Sorry for the stupid question! But to my credit, I was able to find a reference to the other mystery acronym (HRM), so I at least answered one of my questions!
I searched google as well a search on this forum, and I've found lots of posts which use the acronym "PAB", but I cannot for the life of me figure out what it refers to.
Nick is right. What it does is defeat the tonestack and it's resulting losses, giving a boosted signal. I believe Dumble amps have several variants on how this is done.
There are several ways to do it. The easiest (that I know, anyway) is to just hang a switchable resistor on the bottom of the mid cap in a TMB stack. In the attachment I show a 250k trim pot soldered to the lugs of a push/pull mid pot. Season to taste. Completely opening the circuit is a bit overpowering to my ears so I like to be able to fiddle with the boost level.
You could also just use a 250k mid pot with a 10k resistor off lug #1 to ground so all the way off would be normal and increase resistance to ground by turning the knob. sh
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Nick is right. What it does is defeat the tonestack and it's resulting losses, giving a boosted signal. I believe Dumble amps have several variants on how this is done.
I'm pretty sure it's Pre Amp Bypass.
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