PPIMV question

[dude]

I install a PPIMV in a fixed biased 6v6 Plexi, Hoffman and Sluckey schematic. A version of the La Mar. I removed the 220k grids on the board and added a dual gang 250k pot, the pot is well balanced, (225k/222k) and works ok but movement needs to be around halfway down to really notice a cut in volume.
My question is Metro Amps, where l got the schematic says, pot is “not grounded” and they use a 500k dual pot. Well, just adding the pot to the chassis it’s grounded. The ground connections on my pot to bias are not in anyway grounded by the pod to chassis but to bias point. My pot is grounded though to chassis. Am l missing something here by grounding the pot to the chassis? Also, the PPIMV needs to be at least half way down to notice a cut and another quarter way down to get anywhere near bedroom volumes. The Metro schematic called for not removing the 220k grids on the board. I figured leaving them is why Metro used a 500k pot, as being in parallel would be close to 220k. My calculations on leaving the 220k grids with the 250k pot is about 150k wide open, hence l removed them and used a 250k pot. I did not use the 2.2meg safety R’s.  Mainly asking about the pot being grounded to the chassis...? And question the PPIMV having to be turn way down to get any noticeable cut. Any info appreciated.

[Latole]

That is what you do ?

https://robrobinette.com/5F6A_Modifications.htm#Frondelli_Master_Volume

[sluckey]

It's fine for the pot body to be in contact with the chassis. But none of the six solder lugs can be connected to ground in a fixed bias amp when the bias voltage is connected to the pot.

Now if you have a cathode biased amp two of the pot lugs would be connected to ground. Or if you use a second pair of coupling caps to block/isolate the pots from the bias voltage then you would connect two of the pot lugs to ground.

[dude]

Well, l solved my issue of the PPIMV not cutting the vol till almost fully CCW. I switch the coupling caps to the end tab and tube grids to the wiper, fixed the issue. Now slightly turning starts to cut the volume immediately, seems to work very well. I had the wirings as in the Robrobinette PPIMV above but had the coupling cap and tube grid connections switched. I assume l need to use 1.2 megs across the bias and wiper for safety in case the wiper fails.
Wondering why this solved my issue....

[Big_Mike]

Are your leak resistors on your 6v6 plexi 250K?  I know on my Hoffman 6v6 plexi they are 100K.  Not sure if that makes a difference as to the resistance you want on the dual gang pot.

I know Metro uses a dual 500K pot on his PPIMV for a regular plexi.

[dude]

I used a well balanced 250k DP, removed the 100k, or 220k grid leaks (not much difference in this amp what’s there), l used 2m2 safety R’s, which makes the grid leaks about 200k (the dual pot is now 200k). You can do it your way too.I’ve noticed that with PPIMV down, pre-MV up, lot of distortion, my tone stack seems a little inactive, l believe this has to do with negative feedback, takes the tone stack out as you cut the PPIMV...? Is this true?And would something like this help

[dude]

I couldn’t get the schematic to post but was a  variable Neg feedback loop with a 100k pot between the NF resistor and spk tip. Also possibly a frequency cap going from spk tip to pot, wired like a VR. Could use say .01 or .001. Would this help the tone stack be more responsive? Only have the issue with PPIMV turned down at least halfway. Seems to be a lack of high frequencies.

[eurekaiv]

I'm going to risk trying to explain how I imagine the PPIMV doesn't make a huge difference in volume outside of the very low end of the control. I might be wrong and if so, smarter people here will hopefully correct me and/or elaborate. But basically, let's say the volume is all the way up on the pre and volume is all the way up on MV. You're driving the full volume through the amp and it's likely the preamp is now distorting the power section. As you start turning down the MV the same full volume signal from the pre gets attenuated before the power section and as you continue to attenuate the signal at this position, it will eventually stop being clipped. I would not expect any overall volume reduction until the MV has been dialed back to the point where power amp clipping is no longer occurring. Continuing to dial back the PPIMV from this point should then start reducing volume noticeably. Also, depending on the amplifier design, I suspect this could be at a wide range of places along taper of the potentiometer.

FWIW, I like this dual pot PPIMV design the most of the few styles I've tried. Also FWIW, I like PPIMV master volumes in general as they allow one some control over the sweet spot of the power section and the preamp separately. What they're not necessarily great for is taking a plexi and reducing it to bedroom levels, although, I find a great sounding amp with a PPIMV can still sound a lot better than a distortion box through a clean amp at bedroom levels, if higher gain is the sort of sound one's after.

[dude]

If l had no pre-amp MV and only the PPIMV, then your scenario says that the PPIMV doesn’t come into effect till half way down. Or on the other hand, the two other volumes turned down to just before the onset of distortion and no pre master, then the PPIMV should start to cut right away.., ??
My PPIMV does not have this issue since l swapped the coupling caps from the PPIMV wiper with the grids to power tubes. After the switch, a slight turn of the PPIMV starts to cut the volume. Fixed that issue.
I did add 2.2M safety R’s after the switch. I’m not into bedroom levels, just want a little more crunch from power tubes rather than preamp. Turning PPIMV down a third, pre master at half plus, other two vol at 3/4,  l get grainy distortion and seem to lose some high frequencies. I’m thinking Negative Feedback has something to do with this not so nice distortion, maybe l should just disconnect NF..?
B+ is about 403v, normal Plexi rail R’s. Using an 18 watt OT tranny.
Was just looking for suggestions to get the PPIMV distortion sounding better. Several people have build this 6v6 Plexi with PPIMV and rave about the tone..
I apologize for beating a dead horse about this issue.

[dude]

After changing caps, fooling with this 6v6  amp. I played another l have, no issues, everything worked as it should, nice break up, PPIMV works well. But this one had a harsh break-up when PPIMV was lowered. l pulled the preamp tubes one by one, not them but a different set of 6v6s and bingo harsh distortion gone. I’ve had this happen with crummy speakers too. I have the habit of buying various used US made power tubes, matching them by current draw in an amp. Well, their used and l used a worn set or one was beat, live and learn. Could be your tubes especially if you use used tubes off eBay....

[pdf64]

...I’ve noticed that with PPIMV down, pre-MV up, lot of distortion, my tone stack seems a little inactive, l believe this has to do with negative feedback, takes the tone stack out as you cut the PPIMV...?

The tonestack's response per se is little affected by a master vol's setting. However, if the signal after the tonestack is heavily clipped, the effect of the clipping will be to reduce the apparent effectiveness of the controls, as all the signal gets compressed back down to a uniform level. Hence the graphic EQ of the Boogie MK series amps being pretty much an essential option, as it's after the clipping stages, whereas their regular tone stack is early in their signal path, after the input stage, same as a BF Fender.

The presence control is a different matter though; it's an 'active' control, relies on negative feedback for its effect. For negative feedback to operate, there must be sufficient gain within the loop. However, fitting a gain control between the phase splitter and the power valves (as per the LarMar) will inevitably reduce that gain. Hence as its setting is reduced from max, the effectiveness of the presence control (and resonance etc) will rapidly reduce.

[dude]

Yes, pdf64, l get carried away looking to get something out of an amp circuit that can’t be found. You can’t turn up the preMV, cut the PPIMV and expect to get Marshall crunch in this circuit. Slamming the PI, then cutting  the PPIMV isn’t going to give a good distortion plus don’t expect the tone stack to as responsive at the his setting. The new 6v6s, played well but l didn’t slam the PI either. Sorry, for the crazy thread.