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Old 29th June 2007, 05:50 AM   #1
spuloo is offline spuloo  United States
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Join Date: Jun 2007
Default Splitting up the Power Amp

So, I just bought a used B-52 AT-212. The tubes in it are as follows: Power Amp - 4 x 6L6 / 5881, Preamp - 4 x 12AX7, Rectifier - 1 x 5AR4, and the Reverb, Effects Loop, and Phase Inverter tubes are all 12AX7s. Anyway, I know I'll probably have to change the tubes soon, and as a student of electrical engineering and repair, I was thinking about doing it myself. I already know how to bias tube transistors with given voltage specs, but this impending project made me curious as to the possibilities. First off, the amp is extremely versatile, offering a crystal clean channel, two gain channels (that share eq), class switchable rectifier (class a, class ab, and solid state) independent volumes for all three channels, master vol, assignable contour, 100w, foot switchable reverb and effects loop all for under 700 new. (just wanted to be thorough)

Anyway, I've been reading up on the differences in the sound of power amp tubes, particularly 6L6s vs EL34s. I've also been reading about the sweet tone created by overdriving the power amp tubes and that it is easier attained at a lessened volume using a lower voltage.

Now my question: Would it be possible to split the power amp section and re-bias one half to put in EL34s and then provide separate outputs for use with two separate cabs? Now I don't currently have the schematics, which if this is just a horrible dumb idea, that's why. I'm confident I'll be able to get them from customer service though.

I'm would also like to take recommendations as to what brand of tubes to install for whichever channel/section. I like my cleans clean, and my dirtys dirty. I play anything from indie emoish rock to hardcore, southernish metal (think southern rock then push it into metal).

Thanks,
Josh
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Old 30th June 2007, 03:26 PM   #2
spuloo is offline spuloo  United States
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Join Date: Jun 2007
I've been looking around and found this:
http://www.schematicheaven.com/mods/6l6toel34mod.htm
That should get me through the mods for putting in the EL34s, but I'm still fuzzy as to how to actually split the power amp. I can't find anything on how the tubes are generally arranged in a four tube 100w power section (series, parallel, inputs and outputs and such). Would I have to get two 50w output transformers in place of the 100w one? I know the circuit mod is possible since the mesa boogie road king features a switching power amp between 6L6s and EL34s.
Thanks again,
Josh
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