Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: G._Hoffman on June 22, 2025, 03:00:41 pm
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I'm drawing out the Mesa Boogie Mark IIC+ schematic in KiCAD, probably just as a learning exercise, but maybe to make one some day. But there is something which seems odd to me. Some of the volume controls (Lead Master and the overall Master that I've noticed so far), they are drawing the pots as variable resistors, using a fixed resistor to create the voltage divider. See the attached picture - the switch is because this is for the lead channel, and gets switched off.
The only thing I can think of is this is varying the load on the previous gain stage, where wiring a pot as a voltage divider would keep a fixed load on the previous gain stage. Given there are other places they are using a more conventional voltage divider, I am curious what the reasoning behind this would be.
Thinking a bit more, I can see how it would effect the way the lead master works when out of the circuit, but they also implement it like this in areas which aren't switched.
Any way, just something I'm finding curious.
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Is it just me, or is the Reverb control on here grounding out the FX send, when it's on zero? If feels like this needs an extra mix resistor in here somewhere. This is a redrawn version someone did, but I compared it to the Mesa official version, and it's the same. There's gotta be something wrong here, right?Ok, I figured this one out - I found another, better copy of the schematic. It is a voltage divider, wired to have a constant impedance on the EFX send. Like a Jazz bass volume control.