I never thought this would happen to me! (PT failure)

[eschertron]

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]

with a 100ma transformer you would be pushing it. ( from 6v6-gt tube data ab1 )
mabe a 120ma would do better.

[dstrat]

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]

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]

I'm sure a PITA and expensive, but it was at least fun running the hell out of it?

[eschertron]

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]

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]

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]

Ahh the amp designers worst enemy...

HIMSELF!



Seriously though, I hope a rebeefed PT does the trick!

[drewl]

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]

Sometimes these things happen.

^^^^^^^^^^^^^^^^^^ That too.

[nsubulysses]

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]

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]

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:
Jazz Amp 222 2Mar2018.pdf
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]

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.