Fender Deluxe Reverb Silverface - No Sound

[rmeadows]

I am taking a look at my friends Silverface Deluxe Reverb which recently quit producing any sound.

The present condition is:
1. All tube filaments light up but only the rectifier tube gets hot. All other tubes (power and preamp) barely get warm.
2. AC heater voltage is present at all tubes.
3. Plate Voltages are present at all tubes. They are reading high on the power tubes at 507 volts DC and high on the preamp tubes.
4. The amp has new filter caps

I have checked and verified the following:
1. The speaker works
2. The power tubes work (I tried them in a different amp)
3. The standby switch works
4. I replaced the rectifier tube with a known good one (no help)
5. I took voltage and impedance readings on the power transformer and all check out
6. The output transformer has been replaced (no help)
7. The choke is not open

There is a multi-volt selector switch on this amp and I do know someone had it in the 220V position while operating at 120V. After the switch was put back in the 117V position the amp worked for a while and then quit.

Any suggestions would be helpful.

Thanks.

R

[Alex R]

- may as well also check for bias voltage on pin 5, screen volts on pin 4 of the power tubes.

That's quite a high voltage even for a s/f. Is the power tube cathode ground ok? Give it a good poke around, you can get dry joints there.

When you say no sound - not even a bit of hum/hiss?

[rmeadows]

Update

Alex:

After taking a closer look. There was no cathode ground on pin 8 of either power tube. I can't imagine why that would be. I grounded the cathodes and now the amp is working great. All voltages came down to more desirable levels.

There is a ground on pin 5 of each power tube through a .001 mf cap. Does anyone know why that would be there? Should it be removed?

Thanks for your help.

R

[Alex R]

- glad it's sorted. Those caps to ground are probably to suppress oscillation. See how it goes without them.

[Amp Kat]

Can't say I've ever seen that and what good it could do besides what Alex said. That is the bias point and also a signal point so a cap depending on how it's configured in the topology could be frequency dependant.

[Alex R]

...sure those caps aren't on pin 3? (sorry to ask...)

[rmeadows]

Caps

Nope. They are definitely on pin 5. I did some research and found the same oscillation answer that you did.

Again, thanks guys for all your help. You saved me a considerable amount of time and headaches.

R