Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: NotSure on May 18, 2014, 08:54:48 pm
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Hey,
Do you bias a Divided by 13 SJT 10/20 as you would any cathode bias amp? I know there has been some discussion about cathode biased amps. I'm wondering about biasing for both A and AB modes this amp is capable of. Any advise will be greatly appreciated.
Dave
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As a rule, there is no such thing as "biasing a cathode biased amp" aside from simply checking the tubes are within their maximum plate dissipation rating.
That's because cathode bias is inherently self-adjusting: if the tube draws too much current, there is a bigger voltage drop across the cathode resistor, which biases the tube cooler, which reduces tube current.
So the choice of cathode resistor is a design decision, and you don't swap the resistor when you change tubes. You just plug-n-play. Some tubes will naturally idle with a greater plate current than other tubes, but that has to do with the tube having a different transconductance than other tubes.
So you would go in and adjust bias in the 20w setting (I assume this is fixed-bias), and it should be automatically okay in the 10w setting. Hopefully, /13 provided some guidance on this. Otherwise, I'd prefer to look at a schematic or peek in the amp to know how to steer you.