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Post subject: Step-by-step guide on biasing a '65 Princeton Reverb Reissue
Posted: Sat Oct 02, 2010 10:36 am
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Does anyone know where I can find a step-by-step guide on biasing a '65 Princeton Reverb Reissue? I found one for my SCXD, but also need one for my PRRI. Thanks!


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Post subject: Bias a PRRI
Posted: Sat Oct 02, 2010 11:47 am
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Hi tom310,

I'll give it a shot, I've done it both ways for my amps. I don't have a PRRI though. :(

With the amp on and warmed up, turn the "speed" and "intensity" and "volumes" to 1 (all the way down) and back cover removed, find TP 16 or TP 17 on the schematic board pictorial, and locate on your amp. Using a Digital Volt Meter (DVM), scale on DC Volts, put black lead on the chassis (ground) and put the red lead on either TP 16 or TP 17 and read the millivolts (mV). This mV reading can be directly translated to milliamps (mA), because you are reading the Volts across a 1 ohm resistor, V/R = I (current). So if you read 0.023 volts (23 mV), that is 23 mA of current flowing through the Cathode of the tube. The Cathode current is typically a few mA higher than the plate current (about 5 mA higher). For a 6V6 at 440 plate volts, recommended bias is about 17 mA of plate current for an AB amp. The reading of 23 mV at either TP would be about 18 mA of plate current (close enough). Adjust the bias pot (R22 on chassis) to achieve the desired reading at one of the TPs. Of course if you like the bias higher or lower just change the adjustment (I wouldn't go lower, maybe a little higher though). You may find an imbalance between the to TPs, but a couple of mA difference is OK.

Alternately, with the amp off, unplugged from electricity and caps discharged, using the ohms scale on the meter, measure the resistance of half of the output transformer (P13 to P12, or P13 to P14) and record the ohms reading. Then turn on the amp and warm it up, with controls set the same as above. Using the DC volts scale, measure the voltage across the half of the transformer that you recorded the ohms value for and record. Now divide the voltage measured by the resistance measured and that is your plate current to that tube. Example, if you measure 2.5 volts and measure 100 ohms that is 2.5/100 = 25 mA of plate current. Adjust R22 and measure and calculate again. No need to remeasure the ohms of the transformer, it won't change, but the voltage across the transformer will change when R22 is adjusted.

Of course if you use a bias probe, with the amp off an unplugged from electricity, just plug it in, turn on the amp and let warm up and read the cathode current. Keep in mind that the Cathode current is higher than the Plate current, and the Plate current and voltage are what matters (plate dissipation in watts). Tubes are rated in max Plate dissipation in watts, NOT Cathode dissipation. The 6V6 is typically a 14 watt tube, so the Plate current multiplied by the Plate voltage equals the plate wattage. With the recommended plate idle current of 17 mA, and at 440 plate volts, the 6V6 will be idling at about 7.5 watts, which is a little over 50 percent of the max. The B+ voltage measurement minus the voltage measured across the transformer equals the plate voltage. :)

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Posted: Sat Oct 02, 2010 1:21 pm
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+1 Shimmilou,

Tom310, there is also indications at the bottom of the schematic http://support.fender.com/schematics/gu ... Reverb.pdf

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Post subject: 65 PRRI bias
Posted: Sat Oct 02, 2010 2:10 pm
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Thanks Tissan, that is the same schematic that I was looking at. 8) I like that the PRRI has a separate 1 ohm resistor for each tube Cathode, so that you can see any imbalance between the output tubes while using the Cathode measurement method. I usually use the second method so I can see any imbalance when there is only one Cathode resistor. How hot would you run the 6V6 in the PRRI? 70% would be just under 10 watts idle, about 22mA plate current, or about 27 mV at test point. Would that be too hot? :)

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Posted: Sat Oct 02, 2010 6:29 pm
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Very good tutorial, Shimmilou ! If I could interject one observation, the PR needs the bias to be a little warm, for the vib to work at a sweet level. Not being a big fan of the 70% school, I'd start at 50% and adjust up from there until it suits your fancy. Art

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Posted: Sun Oct 03, 2010 10:57 am
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Thanks to all who responded. The information you provided is very helpful. Looks like the method is very similar to how I bias my SCXD. Thanks again!


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Posted: Mon Oct 04, 2010 8:02 am
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Go to www.eurotubes.com & check out his video biasing your amp. Very easy on this amp, did mine yesterday.


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Post subject: PRRI biasing
Posted: Wed Oct 06, 2010 9:04 am
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Hi dtjacob,

Those are nice videos, very helpful. But tom310 has a PRRI, with different test points, and different readings than the ones in the videos. The PRRI readings will be different, because it has two test points. With only one test point, you have to divide the mV reading between two output tubes. :idea:

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Posted: Wed Oct 06, 2010 4:36 pm
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Hi:

The PRRI has a bias pot near the PT on the bottom of the chassis. Used a dual bias probe, very simple job.


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Post subject: Re: Step-by-step guide on biasing a '65 Princeton Reverb Rei
Posted: Wed Oct 17, 2012 6:38 am
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I recently replaced the original power tunes with JJs, biased my '65 Princeton Reverb and it sounds unbelievable. I play a Gibson ES-175 through it and the sound is just perfect with this slight adjustment.


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Post subject: Re: Step-by-step guide on biasing a '65 Princeton Reverb Rei
Posted: Wed Oct 17, 2012 7:32 am
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Did mine 2 years ago (see previous post) - still sounds GREAT!!!!


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