SS rectifumcation

[Toxophilite]

Hi there I'm messing around with a straight up SS rectifier in my AB763 build


I had a 100 ohm resister built in in series to give it a more GZ34 quality that was pretty good


Without the 100 ohm resistor I get 423 on the plates of the 6V6s and about 420 at the OPT primaries


Is that too much juice to put into the 6V6s?


that's with the bias at around 27ma at the cathode resistors.
If I go any lower my voltage will go up more


Is that okay or too hard on the poor old 6V6s ? (smoked glass RCA 6V6GT)

[sluckey]

They are running about 82% of max. I'd decrease the cathode current a bit. I like to run at 70%.

[Toxophilite]

Cool, and thanks!
Out of curiousity

I've read that the bias at the cathode resistor includes the screen bias too
Maybe anywhere from 3-5 ma
Should be subtracting that from the ma/mv reading I get at the cathode resistor?

[lkrasner]

Cool, and thanks!
Out of curiousity

I've read that the bias at the cathode resistor includes the screen bias too
Maybe anywhere from 3-5 ma
Should be subtracting that from the ma/mv reading I get at the cathode resistor?

Often we ignore that current, because it is usually small. You can also measure the Plate current directly using the OT primaries. Just measure the resistance on each side (it might be slightly different). Make sure them amp is off and caps are drained for this. Then power it up and measure the drop across the primaries. Use ohms law. This is very accurate assuming you have a good multimeter.

[tompagan123]

I've read that the bias at the cathode resistor includes the screen bias too
Maybe anywhere from 3-5 ma
Should be subtracting that from the ma/mv reading I get at the cathode resistor?

That's the way I do it.  I calculate the screen current by measuring the voltage drop across the screen resistor.  For example I measured 1.12VDC across the 470 ohm screen resistor which comes out to 2.3 ma.  I subtract that from the cathode current and multiply  the resulting value by the plate voltage to get the idle power dissipation.   I then adjust the bias to achieve an idle plate dissipation of 70% of 14W, at the most.   That 70% number is not hard and fast, I typically adjust it lower until it sounds the best.   I find that around 63% usually sounds good.  Too high, and the sound gets "gainy", and too low it gets "cold".   Fender often biases really cold like 55%, which extends tube life, but doesn't sound quite as good when pushing the amp.  But this is just one guy's approach.     

[Toxophilite]

Cool
THanks for that
More good info and methodology!