Need some advice to trace problem

[fusionbear]

Hello All. I am pretty sure a client brought me an amp with a blown OT, but I want to confirm it since it is a very rare Marshall Super PA.

Symptom: When you fire it up, it reads all the right voltages, but as soon as you play a power chord, it seems to drain and and the next power chord sounds like farts...

I put a 1K sine wave and everything looks great all the way the speakers. That is what is baffling me. All filter caps tested ok with my cap tester, but I replaced them temporarily to see if the problem went away and still the same thing. What I am confused about is that it could be the PT if there is a winding that is just broken, no? Or maybe there is a winding broken in the OT and that is the culprit? My gut feeling is that it is the OT, but I currently don't have a spare one to test with...

I maxed the amplitude on my signal generator to try to push the signal, but couldn't get it to do the same as with a guitar.

Thanks for your help...

[pdf64]

The insulation between windings on an OT might only fail under load, eg across the primary.
The way to test for this is to set it up on the bench with a dummy load, scope the output signal and gradually increase the input signal, past the point of overdrive.
With such a fault, at some point the output will collapse.
Pete

[Stevem]

Install 1 ohm 2 watt resistors in at least one of the output tubes cathodes and drive signal thru the amp, if the OT has a short the tubes will pull far more current that at idle as they try to drive the failed OT but can't!
In fact you will see the power supply voltage start to sag also.

[fusionbear]

Thanks for the help. I added a 1 ohm resistor to the cathode of one of the tubes and found that the OT has a open. The amp will bias, but as soon as a high signal is given to the amp, instead of the current increasing, it tries to increase for about 500 ms, but drops instantly. The voltage never sags. I pulled the OT and will be delivering it to a guy who does rewinds who is extremely well regarded.

Thanks all!

[xtian]

Very interested in what it costs to rewind, Carl. Please let us know!

[Stevem]

Five months ago Mercury charged me well over 400 to re-wind a 200 watt Marshall Major OT and I got a ball park for them to do my 1954 Gibson GA77 25 watt OT of 300 bucks!
Let use know what prices and who you use?

[gui_tarzan]

Have you guys checked out Heyboer here in Michigan? I've dealt with them a little and they seem to be very fair price-wise but I have not had one re-wound.