Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: roseblood11 on August 20, 2023, 05:27:24 pm
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On my workbench is an early, US made Fender Blues Jr. (green board), which has most of the well-known mods from Bill Machrone. Tone stack and coupling caps have Blackface Princeton/Deluxe values (with mid pot as variable resistor ("twin stack mod"), the first filter cap is doubled, there`s a bias pot, the reverb insertion point has been moved to where it is in all later versions, the amp has got a very nice 6600R prim Heyboer OT with 4 and 8 ohm speaker jacks, and the "clean boost mod" is installed, a foot switchable bypass cap at the reverb recovery stage (similar to the fat boost switch).
And the amp has what Machrone sold as the "high voltage preamp" mod, the preamp plate resistors (R3, R10) are 91k. And somebody added a cathode follower, as shown here:
https://mhuss.com/BluesJr/inputMod.jpg
My questions are:
- Do I have to adjust the 100k cathode resistor at the cathode follower V2A, when the plate resistor of V1A is smaller than that? V2 is a 12DW7 (ECC832), with the 12AU7 side used as cathode follower.
- The owner wants to use the amp at home, as a small practice amp for bass and baritone guitar. He already installed an appropiate speaker, an old Carvin made by Eminence, that they used in their bass combos and stage monitors, really nice and woody sounding. What else could be done to optimize the amp for him? Maybe use a lower gain tube (5751) as V1?
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My questions are:
- Do I have to adjust the 100k cathode resistor at the cathode follower V2A, when the plate resistor of V1A is smaller than that?
no