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Vintage threads from the first ten years

6L6 Class A Cathode Bias

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6/2/1999 12:59 PM
CBS	6L6 Class A Cathode Bias What is the highest prudent voltage for 4-6L6's cathode bias. I have a Bassman 100 I want to "mod/play with". It has a huge OT. I read in Funk's book where he changed a SFTR to class A, but from his description I think it was fixed bias. In TUT O'Connor describes a "Simu-class" mod, but it seems to be a mixed fixed & cathode bias. Thnx, CBS

6/2/1999 2:13 PM

Don't know the answer to your question re: max plate voltage ( 390 volts or so I'd guess), but if you are planning to use the existing transformers, it'll be awful hard on that power transformer drawing approx 400 ma all the time from the high voltage winding.Although by doing so you might drop the B+ down into a safer operating region for class A 6L6's . At least until the power transformer dies. The primary impedance of the output transformer will probably be way out of whack too. Do you really want Class A, or just a cathode biased output section?...Jerry

6/2/1999 10:50 PM
Bruce	Didn't that same chapter mention how hard it was to do this due to the horrible amounts of current going through the stock OT and humming like crazy? I'm not sure about 400ma but I bet 300ma would not be far off. Bruce

6/3/1999 6:57 AM

Bruce, 400ma was a guess based on my recent experience with a Matchless Clubman which drew about 85 ma per EL34. I'm guessing that it wasn't quite class A, but close, so true class A would be drawing even more current. Again I'm guessing that the operating point for the 6L6's would be similar to the EL34's. The owner said that he could run either 6L6's or EL34's in his amp, and switching between them required no adjustment. Just plug them in and play....Jerry

6/3/1999 9:46 AM
Bruce	OK...I get it now. 25 vs 30 watt tubes and a different B+ Vdc... I was just thinking a little differently. If the power supply could still hold 400vdc and keep the four 6L6s biased at about 30 watts, it would be about 300ma. Same thing only different. :>) Bruce

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