Trying to add a PPIMV to Vox AC30

[ninjadave7]

Hey all,
I'm trying to add a master volume to a Heritage AC30 and I'm having a little trouble. Here's the story:

I added the MV from the DC30 schematic, wiring the pot from the same 2 turrets that the cut control comes off of. i put 10k resistors in line to isolate it (per instructions I found elsewhere here) double-checked the schematic and my pot wiring, and as far as I can tell everything was good.

Problems: at first the pot didn't do anything really. it should have shunted signal to nothing (one side of pot was empty) but didn't. I removed the 10k's and then it worked, but 0-100% volume took place in the first 20% of the pot. Also, the cut control stopped working.

Obviously there is interaction between them that isn't good, and the "isolating" resistors didn't seem to do anything. What could be going on?

[jrfrond]

The "proper" way to do this, and still have function of the Brilliance control is to use a dual 250K audio taper pot. The resistive element (outer legs) of the pots replace the 220K grid-leak resistors off the grids of the power tubes, and the wipers (center legs) of the pots are from the outputs of the PI coupling caps.

If you try to use a single-pot PPIMV configuration, the Brilliance control will be inactive when the master is turned down due to loading by the MV pot. The dual pot configuration I describe maintains a constant load on the side of the Briiliance pots, just as fixed resistors would, because you pot elements are, in fact, acting as fixed resistors in the same application.