Aa864 bass channel - less gain, more headroom?

[Erik Aa]

Hi. I have cloned the aa864 circuit in a Peavey Classic 50vt. I skipped the normal channel and v3 since I plan to use it for bass guitar. Here is my question. The amp has too much gain and no headroom. It distorts right away. Any suggestions as to how I can reduce gain/distortion and get more clean headroom? Thanks!

[shooter]

have scope will solve!


guessing at which stage is overdriving with math is a little tedious, with a scope it's a no-brainer.


find a tube datasheet like this for the tubes you're using, and tweak the values to give you less gain.
"headroom" as I see it is being able to get the biggest sinewave without distortion.  you can increase the voltage range that the signal swings between, or you can decrease the signal size to swing clean between cutoff and compression.  none of its easy when you're "blind" to the range, the signal....

[mresistor]

"headroom" as I see it is being able to get the biggest sinewave without distortion

you must be related to Max   

[Erik Aa]

Thanks for the suggestion. Unfortunately I don’t have a scope (yet). Are there other ways to achieve this? Could another solution be to drop a gain stage? E.g. V2 and only use the two triodes in V1 in the preamp? Or do you guys have other suggestions? Thanks again.

[mresistor]

One could try a lower gain dual triod in V1 slot. 12AY or 12AU.  Are you using stock values for coupling capacitors?

[mresistor]

Also I think people would want to know where the 6L6 bias is set, and the Voltages of the K, G and A .


Another way to check the signal through the amp is with a listening amp, Doug has the instructions on how to build one.

[Erik Aa]

Thanks mresistor. I will look at my small tube stash for a different tube for V1. I use 12ax7s now. I can measure accurate voltages shortly. So far I have only brought the amp up carefully on the variac. To do some very preliminary testing. I was so curious. Couldn’t wait. Thanks. And yes, stock cap values with the exception of the pt filter caps which are 100uf.

[PRR]

.

[Erik Aa]

Thanks PRR! Do basically drop or bypass the triode in V1 after the tone stack and go straight to V2? Thanks again.

[Erik Aa]

I just rewired to PRR’s suggested mod and it was a significant improvement. I will now try to find a 12au7 for V1. Thanks!

[shooter]

you must be related to Max

my new redneck camper is limited to ppl less than 6ft tall and no wider than 36" 
my plan is to put the sign from the movie above the entrance   

[mresistor]

I will now try to find a 12au7 for V1. Thanks!

I would try an AY first   they are stock in a 5F6A ..