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Post subject: Cool Bias
Posted: Fri Oct 10, 2014 7:44 am
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Upon installing Ruby Tubes JJ 6L6GC'S from Dougs, my hand wired blues deville shows plate voltage of about 480, and max adjusted plate current of 36ma.. Can't adjust any higher.
That's a hair over 50% idle bias.
The sound? Sounds decent.....but I've got to replace a damaged speaker before knowing for sure......Can't turn it up w/out speaker cracking up.
I've heard that with other high plate voltage amps, you can throw the "up to 70% idle bias" out the window....to some extent.......and some high voltage Fenders run with cooler biases/higher voltages that max out at around 50% idle adjustment.
Does 51 or 52% idle bias at 482volts plate sound good enough? (i dont have an oscilliscope to check clipping)


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Post subject: Re: Cool Bias
Posted: Fri Oct 10, 2014 1:54 pm
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To me 36 ma look more close to 60 % than 50 %

http://www.webervst.com/tubes1/calcbias.htm

Most if not all Fender amps I work on sound at their best with colder bias; 50 to 60 %


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Post subject: Re: Cool Bias
Posted: Sat Oct 11, 2014 6:40 am
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You're right.....about 58%...(lost my sliderule).
Builder has offered send me two 10k resistors ....instead of original 15k .....to change the bias pot resistors, which will offer increased idle current..... but make things more touchy.....even the chance of redplating certain 6L6's if not careful.
If i were certain the JJ's wouldn't over time drift down .. (colder)....i would leave things as is.


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Post subject: Re: Cool Bias
Posted: Sat Oct 11, 2014 8:34 am
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If you like the sound of the amp leave it like that .


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Post subject: Re: Cool Bias
Posted: Tue Nov 11, 2014 7:12 pm
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You wouldn't gaim much with a higher bias than what you have at present. ..

.. but your tubes will cook, and you don't need that for sure.


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