Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bigampman on July 02, 2018, 07:59:44 pm
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Hello everyone, Doug suggested I ask this question here, so here goes. Can Doug’s 50 watt Plexi board be converted to a JCM800 style? Looking at the schematic for the Hot Switch mod it looks like it can be done by leaving out the switch itself. Also I need to add a low input so how would that be done? Thank you in advance for all help and suggestions.
Bigampman
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Looks like you're right about wiring it up like the hot switch mod but without the switch with a couple of components added.
You'll need to add a 470k resistor with 470pf cap in parallel between the .022 cap coming off of V1 pin 1 and the shielded cable going to the left leg of the Volume pot on Hoffman's hot switch layout. You can connect these components directly to the pot leg and then to the cable or add a terminal strip in front of the pot for better support.
You can add a .001uf cap between this pot leg and the wiper, but I've heard it's often too bright for most folks. Try it out and see what you think!
Then connect the shielded cable on V1 pin 7 to the volume pot wiper directly, and jumper from the .022 connected to V1 pin 6 to the 470kohm connected to V2 pin 2. Instead of adding in the 470k resistor to ground from that .022 (like it is in Doug's hot switch layout), you can put it where the empty turret space is and ground the end not connected to V2 pin 2. A 470pf cap can be added across the 470k connecting the .022 and V2 pin 2 as well.
The cathode resistor on V1 pin 8 can be changed to 10k and you'll be right there with JCM800 values.
As for a low input, you can just use a standard fender hi/low input jack scheme as shown in Doug's AB763 files.
Sluckey just posted an awesome Dual Plexi 50 amp that has a 1987 style section and a JCM800 section all in one amp. It'd be worth checking out if only to reference the differences in component values between the two preamps.
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That is very helpful, thank you. I will check out the post you mentioned.
Bigampman
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Reply #1 here: http://el34world.com/Forum/index.php?topic=3489.0