My questions are: Should I be worried about the (what I consider) the low plate voltage as 460 to 480 is the average ? Should I be concerned about overloading anything running this amp at higher master volume levels at this 40MA Bias with the plate voltage at 393 ? If the plate voltage was 460 and I wanted to run at 60% the Bias current would be 33MA. The amp sounds great but the Bias pot is adjusted to it's minimum. I read another post from here that someone was having similar issues and it was suggested to add (in parallel) a 33K ohm resistor to the existing 56K ohm at R28 and that would bring up the bias current to where it should be. I did the calculation for 393 plate voltage and at 60% the Bias should be 38MA. Anyhow, I read the Bias current and with the POT maxed it was at 16MA. I read some where that the average plate voltage for this amp is 460 to 480. I hooked up the probe to one of the E34L's and took it out of standby and the plate voltage read 393. I bought Matched JJE34L Power tubes and ecc83s (High Gain) preamp tubes. This time I bought the Eurotube probe and meter which meters the Plate voltage and Bias current in real time. ever since the last time I had this done, I noticed a significant drop in Gain and basically thought maybe it was time to retube again. I get it retubed and biased about once every 2 to 3 years. Shame on me, but have been taking my amp to technicians to replace the tubes and Bias it all these years but have never found one that I could trust knew what he was doing. I have an electronic background so I know just enough to be dangerous.
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