Proper Bias In mA for a 65 PRRI

That would be a bit too high, in a fixed bias amp, as far as I'm concerned. Do you know what your plate voltage is? Let's say the transformers used were correct for today's voltages and somewhat match the AA1164 schematic; 410v plate. 65% of a 12 watt 6V6GT would be 19mv; 70% 20mv. A 6V6GTA is a 14 watt tube and 65% would be 22mv; 70% about 24mv.

 

Following the 70% role, at 14W per plate max dissipation and 410V plate supply, 24mA would be the maximum safe value.

I disregard the 70% rule as urban myth (there is some truth to it but it's a safe limit, not a target) my own Princeton (a '66) runs beautifully at 15mA. And boy does it sing the song!

I would even consider 28mA too hot. Do you really prefer the sound now ? If you want clean, go lower. Besides, the vibrato won't work well if the am runs too hot!

Also, remember the Princeton uses the same power transformer as the Champ, so this is already close to overload. I am not sure whether this also applies to the reissues.

 

Synchronicity! I just biased my PRRI 65 yesterday after installing a new set of power tubes. Plate current is about 19mA (cathode current at 24mA). When I bought the amp I could tell that it was souped up from the factory, so I did a 'by ear' adjustment last year to pull it back a little, but with the rather expensive Tung Sols in it now I decided it do it properly with a DMM at the test points. Although I'd estimate that the overall relative gain has dropped a bit, it does sound sweeter and the vibrato has more play with the intensity.
I still can't get above 4 in the apartment

 

On my PRRI I fried a JJ 6V6 at 28 mA, but it sounded very good for a while .. I assume, tube values vary within production.

 

I have the same amp. It Has Groove Tubes And Was Factory Set To 23 mA. I Lowered It to 20 Where it Sounds Better. It Is Important To Measure Plate Voltage To Calculate Range For Current. A Higher Plate Voltage (pin 3 Of 6v6gt Tube) Calls For A Lower Idle Current And Vice Versa. I Modified Mine With a Switch And Two 3-ohm Resistors To Drop Heater Voltage From 6v To 5v So NOS 5V6GT Can Be Used . These Tubes In Blackplate Can Be Had For $5 A Piece And Sound Great when Properly Biased . I Was Surprised To See Such A Tiny Power And output transfer in this amp.

 

My Princetons always end up in 17-19 mA range. They sound good and don't run out of swing room when the vibrato is on. You get too hot on the bias in those circuits and it makes the vibrato weak sounding ,and that bias vary vibrato is the heart of those amps to me.

 

I have to say I usually bias around 60-65% myself. Just like being conservative with New Production tubes.