Fender Super Reverb silverface Hum

[52 Bill]

V3 is the reverb driver (12AT7) and it will run quite hot. It idles at higher current than the other preamp tubes and loads down the B+ to a certain extent. When you pull it the B+ will increase a bit to the other preamp tubes and that may be responsible for the tone difference you are hearing.
But maybe it's running even hotter than usual, so check those voltages Jazz mentioned.

Is the tank connected to the amp? If it isn't, there is no load on the output of the reverb driver, so the tube could be trying to drive an open load.

[Ampzone]

There is no load on it, maybe this would explain it being so bloody hot.
removing it improves tone.

I'll connect the reverb tank back up to it and then take a voltage check and report back

[Ampzone]

Ok voltages for V1-V3, So the reverb sounds normal FYI.

V1,
1196v,
2 0
3 2v
(4/5 heaters)
6 209
7 0
8 1.8v
9 (heater)

V2
1 218v
2 0v
3 1.71v
4/5 heaters
6 232v
7 0v
8 1.8v
9 heater

V3 Reverb
1 406v
2 1.12v
3 6.1v
4/5 heaters
6 405v
7 1.04v
8 5.9v (this ties to a 470r resistor on the board).
9 heater

[nickb]

Ok voltages for V1-V3, So the reverb sounds normal FYI.

V1,
1196v,
2 0
3 2v
(4/5 heaters)
6 209
7 0
8 1.8v
9 (heater)

V2
1 218v
2 0v
3 1.71v
4/5 heaters
6 232v
7 0v
8 1.8v
9 heater

V3 Reverb
1 406v
2 1.12v
3 6.1v
4/5 heaters
6 405v
7 1.04v
8 5.9v (this ties to a 470r resistor on the board).
9 heater

V3 is odd:
(1) Pins 3 and 8 should be the same
(2) Pins 3 and 8 should connect to a 2.2K ohm resistor
(3) Pins 2 and 7 should be close to 0V
(4) Pins 2 and 7 should be the same

Recheck your numbers and that cathode resistor value.

If you are now sure that pins 2 and 7 are not close to 0v, then
(a) find the 500pf cap that connects to them and measure the DCV on other side.
(b) remove the 12AT7 and remeasure pins 2 & 7.

[Enzo]

The voltages that should be the same on two pins are close. One or two tenths of a volt. Since they are wired together, I tend to think the differences are due to meter probe placement.

Yes, the positive volt on the grids is a problem.

[Ampzone]

The voltages that should be the same on two pins are close. One or two tenths of a volt. Since they are wired together, I tend to think the differences are due to meter probe placement.

Yes, the positive volt on the grids is a problem.

I will re-check and re-post directly. With the positive voltage on the them where could it be from I wonder?

[nickb]

With the positive voltage on the them where could it be from I wonder?

That is why I said
"If you are now sure that pins 2 and 7 are not close to 0v, then
(a) find the 500pf cap that connects to them and measure the DCV on other side.
(b) remove the 12AT7 and remeasure pins 2 & 7."

Most Likely Possibilities:
measurement error - that's why we recheck
leaky capacitors - open the circuit -see (a)
bad tube - remove it to see the effect (b)

[Ampzone]

ok voltage re-take for V3

with tube 12at7 in:
1 402v
2 0.815
3 5.6v
4/5 heat
6 402v
7 0.815
8. 5.6v
9 heat

without tube
1 423v
2 0.077v
3 0v
4/5 heaters
6 423
7 0.077v
8 ov
9 heater


either side of 500pf cap from pin 7
0.458v at .022uf junction and 0.815v at the junction of the 1 meg resitor.

[Ampzone]

ok voltage re-take for V3

with tube 12at7 in:
1 402v
2 0.815
3 5.6v
4/5 heat
6 402v
7 0.815
8. 5.6v
9 heat

without tube
1 423v
2 0.077v
3 0v
4/5 heaters
6 423
7 0.077v
8 ov
9 heater


either side of 500pf cap from pin 7
0.458v at .022uf junction and 0.815v at the junction of the 1 meg resitor.

you can see the 470r resistor and it appears stock. On the old Blackface schemo I see a 2200 there??

[nickb]

Those voltages make more sense
You can see that when the tube is pulled that the grid voltage falls to zero telling us the the current is coming from the tube. The grid leakage current for a 12AT7 is roughly 10uA according to the datasheet for the old GE tube. With the two grids connected to a 1 meg resistor then a 0.815V drop looks perfectly reasonable to me.

On the other hand with a 5.6 drop across 470 ohms means a current of 5.6/470/2 ~= 6mA per tube which means each section is dissipating 2.5W - the limiting value. That's why the tube is so hot! I too see 2.2k on all the schemos I have. You can understand why the higher value was chosen in light of the dissipation. So the bottom line is that it's OK as is, but only just. You could change to a higher value but you'll need to add the bypass capacitor at some point else you start to loose gain.

Personally, I think I'd be inclined to swap the 470 for a 680 to keep it close to the original but add a little safety margin. That will reduce the current to ~4.7mA. The price you pay will be very slightly reduced drive to the reverb. But's it's your call. And I'm sure there will be other opinions too

[Ampzone]

Those voltages make more sense
You can see that when the tube is pulled that the grid voltage falls to zero telling us the the current is coming from the tube. The grid leakage current for a 12AT7 is roughly 10uA according to the datasheet for the old GE tube. With the two grids connected to a 1 meg resistor then a 0.815V drop looks perfectly reasonable to me.

On the other hand with a 5.6 drop across 470 ohms means a current of 5.6/470/2 ~= 6mA per tube which means each section is dissipating 2.5W - the limiting value. That's why the tube is so hot! I too see 2.2k on all the schemos I have. You can understand why the higher value was chosen in light of the dissipation. So the bottom line is that it's OK as is, but only just. You could change to a higher value but you'll need to add the bypass capacitor at some point else you start to loose gain.

Personally, I think I'd be inclined to swap the 470 for a 680 to keep it close to the original but add a little safety margin. That will reduce the current to ~4.7mA. The price you pay will be very slightly reduced drive to the reverb. But's it's your call. And I'm sure there will be other opinions too

Thank you for all the input! Much appreciated. I have another 25uf 25v cap. Should I just add a 2.2k and be done with it. If this would make is safer and less hot.

Did the voltages around the 500pf cap look ok to you?

[nickb]

Thank you for all the input! Much appreciated. I have another 25uf 25v cap. Should I just add a 2.2k and be done with it. If this would make is safer and less hot.

Did the voltages around the 500pf cap look ok to you?

The other side of the 500pF is OK. That is a tiny grid leakage current through the 3.3meg. Not a problem.

[Ampzone]

The other side of the 500pF is OK. That is a tiny grid leakage current through the 3.3meg. Not a problem.

ok good, and my V1-V2 voltages check out ok on your radar?

[g1]

Here's a schematic that shows 6V across 470R cathode resistor for reverb driver. So yes, I believe the heat to be normal.

[nickb]

Thanks G1! Nevertheless, I'd still change to 680 ohms 'cos that's just the way I roll