Low Power w/ SF Bassman

[hasserl]

Looks like your PA is indeed OK if you can get the 40 watts from it. But I find it off you can get the 40 watts strong from it with 6,5v from your gen, but with 6.9v from the normal channel at the same point, it is weak.

Just to clarify, I'm making these measurements with the Volume control at 10, with it set to below 5 the power out is much lower. IME an amp like this will develope max clean power with the Volume control set to around 2 - 3, above that it starts to distort. I would expect a healthy Bassman to make 40 watts power clean. So when all I can get out of it is 40 max I think it's low power.

Also, with the Bass channel set up like it is I'm only getting just a few watts of power out of it with the Vol dimed.

Between the 0.1 of the normal channel output and pin 2 of the PI, you are losing 80v. Is it droppng across the 250k or across the .02?

It's dropping it across the 250k resistor.

[Enzo]

Hmmm

Well, we can brute force the signal into the PI and drive the thing full out. Any chance the grid resistors on the PI are wrong? I am wondering if somehting is making a voltage divider with the 250k.

[hasserl]

Sorry, been out of town on business, couldn't work on this thing. And I've had a customer paying job show up while I was out so I need to set this one aside for a few days while I work on the other one. Next week I'm out of town again all week. So this is going to have to sit for awhile.

In any case, the PI grid resistors are the stock/original resistors. I'll have to check them later. Thanks for the help so far!

[hasserl]

Finally had a chance to get back on this one today. I decided to rewire this circuit so that both channels use the third gain stage. To do this I moved the 250k channel mixing resistor of the top channel over to the junction of the 220k series grid resistor and the 470k local fb resistor at the grid of the third gain stage on the bottom channel. So both channels feed into that gain stage. I then deleted the .1 coupling cap ands the 250k channel mixing resistor of the bottom channel, and instead ran a wire directly to the .022 input coupling cap at the PI. And that seemed to have done the trick, it now drives the PI hard and the power tubes are getting a strong drive signal and I'm getting about 35 - 40 watts power out with a .250v 1kHz sine wave at the input of the Normal channel, and about 55 watts max out put. That's good enough for me, it's louder than I'll be able to use at most places and the Master Volume will come in handy.

I now need to tidy things up and maybe fine tune the two channels. I still don't understand why the individual channels wouldn't drive through the mixing resistors the way I had it wired up originally. I just lost all drive across those 250k resistors.

Thanks to all for their help on this, I appreciate it. You guys are the best!