HELP! MV hose-up

[Fred G.]

I'm trying to put a simple master (pre-PI) MV in my bandmaster.

Here's what I did: 1 meg pot, from 220K junction to middle lug of pot, output to PI input cap from left lug (facing front of amp), grounding from right-most lug of pot (again facing amp from front).

It's not working right (LOL), sure isn't working like an MV.

Shoot me I'm stoopid.

Thanks in advance,

Fred G.

[Enzo]

How about describing it in terms of the schematic rather than left and right in the chassis. I have lots of schematics, but I don't have a Bandmaster handy to look at. ANd then you can tell us whuch Bandmaster you have, the schematics are different.

The old 5E7 has a 220k resistor before what is actually the tube before the PI runing to the grid, but it is the same 12AX7. The very different AA763 and similar have a pair of 220k heading for the grid of the PI through a cap.

You COULD put a volume control after every stage pretty much, so putting one before the PI is no different from putting one after the first stage.

If you have the AA763 type (seems like), the TWO 220k resistors join, mixing the two channels. This point is the preamp output. Disconnect the 500pf cap that feeds the PI grid from that point and connect a wire between the 220k junction and the end of the pot the wiper sits at when the volume is all the way up. Now ground the other end of the pot. The center leg of the pot is the wiper - the variable element - connect that to the end of the 500of cap that was left unattached.

Can't tell you how well it will sound, but that would be a way to wire what you are trying to do. And you could also wire it up guitar style I suppose. Take the way I just described and swap places with the two not grounded legs of the pot, though that seems like maybe what you did.

It might be helpful for you to tells us what is the problem with it. "Not working like an MV" isn't very descriptive.

[Fred G.]

Hi Enzo,

The amp is an AB763. I have hooked it up exactly as you described.

When the pot is all the way off, there is still audible output, just quieter, thin, and trebly. When the pot is rotated to about 2 or 3, BOOM! The output jumps to about full-blast.

I'm using 1/2 of a ganged dual pot, 1 M audio taper. I wonder if the pot is defective or mis-labeled, as it behaved similarly in an experimental X-line PPI master I hooked up a while back...

What do you think? Sound like a defective pot, maybe?

Thanks,

Fred G.

[Enzo]

Don't ask me, you're the one with the pot. Unwire it and get out your ohm meter.