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Post subject: Pro Junior III Bias
Posted: Fri Nov 21, 2014 6:36 pm
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Joined: Fri Sep 26, 2014 8:49 pm
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Hi, if I change any of the power tubes in a Pro Junior III...

1. Do I need to do some biasing if I get the same brand and model of stock tubes (Groove tubes)?
2. Do I need to replace the power tubes as a pair?

Thanks!!!


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Post subject: Re: Pro Junior III Bias
Posted: Sat Nov 22, 2014 10:23 am
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Joined: Tue Jun 29, 2010 1:20 pm
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Location: Indiana
It is always a good idea to check the bias when changing output tubes in a fixed bias amp, and yes you should change output tubes as a matched pair typically. The idea with using GT is that if you use the exact same output tubes, grade and type, and your bias is good with the existing set, you won't have to adjust the bias. However, checking the bias is the only sure way to know if they are good tubes and matched. With amps like the PJr and BJr and other similar amps, there is no bias adjustment pot. The bias can be adjusted by changing the value of the bias resistor, R30 in the case of the PJr, or by adding a bias adjust pot if you have the skills to do so.

Generally speaking, most PJr and BJr owners change output tubes as you've described, and check that the tubes do not red-plate, with no further worries.

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