rockstah wrote:i was very impressed with it - no Parasitic O and it doesnt even happen when you turn it up either! yes it was ONE - bedroom level.
my "'Larry's PPIMV" rocks!
In fact I've never seen it before, that anybody else modded an amp with a PPIMV with a 250K pot - but after I've noticed, that PPIMV's with 500K or even 1M pots are vulnerable for PO first it's been a pure theoretical construct.
The goal was:
- to lower the sensitivity of the PPIMV, that's no longer so much prone for catching other strayfields around the PI and OT wires
- not to drop below the usual bias feed resistance of 220K, seen from the PI's coupling caps
- to keep the resistance between the PI's coupling caps and the output tube's grids as less as possible, to prevent high losses when the pot is set to low volume settings
- and to prevent the output tubes against possible wiper failures of the pot, what would end up with dead output tubes due to bias disconnection
So the result was this type of PPIMV with a 250K double pot w/ 2.2M resistors soldered between the wipers and the outputs (bias voltage supply) - and NO bias feed resistors remaining on the board.
I've always done it w/o shielded cables w/o any problems, just twist the two wires from the coupling caps to the pot's inputs - they aren't far as sensitive, as the wires from the wipers to the tube's grids are.
But when you've mounted the pot in a speaker jack hole, then you can run the wiper's wires simply THE SHORT WAY to the tube's grids! So also these wires not imperatively have to be shielded - just twist them also and keep them away from the output tube's plate wires and the screen resistors as far as possible...
... and all is working properly. No PO, no fizz, just sound
Larry
well the things we try to type sometimes come out funny in text. 