JMP Low Output

[brsmith21]

Hey guys, got a slight problem.

The amp in question is a 50w, 4 input from 1976. It had been previously modded with a master volume. My buddy wanted it returned to stock. I did this and it sounded great. The next time I turned it on, however, there was very low output. The pin voltages are all pretty close, though the B+ is lower than I would expect at 402v. Also, whenever I run sound through it, the output tube bias jumps like crazy, from 20 to almost 100. I'm hoping that this might give a clue as to what the matter is. I've changed tubes but the results are consistent. Any idea where else to look? Thanks in advance for any suggestions.
Bryan

[PRNDL]

Are you sure that there wasn't another mod, such as an extra gain stage?
(Maybe the output tubes are being driven way too hard).

I'd also check to make certain everything is in there that supposed to be there (such as grid resistors). Maybe something was removed when they added the master volume.

Checking resistor values of the output section (plate/cathode) might reveal something.

It's hard to tell without more symptoms and testing.

[pdf64]

Hi Bryan
'I run sound through it, the output tube bias jumps like crazy, from 20 to almost 100'
Bias is the steady dc grid to cathode voltage applied to achieve sufficient no signal cathode current to give acceptable crossover distortion. Are you refering to cathode/plate current in mA with '20 to almost 100'? If so, it is neccessary for a tube which isn't in class A to pass more current when presented with a signal input, until it reaches saturation, so that's normal. Think of a tube as a voltage controlled resistor, so when the grid to cathode voltage increases, the plate to cathode resistance decreases, and visa versa.
Back to your problem of low output, the above is telling me that your output tube are working ok, so the problem lies somewhere between the OT and speaker. Worst case is a bad OT, but most likely it's a bad connection or partial short somewhere between the OT secondary and speaker.
Hope that helps - Peter

[brsmith21]

Thanks guys,
What I'm referring to is in fact the cathode/plate current as measured through my bias meter which plugs in between the tube and socket. I didn't check the actual negative voltage as I don't have a way to do that as I play.

I've been going through the circuit now and double checking the components to make sure other, less obvious mods weren't performed.

Also, I double checked all the pin voltages and found that V2 does indeed have some odd voltages:
V1 pin1 176v, pin 3 1.45v, pin 6 182v
V2 pin 1 206v, pin 3 1.86v, pin 6 286v, pin 7 206v, pin 8 206v
V3 pin 1 219v, pin 2 244v, pin 3 39v, pin 6 208v, pin 7 25v, pin 8 39v
V4&5 pin 3 404v, pin 4 399v, pin 5 (-37v), pin 6 403v

I'm also checking the connections between the OT secondary and speakers now. Thanks again!
Bryan

[Enzo]

Bias readings are like idle speed on your car. Bias is taken AT IDLE. If yuo send signal through the amp you are no longer measuring bias, you are just measuring the operating current through the tubes. You cannot measure bias and play at the same time.

You cannot set the idle speed on your car while driving down the road can you?

The "funny" 12AX7 readings are for a cathode follower stage.

[brsmith21]

Bias readings are like idle speed on your car. Bias is taken AT IDLE. If yuo send signal through the amp you are no longer measuring bias, you are just measuring the operating current through the tubes. You cannot measure bias and play at the same time.

You cannot set the idle speed on your car while driving down the road can you?

Understood. The only point I was making with this is that the current varies wildly when playing on this amp, while it only varies a little on my other amps when playing.

[slowfingers]

Could be the two 8uf/10uf bias supply caps are shot.
slow.

[pdf64]

If your output tubes at idling at 20mA each (or total for the pair?) they are biased on the cold side, 27% of max plate dissipation. That's why the current is varying so much between signal and no signal. Most people would prefer the tone when the bias (ie idle current) puts the idle plate dissipation between 50 and 70% of max plate dissipation.
But that doesn't help your low output problem. Have you checked that the output impedence selector is good, not giving you an open or intermittant open circuit? Peter