Fender Blues Jnr Mk3 Mod

[blindboybenton]

Hi all

I was given last night by the guitarist of my band his Fender Bj. He wants me to check it over and revalve.
But he has asked me if i can give it some more gain. he say he has the Volume almost flat out and adjusts the master.
Fender BluesJuniorIII_schematic_Rev-D.pdf

Can anyone offer some sound advice ? Anyone done a mod like this The guy really likes the amp and wants just too improve it

Blindboybenton

[Leo_Gnardo]

A quick blink whilst I wait a customer to arrive in 5 minutes -

Looks like putting a bypass cap across the third triode's cathode R17 is a dandy place to whip up a bit more gain. At the same time I'd thin the low end on the first cathode by swapping in say 2.2 uF instead of 47 uF for C1. Too much capacitance here will swamp the low frequencies. On the third, across R17 I'd suggest 4.7 to 10 uF.

[cbarrow7625]

Good suggestions. Additionally, to add more gain earlier on in the circuit you can try removing or increasing the value of R6 (130k) that is placed across the 1M gain pot when it is turned all the way up.If you cut it out altogether you may need to reduce the coupling cap value (C2) to maintain the same low end response. If you reduce the stage 1 cathode cap as Leo suggested, you probably can leave C2 alone.

Question: Does he use it with the Fat switch engaged? He may not because it makes the distortion too muddy because of the large value cathode cap switched in across the Stage 2 cathode. Reducing the cathode cap value to around 2.2-10uF will tighten things up. Smaller values will work better as a gain boost if he does not engage the Fat switch currently.

Looks like a lot of addition gain is there for the taking. Try each change one at a time to judge the effects in isolation. Once you know how each change affects the tone and gain start adding a couple together.

The trick with more gain is avoiding too much flabby low end and avoiding squealing oscillations from too much gain.

[66 Kicks]

Also, you can change R3 to 220K or 330K. The problems associated with larger plate resistors don't apply here because the signal is small. If you do raise R3, you should also delete R6 or change it to 1M. You can adjust the low frequency response later with C1 and/or C2 as cbarrow7625 said.

[blindboybenton]

[QUOTE.

Question: Does he use it with the Fat switch engaged? He may not because it makes the distortion too muddy because of the large value cathode cap switched in across the Stage 2 cathode. Reducing the cathode cap value to around 2.2-10uF will tighten things up. Smaller values will work better as a gain boost if he does not engage the Fat switch currently.
.[/QUOTE]

Many thanks for the info. He don't like the FAT s/w

[blindboybenton]

Many thanks chaps.... There is a few ideas for me to try

[Leo_Gnardo]

He don't like the FAT s/w

Might like it better if you put the fat switch on an instant diet, and swap out its 22 uF cap for say 1 or 2 or 3 uF instead. Even less if he's looking for a boost in mids/highs only.

And there's a whole triode sittin' in there unused if you want to add yet another stage. (Put kookoo-nutty smiley face thing here. isn't enough.)

[66 Kicks]

Or leave the Fat Switch as is and permanently bypass the 1.5K cathode resistor with a 0.47 or 0.68uf cap. Now the switch would actually fatten-up the sound without changing gain. Tweak the two bypass capacitors to get two usable sounds.