Re: Bias for 6L6?



Phil_S wrote:
Well, fellas, looks like those pulls from the Sears organ chassis are going to be the ticket. I've got them in the amp. Va=~360 and Ia = 61mA on one tube and 62.4mA on the other tube. I'm making this for about 22W per tube or 44W for the pair. Grids are about -26V. I can move grid voltage down to almost -22. Where do these "belong"? The OT is a Stancor A-3801, the fabled Trainwreck iron, 6600 ohm primary.

Screen voltage is around 350V.

mA are calculated based on the voltage drop between the plate and CT of the OT. I know the Ohms on each half of the winding. I'd call this the modified shunt method. I freely admit that I'm too chicken to put the meter in amps mode. Last time I tried doing it, I slipped and it cost me both the OT and the PT. I had tip jacks installed in the chassis and darn if I didn't slip while putting the probe in the tip jack. Some things are very unforgiving and defy idiot proofing.

I have the idea from looking at the RCA Receiving Tube Manual that a pair of 6L6GC might be happier with a more mA. But all the tabled data show a big spread between plate voltage and screen voltage.

Thanks.

Phil

22W/tube is good. You can run up to 23W/tube and still be safe. After that, you'll be shortening the device life.

There is no "magic" number where the tube stands up and sings. Its your amp, your ears. If you want to run them a bit hotter for the tone you like, do it. Just be prepared to replace them sooner. Big time players who can afford it run their tubes hot enough to need replacing after each show. Whatever.

I always ask my customers to come with their guitar for the bias setting.

And they always love the tone they choose.

Hope it helps.

John
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