Park 75 Bass Amp Biasing Question

[rf7]

I've got an early 70's Park 75 Bass on the bench that biases up funny. If I bias for the crossover notch on the scope, I only get 16W out of it. When I bias it up using the current draw through a 1 ohm cathode resistor I get better results, about 66W at clipping with the plate sitting at 425V. Basically, the crossover notch goes away very early, before the amp is pushed. Has anyone seen something like this before? What is that due to?

[Enzo]

It is due to setting the amp up like a hifi amp - crossover notch - instead of setting it up as a guitar amp.

[rf7]

Aw, ok. Thanks Enzo!

[g1]

I'm a little curious about your crossover bias method. The amp puts out approx. 60W. You would run it up to near full power and observe the waveform. Then you would adjust the bias till you see a bit of crossover notch and remove it. I don't know how at this point the output would have dropped to 16W. ?
I'm wondering if you had the amp set up for low output when you removed the notch?
I usually monitor idle current and check the crossover notch when adjusting bias. I have never seen what you have described so it's quite puzzling.

[Jazz P Bass]

I don't really see a 'notch' per se on tube amps.

More of a crappy 'hollow cheek' sign wave.

Maybe I never took one low enough.

[Chuck H]

I'm a little curious about your crossover bias method. The amp puts out approx. 60W. You would run it up to near full power and observe the waveform. Then you would adjust the bias till you see a bit of crossover notch and remove it. I don't know how at this point the output would have dropped to 16W. ?
I'm wondering if you had the amp set up for low output when you removed the notch?
I usually monitor idle current and check the crossover notch when adjusting bias. I have never seen what you have described so it's quite puzzling.

What he said!