Fender Twin Reverb Massive Hum and Resistor 470ohm destroied

[Stratotwin]

I have a Fender Twin Reverb from 1975. One month ago when i switch it on a "hum" noise started very strong and also some other noises inside.
I switch it off completly and I openned it.
The 470 ohm resistor from V8 completly blow/burned out and V8 was looking damaged.
I just repair the resistor and put a new V8.
When i switch it on again, the problem was still there. Resistor 470 ohm did not burned out ( it is a 5w now) but V8 looks bad (the bright color on top is gone).
Also there was some light comming out the "hum potentiometer" on the back part of. It shows also a burn color now.

What shoudl i look for now?

Thanks ins advance.

[Enzo]

Hi, welcome.


Most times, when a tube fails and burns up that resistor, replacing both things is all that is needed. You seem to have more problem. Leave V8 socket empty. Have the amp chassis in front of you on the bench so you can see inside. Now turn off the room lighting and watch the amp in the dark. Turn on power. Do you see any sparks or flame?

I am concerned you have an arc on a tube socket, probably V8 socket. Most likely the arc betwen pins 2 and 3. Pin 3 is the plate, with high voltage, and pin 2 is the heater. So the arc I suspect would connect those together. And that would be consistent with your hum potentiometer damage.

Such an arc could form on top or bottom surface of the socket, but most likely between the solder terminals. But in the dark it usually looks like a tiny blue electric spark.

[Stratotwin]

Hello and thanks for your tips.

You are right. I removed all 4 tube from V7 to V8 and the problem was still there.
When I removed wires from pin 2 ( keeping both conected), problem was gone. No more sparks on V8 or Hum Pot.
After removing the socket I tested pin2 to pin3 and i found some hundred ohms between. There was a burned dark area. After deeply cleaning it was a clear open conection of >1000kohm's.

Then i tested the hum potentiometer and it is KO! clearly open circuit. I have to change it also. I could not found the resistor value of it.

Then i follow you sugestion and tested again without tubes V7 to V10, and room lights off. A bad surprise came up!

V3 was burning inside glass tube. After switching all off, a smoke was coming from resistor 680ohms conected to pin8 of V3 or capacitor (25uf/25V) comencted to pin 8 V2.
V3 heater filament is no longer working.

All tests were performed with speakers in, but without reverb unit.

Do you know what could be this new problem now?

I will need a new 6l6gc tube, a new socket, a new hum pot, a new V3 and probably some more.
Do you know where i can find these parts?

Best regards and thanks in advance.

[Stratotwin]

Hi, welcome.


Most times, when a tube fails and burns up that resistor, replacing both things is all that is needed. You seem to have more problem. Leave V8 socket empty. Have the amp chassis in front of you on the bench so you can see inside. Now turn off the room lighting and watch the amp in the dark. Turn on power. Do you see any sparks or flame?

I am concerned you have an arc on a tube socket, probably V8 socket. Most likely the arc betwen pins 2 and 3. Pin 3 is the plate, with high voltage, and pin 2 is the heater. So the arc I suspect would connect those together. And that would be consistent with your hum potentiometer damage.

Such an arc could form on top or bottom surface of the socket, but most likely between the solder terminals. But in the dark it usually looks like a tiny blue electric spark.

Hello and thanks for your tips.

You are right. I removed all 4 tube from V7 to V8 and the problem was still there.
When I removed wires from pin 2 ( keeping both conected), problem was gone. No more sparks on V8 or Hum Pot.
After removing the socket I tested pin2 to pin3 and i found some hundred ohms between. There was a burned dark area. After deeply cleaning it was a clear open conection of >1000kohm's.

Then i tested the hum potentiometer and it is KO! clearly open circuit. I have to change it also. I could not found the resistor value of it.

Then i follow you sugestion and tested again without tubes V7 to V10, and room lights off. A bad surprise came up!

V3 was burning inside glass tube. After switching all off, a smoke was coming from resistor 680ohms conected to pin8 of V3 or capacitor (25uf/25V) comencted to pin 8 V2.
V3 heater filament is no longer working.

All tests were performed with speakers in, but without reverb unit.

Do you know what could be this new problem now?

I will need a new 6l6gc tube, a new socket, a new hum pot, a new V3 and probably some more.
Do you know where i can find these parts?

Best regards and thanks in advance.