carvin vintage tube

[Andy Le Blanc]

Came over the bench with a bias issue.
Love the fax-distortion circuit, Ive seen the mid control elsewhere.

Interesting... the 56p between the inverter grid and plate VS say between the
inverter plates your accustomed to seeing in other amps.
I've seen FB applied there in much older amps but usually with a resistor and large uF valued cap.

The bias method posted on the web as the preferred method is to clip into the
standby switch with a meter or a shunt to measure ma/mv over and then turn on the amp.
You leave the standby switch on standby. They say 75 ma for 4 6bq5
That leaves it at around 7.3 w per tube, chilly....

Any body find sweet spot with these cheap trannies ....
any "biased" opinion will do...

[Structo]

I have a Belair.

There is a guy that goes by Hasserl that has published quite a few mods for these amps.

Not sure if this is the same document you have but it outlines the biasing procedure.

Those amps have a pretty good clean channel but the soak channel isn't so hot.
I have some mods for that if you are interested.

[Andy Le Blanc]

that's a nicely considered article..... hadn't found that one ... thanks...

I ended up inserting a 1 ohm in the plate circuit and grabbed a dissipation
figure there. The real issue with the amp was the bias pot.

You go back and forth from a dry cold band wagon to a fairly humid bar/gig,
The condensation is nutz, I brought an amp in from a negative cold barn
one day and got half an inch of frost in less than 5 min. Took half the day
to thaw an dry, next to the wood stove.

bumm trim pot. I contacted the client and he wanted a hotter bias.
didn't push it.... about half way between the carvin recomended and 70%
small envelope tubes wear more quickly.

Mods are always fun.

It was nice to connect the scheme with the sound of the tone control function too.
That style tone has a -20 loss, useful for carving up a gain structure.