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Wondering on how I could modify a stock ST-70 so I could adjust the bias on each output tube individually using a "balance" pot similar to Shannon's design? I understand the circuit needed to measure the current on each tube, can copy that off my diytude board design.

What I am looking for is info on if a 10k or 20K linear pot can be used to adjust the current from side to side? Basically I want to come off the existing pot and go into a second pot that adjusts the ratio.

Any ideas or suggestions guys? The chassis I got was already drilled for the extra pots and I like the idea of balancing rather than trying to adjust 4 tubes individually.

I don't trust the "matched pairs" I buy today to stay matched for long!

Thanks,

Ed the Solder Slinger

Ed,
Go to Dynakitparts website at http://www.dynakitparts.com/store/news.aspx they are coming out exactly with what you are looking for. The BBCU-1 Bias Balance Control Upgrade.

I worked with Kevin on the development of the bias balance mod and it works great. I used 2 unmatched (on my Hickok 539B tube tester) different manufacture EL-34 tubes in the ST-70 and I was able to balance the bias with no problems.

I know you mentioned you already have holes drilled in your chassis but Kevin designed the perfect way to mount the potentiometers without drilling.

Regards,
Sal

I'm installing a failsafe bias on my ST-70, similar to those used on Audio Research amps... one control adjusts bias, other balance.

It's failsafe in that if the wiper lifts on either control, the tube grids hit full V- and cutoff, rather than self-destruct, like all the Dynaco setups.

Cheers!

Ted Weber at www.tedweber.com also sells some ready-made raw bias adjust boards that allow 1, 2, or 4 tubes to be adjusted. I've got a 4 way one to use in my Mesa Dual Rectifier when I get around to swapping it in.

Greg