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 Post subject: Biasing instructions for later model Fender Twin Reverb?
PostPosted: Sat Dec 15, 2012 9:31 am 
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Hi all!

I've searched the threads, and google, for anything remotely regarding proper bias instructions for a post '85 Fender Twin Reverb model - But so far no luck.

I understand the late model manual suggests taking your amp to a tech to adjust the bias. But coming from the audiophile community taking your amp to a tech each time you've sampled (rolled) NOS tubes into your 500 watt tube amp is beyond the norm. Audio tube amps possess user accessible bias points handily adjusted with a voltimeter.

I came across a UK forum thread that displayed photographs of a biasing procedure for a previous Fender model that displayed actual bias adjustment points on the rear of the amps.

http://www.amp-fix.com/fender_twin_amp.htm

Obviously, with the design of the newer models those days are long gone.

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I've just taken possession of a used '99 model Fender Twin Reverb. I'm trying to track down a possible source of hum, which of course could come from something as simple as a bad preamp tube. But in this case the tubes, JJ's, had recently been replaced. And more importantly, I'd like to learn the proper procedure for biasing my own amp, which, on the audio side, I've done for decades.

Any thoughts will be most appreciated. Thanks in advance!

2b


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 Post subject: Re: Biasing instructions for later model Fender Twin Reverb?
PostPosted: Sat Dec 15, 2012 12:55 pm 
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Hi 2bornot2bop,

If you have a '65 Reissue Twin Reverb, you have a bias adjust pot, but no test points, so you would need bias probes to measure the Cathode current for each tube. You can measure the Plate voltage at the output transformer primary, or get a bias probe that also measures the Plate voltage as well as Cathode current.

This video, although for a Hot Rod Deluxe, will give you a good idea of the procedure involved.
http://www.eurotubes.com/eurotubes-Hotrod-Deluxe-How-To-Bias-Video-6L6-6V6-millivolts-mA.htm

The basic idea of setting the idle bias wattage, is to measure Plate voltage and Plate current and adjust the pot to get the current that will give you the desired idle wattage. Cathode current is close enough to Plate current, and is the measurement that most probes use, so just substitute Cathode current for Plate current in the calculation. Typically, you want to aim for an idle wattage dissipation of between 50% and 70% of each output tube's max Plate wattage dissipation. For example, a 30 watt 6L6 tube, you would want between 15 watts and 21 watts at idle. Plate volts times Plate current equals Plate wattage dissipated at idle. If you have 440 VDC on the Plate, and you want 15 watts at idle (50%), you would need to adjust the pot to get approx 35 mA Cathode current measured with the bias probe. 15 watts divided by 440 Plate volts equals .034 amp of current (34 mA). After setting the current, recheck the voltage and adjust as needed as the Plate voltage will change slightly with adjustment. After playing the amp for awhile, recheck setting for any drift that might occur.

I use the "Quadstage BiasPro MQ10" that allows measuring all four output tubes' Cathode current at the same time without a separate volt meter. This probe does not measure Plate voltage, you can do that with a volt meter. Other forum members will probably have recommendations of their preferred bias probes.

http://www.asharpfretworks.com/5543.html

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 Post subject: Re: Biasing instructions for later model Fender Twin Reverb?
PostPosted: Sat Dec 15, 2012 1:01 pm 
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I assume you mean the Twin Reverb Reissue and not the Evil Twin. If so, here is the service manual. The TRRI has a bias pot, R71. It does not appear to have a one ohm resistor in the cathode circuits of the power tubes, so a bias probe will need to be used. You implied you know how to bias an amp, so I'll assume you know about measuring cathode current and plate voltage to come up with the power tube idle plate dissipation. Since 6L6GCs are 30 watt tubes, try and set the plate idle dissipation betwen 50% and 70% of 30 watts. Hopefully your tubes are very well matched since there is just a single bias pot to handle all four tubes.

http://ampwares.com/schematics/65_twin_ ... manual.pdf


Go for the 4-tube octal model:

https://taweber.powweb.com/biasrite/br_page.htm

Edit: Beat me by 6 seconds. :lol:

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