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Note: the pic above is from the green board BJr. Rev B schematic.
For those who'd like their BJr amp to be -
1) loud enough to compete with a DRRI
2) get thee *righteous* overdrive tones and rich harmonics like a boutique amp
3) and have a somewhat more BF vibe
This mod is intended for someone who's done most of the popular Billm mods. So far, you have done Tonestack, Twinstack, Cooler Bias, Stiffened Power Supply, Presence, Clean Boost, Preamp Boost (300 to 320 volts supply to V1A plate resistor) and optionally, a better output transformer (recommended).
DISCUSSION: The stage that gets the most overdrive in a BJr is V1A. Since you have installed the Clean Boost and Preamp Boost, this stage is now the bottleneck to getting more volume to the Phase Inverter and on to the Power Section.
First goal of the mod is to get enough juice to the tube to manipulate the gain and bias point. The Preamp Boost mod does just that.
Next order of business with this mod is to get this stage to open up to take a lot more input voltage without going into cutoff. That buys dynamic headroom. Do that by changing the bias (cathode) resistor on the tube. This new bias point creates lots of 2nd order harmonics like the hi-gain boutique amps do.
The BJr doesn't need all the gain in the low mids (the source of muddy-boxy). Let's pass those cool new harmonics on to the speaker better by taking the opportunity to tune the voicing to deliver a gain boost to those nice high harmonics right where the speaker's treble performance is usually starting to wuss out. The solution here, is to downsize the bypass cap.
When this mod is done, you'll have enough preamp signal to get the Phase Inverter into overdrive. You'll have more, smaller overdrive situations, which equals better tone !! The bass and mids will get a little compression from the now more self-biasing nature of the stage, a la Marshall. The new overdrive sound from the stage will feature more saturation, which is far richer in the musical (even order) harmonics than the cutoff that was featured, before. She'll sing when you push her !!
PROCEDURE: Properly discharge filter caps, and remove PCB from chassis. By now you know how to do this. If not, refer to Billm's excellent Blues Junior Modification pages at http://home.comcast.net/~machrone/bluesjunior.htm Confirm your Pre Amp Boost Mod. This Voicing Mod needs a bit more preamp plate voltage for the new bias point to work its best. If your plate resistor is still 100Kohm, you've not done the Pre Amp Boost mod yet. It'll have to be changed to 82Kohm.
For green board - Change R10 from 100Kohm 1/2 watt to 82Kohm 1/2 watt. Change R11 from 1.5Kohm to 2.0Kohm. Change 22uF @ 25V electrolytic cap at C4 to .022uF @ 400V with the tone cap of your choice.
For cream board - Change R8 from 100Kohm 1/2 watt to 82Kohm 1/2 watt. Change R10 from 1.5Kohm to 2.0Kohm. Change 22uF @ 25V electrolytic cap at C4 to .022uF @ 400V with the tone cap of your choice. This style of the mod defeats the Fat switch, which now should remain engaged for bypass cap C4 to work. FAT has become a "bright switch".
Alternately, on the Cream board, you can solder the new .022uF cap in parallel with the new R10 leaving the FAT circuit cap at C4 intact. Done thusly, the FAT engages as usual the cap at C4 additively with the one you just installed.
FURTHER TWEAKING: If you were using a 12AY7 (44%) in V1 to calm things down before, now try a 5751 (70%). If using a 5751 before now try a 12AX7 (100%). I like to shoot for a strat on 7 to be passing from clean to hair at about 7 to 8 on the Volume control. A guitar with P90's or stout humbuckers might only need 4 to 5.
[YMMV and Fritz_D_Cat or any of his likenesses is not responsible for any damage or injury because you messed with a perfectly good amp]