Bassman PI resistors

[lowell]

Got a silverface bassman on the bench. I have replaced coupling caps and bypass caps, tested all resistors, retubed, set bias 35ma, checked all voltages etc... The voltages on the PI are 30V higher on the plates than my flowchart says they should be. The schematic says that the two grid resistors should be 330k but in the amp they are 1meg. This amp is breaking up WAY too early... could this be why?

[hasserl]

Are all the other voltages correct per the schematic except for these? What about all of the resistor values in the PI, have they been changed? Or do they match the stock values? Changes in the plate loads and the cathode R and tail R will effect the distortion.

But I don't think the grid resistos on the PI will have that great of an effect on the sound. I recall John Kelley Brown's experiments on the old Ampage with LTPI's where he switched in/out various resistor values in all of the position on the PI, and he could not hear ANY difference from changing the grid resistors, none at all, from any value change. In fact, I saved much of that thread, here is what he said:

The grid resistors.
Jeeeze. Can someone honestly tell me they hear something here? Here's where I embarrass myself with something probably everybody can hear, but I have to give up because I'm just not getting it. I have it set up so I can select between 680K, 820K, 1M, and 1.2M, I wasn't hearing the difference. Now, sometimes I don't hear the difference with something subtile but eventualy learn that I was listening for the wrong thing, then it slowly becomes more apparent, like the PI cathode resistor value. So I listen closer, then I listen even closer, then I give a determined listen even closer yet still, I listen so hard my eyes cross, sweat is running down my face and all I can see is that forsaken rotary switch for changing these damn values! Ok, back off, relax, that's it, repeat the mantra, "you are a tone god, you can hear all, you are a tone god, you can hear all...". Then I calmly turn the volume knob on the Strat off, move it to the other side of the room, crank the amp up as Nigel taught us so there's enough white noise, then go to that damn rotary switch and listen the way I would if I were a tone god and could hear all. Oh I'm hearing the white noise alright, and the sound of a rotary switch going through it's rotation, BUT NO FRIGGING CHANGE IN TONE WHATSOEVER!!! Lotta good that mantra did me! So you guys who can hear the difference, can you really hear the difference, even in a double blind test would you hear the difference? This isn't even a blind test, I'm looking at the matriarcal forenacatin' switch move and I'm hearing the imperfections in the switch as it goes through it's rotatin' hitting all the four positions, but the amp tone just doesn't change! Ok, if I can't hear it, and it's my amp, who cares, I'll divine one of the four choices by pulling a number out of a box. And it's not like it's dependant on the other values in the PI, I went through this agonizing process in association with varying the values of all the other resistors in the circuit.

[lowell]

Thanks for that info... good to know. However I'm still stuck w/ a lot of distortion in the amp.

I'm also stupid. I said in previous post that the plates of PI were 30V above, actually they're 30V below. The supply node before the anode resistors is 423V and the plates of the PI are only 304V. This seems like a large v-drop. any clues? Yes all resistors are per schematic. Except a couple plate resistors throughout the preamp are measuring 80k instead of the stock 100k... is this a concern? Anyone have one of these amps? It does not seem possible to get a clean tone.

[Jag]

The schematic says that the two grid resistors should be 330k but in the amp they are 1meg

Maybe you're looking at the wrong schematic? The AA371 schematic shows 330k resistors but the other five versions of that amp all haved 1M.

What's the plate voltage?

[lowell]

yes you are right, must be wrong schem. however problem still remains. as stated in last post, plate voltages on PI are 304v.

[Jag]

Sorry! I meant the plate voltages on the output tubes.

[Twist]

If you have a scope generate a sine wave on the input and look at the signal at the speaker. I've had fender amps that after replacing the output tubes and setting the bias I still couldn't get much of a clean output. It turned out to be a faulty output tranny. It wasn't completely shorted but wouldn't allow full power. The sine wave at the output was oddly shaped and I couldn't get symetrical clipping.