Ampeg Reverberocket II Bias problem

[stereomonostereo]

I'm working on an Ampeg Reverberocket II, Model GS-12R, with low, distorted output. Upon checking the 7591 power tubes, i noticed that i'm not getting any voltage on the grid at pin 5. My 140 ohm resistor is good, and i replaced the capacitor that it's coupled with. I haven't worked on anything with 7591's in a while, so maybe i'm just missing something simple, but if i recall correctly, i should be getting some sort of voltage off of pin 5 to control the bias.

[Enzo]

Always good form to link a schematic so we can follow along.

140 ohm resistor, as in cathode resistor for power tubes? How many volts across that? That is your bias. 0v DC on the grids then would be desired.

Bias is the voltage difference between the grid and cathode of the tube. It doesn;t matter to the tube whether the grid is ground and the cathode is +30 or if the cathode is ground and the grid is -30. Either way, the grid is 30v more negative than the cathode. (Yes yes, ther are differences in plate voltage and stuff between the two ways, but that is separate from expecting to find DC on the grid of a cathode biased tu8be.)

[Chuck H]

Indeed, and as usual, Enzo is correct. I did look up the schem because Ampeg does have a few 7591 amps with both a cathode resistor AND a bias supply. But this isn't one of them. The grids are referenced to 0V. And, as mentioned, bias is the grid to cathode relationship. If the cathode is psitive of 0, and the grid is 0, the grid is indeed negative WRT the cathode.

As to the low distorted output... Waaaay too many things it could be. More info reguired. Maybe take voltage readings at the tube pins and post them.

[stereomonostereo]

After checking my voltages again i've found that i'm getting 356vdc on pins 2 and 7 of the 7591s, which are the heaters. I disconnected the leads and tested again, coming straight off of the transformer and i'm still reading the same thing. When i switch to AC, i'm getting 6.3 volts. I couldn't be getting both AC and DC off of my heaters.

[Chuck H]

That sounds bad. Unplug the power tubes, keep the heaters disconnected and just to be sure, lift the heater CT. Now test for continuity between the HV and filament windings. If you don't have continuity your PT is probably OK. The next likely culprit would be a short in one of your power tubes (note that even with the filament winding ends disconnected, the CT is still connected to the power tube cathode). If you do find continuity with this test, your PT is toast.