Very Low Plate Voltage on 1 Power Tube - Push/Pull

[GypsyDavey]

I noticed one of the 6L6 power tubes being physically much cooler on my home-built Gibson EH150 style amp. I decided to check the plate dissipation and noticed right away that the power tube in question only had a few volts on the plate, while it's partner had about 30 extra compared to the last time I checked the dissipation. I swapped the tubes and the problem remained for the socket in question and did not follow the tube. The resistance values for each half of the output transformer are the exact same as they were the last time I checked dissipation and the plate voltage and dissipation values were fairly normal at that time. I don't see any obvious problems inside the amp. Is it possible the output transformer has failed, but still shows normal resistance between centertap and each plate lead? Thanks for any help or suggestions you guys might have!

[Stevem]

Swap the plate wires from pin 3 to pin 3 and if that issue follows the plate wire then it could only be a bad OT.

Not that the amp May howl like mad from the feedback being of the wrong phase from this swap but testing for the proper voltage should take only a few seconds, if you have a test load resistor then just run the amp into that .

These old Gibson output transformers where not overly design to handle output tubes shorting a few times and red plating.

The one in my 1954 GA77 went south from that issue, but it went open completely on one leg.

[sluckey]

What is the voltage on pin 4, 5, and 8 of each power tube?

[GypsyDavey]

Here are the voltages for each tube-- Tube 1- Pin 3: 1v, Pin 4: 341v, Pin 5: 0v
Tube 2- Pin 3: 330v, Pin 4: 339v, Pin 5: 0v

I used a new Hammond 1760J for the output transformer in the build, but I do like to play it at close to full volume at times.

[GypsyDavey]

Sorry, I left out Pin 8. It is 24v on each tube.

[sluckey]

Do what Stevem suggested.

[pdf64]

If an output valve anode voltage must be checked (why?), do so with the valve in the phase splitter socket removed *
Otherwise oscillation is a distinct possibility.
Due to the probe / probe lead / meter creating the worst lead dress conceivable at the most significant electric field transmitter node in the amp.

* ie the 6N7 of this version https://el34world.com/charts/Schematics ... _later.pdf

[GypsyDavey]

Ok guys, I swapped the plate leads. Surprisingly, I get the same plate voltages I did before on the same tubes, the low plate voltage did not go to the other side. I really expected the OT to be the cause.

[Phil_S]

1) Tube swap. Problem stays with the socket.
2) OT wire swap. Problem stays with the socket.
Conclusion: The socket is the location of the problem.
Check to see that the socket is wired correctly. Look downstream at the PI, too and check that wiring. Maybe you just have a bad solder or unintended ground?
Though I don't think it's likely, there could be a carbon trace on the socket that is causing the problem, in which case, the socket needs to be changed.

[sluckey]

Measure resistance from pin 3 to ground.

[Stevem]

The inherent problem of pin 4 being a higher voltage in these amps is a problem but we can help you correct that once the amp is back to normal.

[blackeye]

How does 330 V just disappear without blowing up the tube, OT, power supply, PT, or just blowing the fuse? I eagerly await to find out.

Is the OP measuring the voltage right off of the OT lead where it is soldered to the socket? I would consider pulling the power tubes and checking voltages all around.

[GypsyDavey]

Resistance for Pin 3 to ground on the problem socket is 98 ohms. It is 89 ohms on the other socket. There are no carbon traces on the tube socket, it looks clean. Also, I have had the amp plugged into my current limiter lamp during this process and it hasn't lit the filament at any point, so I don't think there are any obvious shorts, or at least nothing is pulling an unusual amount of current.

[GypsyDavey]

blackeye, yes I made my measurements directly off the lead where it is soldered to the socket pin. I also measured resistance from the side of the socket where the tube pin inserts with no difference in the resistance from the side where the lead is soldered. Very puzzling indeed.

[sluckey]

Resistance for Pin 3 to ground on the problem socket is 98 ohms. It is 89 ohms on the other socket.

That's incredible! Would you check that again and look closely at the meter display.