Plate voltage rising on PR

[disorder]

Hey guys, just finished building a Princeton Reverb clone and having a few biasing issues. It's completely stock, with the only differences to an original blackface PR being the power transformer... I used the Heyboer equivalent of the Deluxe Reverb PT... #TP40D from Allen Amps order page.

Initially after building I was getting 380v on the plates, I had a pair of crappy Russian 6V6's in at the time and I wasn't sure what they were biased at, just put them in to test. Since then I changed one resistor and a cap in the reverb mixing circuit to lessen the reverb amount and added 1ohm resistors from cathode to ground to more easily read the bias. Now I am reading 437v on the plates with 22mA cathode current. I have 119.7VAC from the wall and I am using a NOS GE 5U4GB rect. What would cause this rise? Did I have the bias way off before when I couldn't read it and it pulled the plate voltage down?

And is a 440 plate voltage something to worry about? Can I still get away with using a pair of Tungsol 6V6GTA's at these conditions?

[acorkos]

Now I am reading 437v on the plates with 22mA cathode current. I have 119.7VAC from the wall and I am using a NOS GE 5U4GB rect. What would cause this rise? Did I have the bias way off before when I couldn't read it and it pulled the plate voltage down?

most likely the bias was way off when you measured 380v. But where you have it set now, you are running the power tubes at 80% of their max at idle....14-19mA gets you in the 50-70% range.

the AA1164 schematic shows 410V on the plates

[disorder]

thanks, I figured that might have been the case. Now that I can read the bias I noticed that my "matched" pair from The Tube Store aren't even close to being matched. I'm getting a difference of about 10mA (or 10mV drop across the 1ohm resistors). Could this be an issue in the amp?

[acorkos]

I noticed that my "matched" pair from The Tube Store aren't even close to being matched. I'm getting a difference of about 10mA (or 10mV drop across the 1ohm resistors). Could this be an issue in the amp?

try each tube in each socket and measure voltage. are your resistors *exactly* 1 ohm?

[disorder]

Yeah they are 2watt Dale resistors. 1 ohm at 0.5% tolerance, I measured them and they are identical down to the hundredth.

I can set one tube to 21.5mV drop across the resistor and that leaves the plate voltage at 440V. This works out to be about 9w dissipation which is good.

However at this setting, the other tube is at 31.3mV drop at 437 plate volts. This is roughly 13w dissipation...NOT GOOD.

I flipped the tubes and the readings follow them.

[acorkos]

those tubes aren't matched....about a 50% mismatch, should be 5% or less