MV mod on 74 JMP50?

[hellwood]

OK, maybe you can clue me in. looking at the photo, the 100k resistor on the PCB is connected to the board and lifted on one side and it appears to go to pin 8 of V1 as well as half of the cap which then goes to lug 3 of the MV. Lug 2 of the MV pot then goes to the bottom of that 33k resistor on the PCB. what do you think?

[hellwood]

please disregard my 1st attempts at drawing out the circuit, even though it could still be wrong!

[hellwood]

please disregard the previous attempt of drawing out the circuit. i think this might be how it goes.

[Enzo]

SO what do you want to know?

The signal used to go from the cathode follower cathode to the input of the tone stack. Now we broke that connection and inserted the volume control in that path.

The cap blocks the substantial DC on the cathode resistor from getting to the pot, while passing the signal along to the pot. The pot is a simple variable voltage divider like any channel volume cointrol. The signal goes in the top of the pot, the bottom is grounded and the wiper takes a variable amount of signal out and puts it on the tone stack.

[Enzo]

Is there some particular reason we are having this same discussion in two places at once?

[hellwood]

thanks enzo! im trying to figure out if there is a tonal advantage to this because it would be much easier just to put it between the treble pot and output driver.

[Mars Amp Repair]

Enzo,
Where's the other place? g

[Mars Amp Repair]

found the duplicate post...my suggestion from the other posting of this thread in Mods & tweeks it why not just put the mv pot AFTER the treble control thereby utilizing the already existing coupling cap.
Other than possible loading on the tone stack...just seems simpler. I'm assuming the ususal 1meg mv pot...wouldn't load it much. g

[Mars Amp Repair]

oops, nevermind...I see the had drawn schemo shows a variation with the MV post treble control..g

[Bruce / Mission Amps]

I think he just forgot to strike the lead from the CF to the tone stack because the included photo of the actual mod clearly shows a bigass orange drop cap blocking the cathode from the MV and tone stack.
Note it is connected to the 100K CF resistor.

[dai h.]

http://music-electronics-forum.com/s...21661#poststop