older Fender Pro

[JC@]

I have an old Fender pro here, and now is working good, but I would like to have more clean headroom, if that is the correct term.

The amp begins to clip a little too early, and the owner wants it to stay clean a little further up the volume control.

Any help?
Thank you.

[MWJB]

How old? If it's got a paraphase PI (5D5 & earlier) & 6SC7s it's never going to be a clean machine.

If it's got a 5U4 then try a GZ34, try larger 1st stage filter cap (40-50uf) & 6L6 cathode bypass cap (100uf/100v), more efficient speaker, you might want to try a 330ohm 10W cathode resistor to tame plate current?

A little more info about the amp might help with more specific suggestions?

[JC@]

MWJB, it is a clone actually from a model Pro Reverb aa1009. It already has diode rectification instead of the 5U4GB, and new filter caps of a 50uF value in it. It also has a real bias circuit not a balance one.

[MWJB]

Aha! A Pro Reverb is a very different amp from a Pro.

You might try a 12AT7 or GE5751 in the preamp stage, or, raising preamp plate voltages by using a smaller value power supply dropping resistor (4.7K on schematic) and/or a 2.2K or 2.7K cathode resistor at the first stage?

[cbarrow7625]

Assuming that it is the 3rd stage and/or the phase inverter that is being overdriven, you could reduce the value of the 220K grid leak resistor going into the 3rd stage (The one at the junction of the 3.3M & 470K reverb mixing resistors).

In addition, you could try changing the 1M grid leak resistors in the phase inverter to something smaller (100-200k?) as well as reducing the power tube grid leak resistors. You will likely have to increase the size of the capacitors in there to keep the same bass response.

I would start with the 220K resistor first though. You won't need to change any caps with that one. Wire up a 250K pot in there and play through it while adjusting until it seems right. You could hook it up as a 'master volume' or just as a variable resitor. I don't think it would make much difference in that circuit.

Chris

[JC@]

Thanks a lot, I have plenty of options now.
Cathode resistors were my move, but I will do it right and check with the scope where it clips and with a little here and a little there I will end up with a solution.

I like a lot the master pot idea.