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Proper Bias In mA for a 65 PRRI

Discussion in 'Amp Tech Center' started by DennisM, Feb 18, 2017.

  1. DennisM

    DennisM Tele-Meister

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    I have a PRRI, Bordeaux with the 12" P12R Jensen. In milliamps, what would you suggest for a bias current? Not too hot, not too cold. I just noodle with it clean, so I'm not looking for any kind of breakup. When I got it, it was at 28mA. I boosted it to about 31mA, but I don't want to fry the 6v6's. 31 too much?
     
  2. slider313

    slider313 Tele-Afflicted Silver Supporter

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    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.
     
  3. jazzguitar

    jazzguitar Tele-Afflicted

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    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.
     
  4. tecelaster

    tecelaster Tele-Meister

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    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 :)
     
  5. DennisM

    DennisM Tele-Meister

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    Great info fellas. While I was waiting for responses here I went in and brought it down to between 24 and 25 mA. Sounds much better! It was kinda harsh at 31mA. After reading these responses, I'm gonna go back in and bring it down to 22mA and see what she sounds like. I was just too lazy to tear the chassis out and check plate V etc. Certainly sounds MUCH sweeter to my ear with the bias down. The stock Groove Tube 6v6's are pretty well matched, Thanks guys!
     
  6. koko1

    koko1 TDPRI Member

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    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.
     
    Last edited: Mar 23, 2017
  7. swaptronics

    swaptronics TDPRI Member

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    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.
     
  8. DennisM

    DennisM Tele-Meister

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    I mistakenly said my speaker was a P12R. It's actually a P12Q...like yours :) I currently am running 23 mA with the plates around 460v. This gives me about 10.5 watts dissipation. I'm comfortable with that and it sounds great there. When I had it running 19/20mA the plates were at 470 and the PD was about 9.5. I might run it back down to 20mA an have a listen. Yeah, I just built a 5e3 and the trannies dwarf the PRRI ones. I was surprised too.
     
  9. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    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.
     
  10. vanr

    vanr Tele-Afflicted

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