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blackarab 4th November 2012 11:49 AM

Hi everyone looking for some help in adding adjustable bias to this amp , as i would prefer to have a choice of brand tubes that can be used , any help with this will be greatly appreciated ,many thanks Black arab.

kevinkr 5th November 2012 12:03 AM

You'd need to at minimum give us a (partial) schematic of the amplifier circuit. A hand drawn schematic of the bias circuitry as it currently exists would be sufficient.

:cop: **Please do not link to unauthorized online repositories of copyrighted material.**

blackarab 7th November 2012 03:48 PM

Hi Kevinkr i enclose a schematic of the circuit for you to look at many thanks Black arab.

costis_n 7th November 2012 04:09 PM

You mean tube types, right? Because if it was designed for STR brand 6l6GC's, it will work fine with any 6L6 gc or other similar variants. @Special Biasing" is 90% guitar tech myth.

Consider this. If the designer thought bias is an important enough issue, why did s/he not provide any means of adjusting it individually per tube?

blackarab 14th November 2012 06:33 PM

what would be suitable trim pot value and resistor value to exchange for the fixed resistor ,many thanks black arab.

blackarab 14th November 2012 06:34 PM

thanks Costis_n for your advice.

Chris Hornbeck 14th November 2012 08:26 PM

You'd probably have a better response to this question in the musical instrument amp section, but, in general, maybe choose a pot and a fixed resistor that together add up to the same as the sum of the two un-labeled resistors in the bias circuit. Make the pot about half of the total resistance and put it on "top". Add a small resistor in the output valves' cathodes to ground as a current sample point.

All good fortune,

DavesNotHere 15th November 2012 02:14 PM

well the philosophy of the method the amp bias was engineer is this:

the bias is set up for the first 40-60% of the tube life. this allows tube swapping at the musicians level without worrying about bias. And to change the tube before a failure arises.

The spec on the output tubes is matching Rp. JJ Tesla matches them for them at the factory and labels them Mesa Boogie for the OEM solution. The phase splitter tube is of course matching in gain as well as rp.

so why change it? 98% of the sound is before the output stage.

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