Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Effects => Topic started by: MakerDP on June 21, 2017, 01:25:03 pm
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I know there are a couple ways to do this but I have a specific question on how to add one to an existing circuit...
BACKGROUND:
I just bought a Fender Champ25 amp for only $80 and I think I want to gut it, basically reuse the tube output section more-or-less-as-is and build a solid state two-channel preamp into it. The Champ25 is a solid-state preamp/tube output hybrid design and the preamp section is pretty awful.
I am considering the Boss FA-1 Preamp as the "clean channel" input and possibly as the "lead channel's" EQ section, post any overdrive circuit I end up using, then possibly a BBE Sonic Stomp circuit at the very end of everything before it hits the tube output section.
I want to add a mid control to the tonestack of the FA-1, which is of the James variety. Here is the circuit I will be working with... http://www.8bitsindgenug.net/BossFA_1.png (http://www.8bitsindgenug.net/BossFA_1.png)
I found a simple implementation of a mid control added to a James stack in the Carvin Nomad Bel Air here... http://www.carvinmuseum.com/pdf/amps...r%20REV-E1.pdf (http://www.carvinmuseum.com/pdf/amps...r%20REV-E1.pdf)
QUESTION:
In the Carvin, the wiper of the mid control is going to ground, but the resistor I would replace with the pot (R9 from FA-1 schematic) looks to me like it's also part of the feedback loop of the output buffer stage (please correct me if that is not what's actually going on.) Would I replace R9 with my pot, connecting pin 1 to the VR2/C7 junction, the wiper to the IC2pin1/C9 junction and leave pin 3 not connected? Or is there another way this should be done?
(I used the Duncan Tone Stack Calculator to determine a 50kB pot would work well here.)
Thanks for looking and please resist the urge to post "why in the heck would you want to do that." This is going to be a "tinkerer's project" platform. I am actually using stomp-box PCBs from Aion Electronics for the preamp sections... they are fairly mod-friendly, easy to swap in and out if I don't like the results and take up very very little space. And yes, I know I could just add a filament transformer to just turn it into an all-tube design. Sorry... just frustrated by all the unhelpful responses I got from another forum with this question :laugh: I know there are some solid-state gurus here.
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I couldn't get your carvin museum link to work.
My question, why hasn't someone asked this question before. It makes sense.
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The Boss has an "active" Baxandall tone stack, hence the feedback loop. The Carvin has a simple "passive" James tone stack. They appear similar at quick glance but are really quite different.