Epiphone Valve Senior problem

[MarkusBass]

Yes, it looks like you are conducting every possible test but not the basic one we suggested: remove V1 and V2, leave V3, V4 and V5 and provide the signal to C13. What happens in this case?

Heater voltage pin 2 or 7 referenced to ground starts at 3 VAC until standby switch is turned off, then stays steady at 2.3 volts before and after the drop.

This information worries me. Can you check it once again? The tubes should be getting 6.3V but it seems that they are getting 4.6V only. Can you repeat the measurement but this time connect the meter to pins 2 and 7 directly? Don't provide any input signal, just turn the amp on and read what is displayed on the meter (AC voltage). Tubes with 4.6V heater voltage may not work correctly.

Mark

[bigdrums925]

Alex: I did that (removed V1 & V2) injected a signal from an HP signal generator to C13 and the problem persisted.

[Alex R]

OK gotcha. If it's oscillation then output transformer leads are the suspect - I see you twisted and rerouted them. If you are comfortable working with the amp on, and can avoid the various high voltage sources, my next step would be leaving the preamp valves out and producing the fault condition, then moving leads around to see if it changes and trying to stop it that way. Swapping in known good PI and power valves also worth a try, especially if they are microphonic. Next try tacking in some suppressor caps< I think I might try say .0022 caps grid to ground on the power amp, not good for the sound but that would stop HF oscillation around the power valves and cut that possibility out. That's not the ultimate cure, or not a good one, it just tells you where the problem feedback is. If that doesn't find the problem you need to eliminate the PI stage, again a cap to ground on the input to see if that stops it. If it's none of those things then we might have to ask what is the next suspect problem - but it does sound very much like oscillation. I think my next suspect would be a bad OT but that's a long way second to oscillation.

There are wiser heads than mine on here and they may adapt these suggestions to good effect.

[bigdrums925]

With V1 and V2 removed, signal injected to C13, I have rerouted the output leads and tried holding them as far away from the power tubes as I could with no change. Next swapped tubes both power and PI to no avail.
Then added suppressor caps like you said. I placed them on the tube side of R20 & R19 which is the control grid. Is that where you meant? There is also the screen grid pin 4, and the suppressor grid/cathode pin 8. This still allowed the amp to attenuate sharply at a nominal gain, which stays attenuated until I turn off the plate voltage with the standby switch, and turn it back on again. This I think is a clue because if it were just oscillation, wouldn't it likely come back to normal with a reduction of the input signal?
How would I eliminate the PI section?

[MarkusBass]

This I think is a clue because if it were just oscillation, wouldn't it likely come back to normal with a reduction of the input signal?

I agree, this may be something else than oscillation.

How would I eliminate the PI section?

This is difficult - you would need two inverted signals with quite high amplitude. I would use PI circuit from another amp, or such a circuit assembled on a breadboard.

But, there is one more test that you haven't performed. You said that the problem occurs when you touch cathode resistors with meter lead in the PI circuit. And I suggested to solder there 1Mega Ohm resistor and see what happens. I would do it without output tubes. If this does not help, I would think of getting a replacement output transformer (but don't buy it, just use an OT from another amp).

BTW, I assume that you checked quality of soldering in this amp. It's because the problem that you have my be a result of a cold solder in the power amp. I would double check tube sockets (and everything else).

Mark

[bigdrums925]

I started this time going back and resoldering just about every joint, under a microscope, and tighting the spade lugs. Also labeled all the wires
Put it all back together, carefully routing the wires as far from the board as possible.
Put all the tubes in and surprise, it didn't attenuate while turning it up & got rather loud.
However this situation did not last but for about 10 minutes.
Now it's reduced all the time and found that V5 is not putting out, or nothings being put in causing nothing to come out.
I'll check tomorrow or Sunday. Found out by removing 1 power tube at a time, both tubes work in 1 socket and neither in the other
AC & DC voltages are the same on both power tubes, both are warming up equally

[g1]

AC & DC voltages are the same on both power tubes, both are warming up equally

Does that include the AC signal going into both power tubes (pin5)? It is equal for both tubes?

[bigdrums925]

I tried it again tonight, all 5 tubes installed, signal input to the input jack
I will say yes, V4 pin 5 AC signal measures 11 VAC while V5 pin 5 measures 11.2 VAC with the same gain setting.
I turn up the gain or master and they both reach approx. 50 VAC before attenuating, at which point they will jump slightly
to about 54 VAC. There is also some distortion that sounds like scratchiness coming from the 4-10" Marshall speaker cabinet nominal 8 ohm load.
The speaker cable is plugged into the 8 ohm jack. This scratchiness seems to be coming from the amp and sounds like crackling
to the amp section, and rises in volume with the output volume.
Once it has attenuated, I can't easily restore it by just flipping on the standby, then flipping it off again like was the case a short time ago.
I'm posting a picture of the o-scope display at the point the distortion starts to appear. Hard to see, but at the upward peaks are some
display lines coming straight down which is the scratchiness I believe. The second shot is of the amp speaker output once attenuated.

[bigdrums925]

Alright Mentors & Students: It appears the problem is finally solved. Apparently the center tap on the primary side of the output transformer partially shorted to one side of the windings connected to the plates of the power tubes. The DCR between the output leads appeared normal, but the center tap was not centered, different DCR from center tap to the other 2 windings. I ordered and installed a new output transformer from Mercury Magnetics and installed it this morning. All appears OK now. Wanted to thank you all for continuing to help those of us who get stuck trying to do these repairs ourselves. You are providing a fantastic resource!

[bigdrums925]

Its been a while but wanted to say thanks for the help on repairing this amp. It turned out to be the output transformer. Apparently the primary winding had shorted on one side as measured by DCR. It seems none of the center-tap transformers are truly in the center but at what point it is too far off is hard to know.

[Enzo]

Typically one turn shorts to the next, and a meter will never detect that.

There are two ways top wind an OT. One is to wind both halves of the primary at the same time, using two pieces of wire. That way both halves are the same length. The two wires are joined as the center tap. That results in even resistance on the two sides. The other way is to wind half the turns, connect a center tap wire, then complete the turns. That results in one side having the outer turns. The outer turns are much longer, since each turn has to surround all the previous ones. So for the same number of turns, one side is much longer piece of wire than the other, and so the resistanbce wiull be much different. Transformer work ob turns rations, not resistance, and so they still work, evn though teh resistance is very different between the two halves.