a 50s deluxe (5e3) at 100ma and the BF, SF deluxe at 120ma.
knew I had seen that but had to look again.
So where are you getting your current consumption numbers from? I need to get a hold of that...
 
dstrat 6/2/2018 6:52 PM
they can be found online Tube Data Sheet Locator
or just google 6v6-gt datasheet,or what ever tube.
some have charts some do not. but general class a1 , ab1 , ect is usually there.
jj has some also, have not seen any for the Russian reissues.

note: I use tube datasheets as general data, some guitar amps do vastly exceed max values. ( I know,..go figure )
but they give an idea of what to expect.
 
g1 6/2/2018 6:55 PM
Quote Originally Posted by eschertron View Post
So where are you getting your current consumption numbers from? I need to get a hold of that...
I think he is getting them from the tube data sheets, like attached. Suggest getting as many as you can for each tube type, as they often give different examples as far as plate voltages used, etc. If you look at enough, you will probably find an example that is somewhat close to your circuit voltages/values. (or at least you can guess-strapolate )


edit: what he said ^^^^
 
dstrat 6/2/2018 7:09 PM
woops spoke too soon.... Russian Tungsol :|

Tung-Sol.com:: 6V6GT Vacuum Tube page

oh boy!
 
eschertron 6/2/2018 8:17 PM
OK. Curious. The two spec sheets agree that the "maximum signal plate current" is 92mA, for whatever B+ (285vdc is mentioned in one document). I wouldn't have taken it as a guideline, but on re-reading it I put it together now as
"Radiotron 6V6-GT maximum ratings are design center values Push-pull AB1 max sig plate current 92mA".
I had previously thought it was a limiting value, so specing a PT that could provide less than that number would be a good idea.
 
eschertron 6/4/2018 6:03 AM
OK, since there's a consensus about plate draw at max, I'm gonna assume that the question of how much (or where to find how much) has been answered. Appears the Old-timey engineers actually put good info on their data sheets. Who knew?

Can I assume that the failure was thermal? I don't see a way that HV spikes at the plates could make their way back to the PT, could there?
 
pdf64 6/5/2018 2:32 AM
Quote Originally Posted by eschertron View Post
OK. Curious. The two spec sheets agree that the "maximum signal plate current" is 92mA, for whatever B+ (285vdc is mentioned in one document). I wouldn't have taken it as a guideline, but on re-reading it I put it together now as
"Radiotron 6V6-GT maximum ratings are design center values Push-pull AB1 max sig plate current 92mA".
I had previously thought it was a limiting value, so specing a PT that could provide less than that number would be a good idea.
It's the designer's job to ensure that the circuit's design is such that the tube's limiting values are not exceeded; the tube won't somehow self limit.
Unless some sort of current limiting circuit was included (RG's MOSFET HT regulator) the max current that the power amp can draw will mainly be limited by the loaded VHT / OT primary impedance / 4.
So even with that info sheet, if the HT is higher (as is likely with your amp) more current will be drawn.
And the PT will supply as much current as the circuit draws, albeit with a slightly lower winding voltage.

In regard of circuit limits on the peak HT current that can be drawn, your amp is somewhat less restricted than a tweed deluxe, eg silicon rather than 5Y3 rectifier, fixed rather than cathode bias, 1k rather than 5k g2 HT dropper (plate current will be lowered as Vg2 is reduced).

Hence the potential for your amp's PT to become overloaded by excessive HT current demand, whilst it may be have been ok with a circuit that had the same idle conditions but a saggier HT and cathode bias.
 
eschertron 6/5/2018 9:15 AM
Quote Originally Posted by pdf64 View Post
It's the designer's job to ensure that the circuit's design is such that the tube's limiting values are not exceeded; the tube won't somehow self limit.
Unless some sort of current limiting circuit was included (RG's MOSFET HT regulator) the max current that the power amp can draw will mainly be limited by the loaded VHT / OT primary impedance / 4.
So even with that info sheet, if the HT is higher (as is likely with your amp) more current will be drawn.
And the PT will supply as much current as the circuit draws, albeit with a slightly lower winding voltage.
From above, current draw in the 6V6's could approach 330vdc/6.6k/4 = 200mA? Yikes!

In regard of circuit limits on the peak HT current that can be drawn, your amp is somewhat less restricted than a tweed deluxe, eg silicon rather than 5Y3 rectifier, fixed rather than cathode bias, 1k rather than 5k g2 HT dropper (plate current will be lowered as Vg2 is reduced).
I sometimes fail to consider that once the zener voltage is exceeded, the current through the cathode resistor is only a fraction of total cathode current.

Hence the potential for your amp's PT to become overloaded by excessive HT current demand, whilst it may be have been ok with a circuit that had the same idle conditions but a saggier HT and cathode bias.
The original design had a HUGE screen resistor and a cap to ground (4.7k/10W and 8uF, I think), which gave me the kind of degeneration that i wanted with an SE version of the circuit. I didn't like the way it sounded here, so I removed it and went with individual 100R stops. 470R or 1k stops are probably in this amp's future, then.
 
dstrat 6/5/2018 9:40 AM
this is a perfect example of everything matters design wise.

Edit: I would say 200ma is overkill for a pair of 6v6 tubes.
 
eschertron 6/5/2018 10:20 AM
Quote Originally Posted by dstrat View Post
this perfect example of everything matters design wise.
I couldn't agree more. And, as a budding 'designer', I'm getting my head around how B+, current consumption, and total power (delivered + dissipated) relate. My too-simple equation of Voltage(B+) x rated HT current = power dissipated + audio output left me holding a PT with shorted turns. Will my equation be more correct if I can measure B+ under full load conditions? Did the old PT simply sag too much and die of a brown-out? Alas, my wayback machine is broken, too, and I can't measure the B+ at full power any more.

We've talked about fusing the B+, but again this is a scheme for protecting PTs in properly-designed amps. I'm still left wanting the magic equation (cue angel's "ahhhh") that predicts the right size PT for the application.
 
dstrat 6/5/2018 10:23 AM
you can get a general idea from this:

Interactive Valve Data Sheets
 
pdf64 6/5/2018 10:54 AM
A bit more sag at the g2 node will reduce max continuous plate current and order may be restored, without the need for big iron.
 
eschertron 6/5/2018 10:57 AM
Quote Originally Posted by dstrat View Post
you can get a general idea from this:

Interactive Valve Data Sheets
Thanks! That was my primary resource.
What I don't see is any calculation of what average current would be at any given signal level. At any point in class A operation, I understand that as one tube is drawing less, the PP complement tube is drawing more, so total current remains constant. As one tube goes into cutoff when we enter class AB1, the "other" tube continues to draw more, increasing total current from idle. I'd really like to be able to model that with some kind of equation.

edit: ... without integrating the area under the curve, I mean...
 
dstrat 6/5/2018 11:10 AM
I agree , with data measured from my 48 deluxe the graphs do seem off, but mabe I am the one thats off..

drawing a load line is not super hard , reading the data gets me.
 
eschertron 6/6/2018 6:42 PM
I emailed Hammond with a query “Can I ask how a designer might best spec the needed current rating of a certain tube complement and topology?” and got an answer of sorts:

I really wish I knew the answer to that question but unfortunately we “Hammond” have little experience in amp design or theory. Those engineers are long gone or retired by now. We make great transformers but the required information needs to be spoon fed to us. If I had a schematic of a Fender or similar amp that was similar to what you are making I could see what transformer we have designed for it.
Which confirms the rest of the sage advice I've received in this thread, basically "find the schem with the closest matching operating conditions and get the replacement PT for that amp". Which is all well and good, but I'm not buying the magical mojo black arts of amp design

On a whim, I calculated the dissipation on the cathode resistor network and the PSU resistors. Came up with a few VA, but nothing near what I expect to need in a real-world fix for this amp. I fully agree that 100mA or more should be supplied to the power amp to make it work as designed, but still a bit hesitant to accept the fact that I can't get a simple '1st law' energy balance equation to work. I just can't find all the missing power. A few W here or there from power resistors, some heat from the trannys, it doesn't all add up. Unless the amp was generating WAY more audio power than I'd expect from the models. sigh.
 
nosaj 6/6/2018 7:01 PM
Quote Originally Posted by eschertron View Post
I emailed Hammond with a query “Can I ask how a designer might best spec the needed current rating of a certain tube complement and topology?” and got an answer of sorts:



Which confirms the rest of the sage advice I've received in this thread, basically "find the schem with the closest matching operating conditions and get the replacement PT for that amp". Which is all well and good, but I'm not buying the magical mojo black arts of amp design

On a whim, I calculated the dissipation on the cathode resistor network and the PSU resistors. Came up with a few VA, but nothing near what I expect to need in a real-world fix for this amp. I fully agree that 100mA or more should be supplied to the power amp to make it work as designed, but still a bit hesitant to accept the fact that I can't get a simple '1st law' energy balance equation to work. I just can't find all the missing power. A few W here or there from power resistors, some heat from the trannys, it doesn't all add up. Unless the amp was generating WAY more audio power than I'd expect from the models. sigh.
You might try the Antique Radio Forums ? View new posts
There are a lot of older older guys there, every year there are fewer. Maybe you can tap one that has the formula or answer you are looking for.

nosaj
 
pdf64 6/7/2018 2:09 AM
Quote Originally Posted by eschertron View Post
I...On a whim, I calculated the dissipation on the cathode resistor network and the PSU resistors...
But at max loading, do you know the current through the zener?
 
eschertron 6/7/2018 6:51 AM
Quote Originally Posted by pdf64 View Post
But at max loading, do you know the current through the zener?
Well, I don't know the actual current through the power tubes, which is how I got here in the first place. But I did calculate the total power lost in the cathode network assuming 100mA current. And what fraction in the resistor, the remaining in the diode.
27v * 100mA = 2.7W total
27v / 470R = 55mA dissipated in the resistor
27v^2 / 470R = 1.5W in the resistor

so the rest, 45mA and 1.3W dissipated in the zener. And yes, if the current was higher, the dissipation in the zener goes up quickly.
 
Dave H 6/7/2018 8:27 AM
Quote Originally Posted by eschertron View Post
“Can I ask how a designer might best spec the needed current rating of a certain tube complement and topology?”
You can get an idea of the DC required by the amp at full output from the tube's data sheet (as in post #15) but that isn't the current value needed to specify the PT. What is really needed is the PT RMS current and using a solid state rectifier the RMS can be 2 x the DC current for a bridge rectifier and possibly up to x1.5 for a full wave CT rectifier so to be on the safe side I'd be thinking of using a 150mA rated PT.
 
eschertron 6/13/2018 2:28 PM
I ordered a Hammond 270DX (104mA at 550VCT up from 75mA, about a 38% bigger current rating). That plus larger screen grid stops may keep me from buying another transformer. I will measure voltage and current under load this time. I have some 1R/1W resistors, I'll put one on the tail of the cathode network.

I popped the end bells of the PT last night, took a visual. Looked OK. I measured the dc resistance, seemed to get the one winding's value to fluctuate from 170R (good) to 43R (bad) as I wiggled the leads around a bit. I am convinced that the winding's turm-to-turn insulation was compromised. I am not convinced that excess current was the sole factor.
 
pdf64 6/14/2018 1:58 AM
Maybe see if Hammond are willing to check it for a manufacturing defect?
 
eschertron 6/14/2018 9:08 AM
Quote Originally Posted by pdf64 View Post
Maybe see if Hammond are willing to check it for a manufacturing defect?
The thought had crossed my mind, but I didn't ask and they didn't offer. Some companies are eager to do failure analysis, others not so much. I held on to it for a few days, then pulled it apart myself. After taking the resistance readings, I started to peel back the tape, to see what I could see. Not much. With the tranny potted in epoxy (as it appears to my untrained eye) I'm guessing a quick visual inspection is the best this will get.
 
mikepukmel 6/14/2018 3:49 PM
Quote Originally Posted by eschertron View Post
The thought had crossed my mind, but I didn't ask and they didn't offer. Some companies are eager to do failure analysis, others not so much. I held on to it for a few days, then pulled it apart myself. After taking the resistance readings, I started to peel back the tape, to see what I could see. Not much. With the tranny potted in epoxy (as it appears to my untrained eye) I'm guessing a quick visual inspection is the best this will get.
definitely ask and more than once if y ou need to. When I bought the transformer set from Mouser (Hammond) and got everything running, the PT madea nice hum. Enough to be annoying at low playing volumes. I asked a few times, got them to take the transformer to their tech center in USA for testing, and they did replace it. Their tech support office seems quite busy, but I kept after and they did replace it. Could be a bad PT. Hmm, so you jiggle the windings and the mreasured resistance fluctuates? If it was a burn in the middle of a winding, less change you'd see that I think. Maybe the connection where the lead wire meets the magnet wire is compromised. Definitely ask about warranty testing.
 
eschertron 6/14/2018 8:09 PM
Quote Originally Posted by mikepukmel View Post
definitely ask and ... Definitely ask
Yeah, and well, no because I'm too nice of a person
I didn't take a reader's poll, but the consensus seems to be that the PT was underrated. I have placed an order for a bigger one. My fault as a designer.
 
eschertron 6/16/2018 9:02 PM
Update:
new PT is in, tested with tubes out, and then again with tubes in at idle. So far measurements match expected design. At idle, bias tester measures about 325v plate-cathode, 26mA cathode current. Matches up with Nick B's tube model nicely, about 60% dissipation. Measuring primary side power, I get .34A * 122vac (line voltage at that moment) = 41.48VA. Subtracting about 19.5VA for the filaments, I get about 11W idle dissipation per tube. OK so far.
I'm thinking about tacking in a 1R/1W resistor on the cathode network to better see how the tubes behave when I put a signal through them. But my primary data point will be primary current, monitoring to prevent exceeding the PT's rating.
 
Steve A. 6/16/2018 11:18 PM
Quote Originally Posted by eschertron View Post
I'm thinking about tacking in a 1R/1W resistor on the cathode network to better see how the tubes behave when I put a signal through them.
By all means do that, especially since we suspect that excessive current through the output tubes was the cause of the PT failure. Definitely old school but what works works...

But my primary data point will be primary current, monitoring to prevent exceeding the PT's rating.
I would have thought that the amp panel fuse would have offered protection but it obviously didn't.

I am a bit surprised that the PT internal thermal fuse didn't open before the windings were damaged. I bought a Pignose 40GV in 1998 which was the first tube amp I had seen that was made in China. There was a strange smell whenever it was turned on and within a month or two it died... there was no continuity in the PT primary. At that time there were no parts available for it so I was on my own (I had already started doing mods to it which voided the warranty.)

In any case I eventually tracked down the underlying problem to be the amateurish-looking power supply pcb which had not been properly etched so there was current leakage between the high voltage traces. I had to manually remove the unetched copper between the traces. As a temporary measure I bypassed the internal thermal fuse in the PT until I could track down an acceptable replacement and the amp worked fine after that and was the test bed for all of the mods documented in the attached PDF file...

[ATTACH]49347[/ATTACH]

http://blueguitar.org/new/articles/b...s/g40v_mod.pdf

Steve A.