Help with taming a 12AX7 and 6AQ5a amp

[DavidAmp]

I just built a guitar amp from an old GE stereo amp. basically just using the xfmers, tubes, and chassis. All else is new. I have wired it similar to a Gibson GA-5 skylark. a 12AX7 in series, into two 6AQ5a power tubes in a push-pull stake. it works! however, I get saturation at about 1/4 volume or less than half volume. Too much distortion to soon for me! I would prefer a nice clean signal and still have all the volume! Can anyone give me suggestions on how to tame the 12AX7 down so not to over drive the pair of 6AQ5 tubes?
I don't know much about tubes. I'm an electronics eng, and this is my first tube project. thanks.

[poobah]

Hey Dave,

Can you post a schem? You know; doing one thig always upsets another... just a scan of a pencil sketch or???

Your 12AX7 must be both and phase splitter so one simple change may not do it.

There is also a guitar tube whiz here "Tubelab"...

[DigitalJunkie]

Hey,Another Oregonian! Welcome Dave!
Where are you located at? (i'm in SE Portland)

[Eli Duttman]

Originally posted by DavidAmp
I just built a guitar amp from an old GE stereo amp. basically just using the xfmers, tubes, and chassis. All else is new. I have wired it similar to a Gibson GA-5 skylark. a 12AX7 in series, into two 6AQ5a power tubes in a push-pull stake. it works! however, I get saturation at about 1/4 volume or less than half volume. Too much distortion to soon for me! I would prefer a nice clean signal and still have all the volume! Can anyone give me suggestions on how to tame the 12AX7 down so not to over drive the pair of 6AQ5 tubes?
I don't know much about tubes. I'm an electronics eng, and this is my first tube project. thanks.

The mu (amplification factor) of the 'X7 is 100. A 5751, whose mu is 70, will drop right in. The 12AY7, with a mu in the 40s, may drop in without parts changes too.

[tubelab.com]

I have wired it similar to a Gibson GA-5 skylark.

There were several different flavors of GA-5. I can't find the schematic for one that has 2 6AQ5's and a 12AX7. I personally have a GA-5 that has one 6AQ5 and I can't find the schematic for it either. Gibson used a 6EU7 in many of these amps (mine also), which is similar to a 12AX7, but usually has slightly less gain.

My question is do you get enough volume before the distortion sets in or is the sound distorted but not loud enough.

If the amp distorts early, but doesn't get 5 to 10 watts worth of loud, then the problem is usually in the output stage. Usually worn out tubes. If the voltage on the plate and screen are 240 to 300 volts, check the cathode voltage it should be 15 to 20 volts). Low voltage usually indicates weak tubes (or a wrong part, or wiring error).

If you get plenty of volume, but the distortion comes on too soon then you might have too much gain. If you can find any of the previously mentioned tubes, it wouldnt hurt to try them. A 12AU7 is the lowest gain of these types, probably too low, but they may work. You can try disconnecting the capacitor from the cathode to ground of the 6AQ5's.

It is also possible that the bias is not set at the optimum point.

I could make a better guess if I had an idea of how you wired it up. If you have a schematic, please post it.

[ivegotmono]

here's a schematic of one like yours I think.