20to20 wrote:Changing the 10R bias resistors to 20R won't provide any additional range. They are only there to provide a small test point voltage drop to read, not create a biasing voltage at the cathode. Instead of changing the biasing pots you could add 1K, 2K, 5K on up a little at a time in series to the 10K R to ground coming off the pots until you hit the new biasing range you want. Remember you must have a higher (-) negative biasing voltage to tame the current. So don't put the new resistors in on the wrong side of the pot or you will get the opposite effect.
This is assuming you have the stock 2 pot circuit, still.
kingmatt wrote:Thanks everyone for the replies. I changed out the bias resistors as directed and have tons more negative voltage to control bias now. After going through a few questionable NOS tubes, I found a set of Sylvania 6BG6GAs that don't redplate like the Raytheons I had did. I'm running them on the cool side (14w dissipation per tube...any more and I get a very minor streak of red on one of the tubes) and am not totally convinced that these tubes are the way to go. They are approximately as pleasing to the ears as the 6CA7s I had in there, but they lack the overall power and bass.
In any event I've learned a ton about how this part of the amp works and thank everyone who replied for their time. All is well for now until I screw something else up!
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