Frontelli PPIMV

[dude]

Install the Frontelli PPIMV, as Rob Robrobinette say's it's his favorite PPIMV. He shows this mod on a Bassman basically the same as my 6V6 PLexi. Works great except you have to disconnected the NFB. Squeal, loud hum, all kinds of crazy noise unless the Presence knob is halfway, 12 o'clock, turning up the MV's knob pass 5 or 6 the loud squeal or turning the presence knob anyway causes it too, totally unusable. I read something on another forum that rob said if the mid ground is on the same tab as the presence you have to ground the presence alone. I tried that, no go. I cut the feed Resistor on the NFB, works great...? My presence pot is not getting it's ground from the mid but I have a floating buss bar the length of the bd. getting it's ground at the input only. So, I tried lifting the presence ground and running it to the input on a separate wire, might be worst...? I still have the .1 cap on presence grounded to that bar which picks up the mid ground in a few inches, any issue here?
This mod doesn't work in my amp unless I scrape the Presence, don't really want to do that. Any, suggestions? I have the NFB 47k R on the 4 ohm tap, tried the 16, probably worst. Disconnect FB loop, no issues but no Presence or NFB, not a Plexi anymore.


One other note, I didn't use shielded wire but with Presence disconnected no issues and the wires are running high over the chassis


Thanks as always

[sluckey]

Try swapping the OT plate resistors. Any better?

[dude]

Ha, I fixed this issue before I read your post. I searched with Google, found an old EL34 forum post with some guy with noise issues adding two PPIMV's. Several answered then you posted this:


"What exactly did you do to the presence control circuit? It's in the NFB loop. Also, when you added the second MV, did you possibly reverse the two signal leads between the PI plates and the PA grids? If so, the NFB loop will now be the wrong phase and cause possible squealing."


Exactly what my issue seemed to be, I didn't change the Phase by changing the OT plates but changed the grid wires on the PPIMV, as the amp played great before adding the PPIMV. And problem fixed. Kind of glad I didn't read your reply, the search and more reading was a beneficial learning.

Thanks

[dude]

After playing for a bit, in this amp I'm not completely impressed with the Frontelli PPIMV, (but does not effect presence, works well up or down and brightness is not really cut that much to not be overcome with the tone stack).  For one thing, the cut on the this MV doesn't do much till it's down to 2 or 3, sounds the same to me from 10 to 4 on the knob, not much range but seems to work.  Just turning the MV volume when it's that low to work, you can't turn more then a hair as all range is 1 to 3 on the knob, I find that strange, but it does work.  I used a 220k dual pot in place of the board grids (removed), the bias is not on the wiper, no safety resistors needed. The gang pot is well balanced using a R on one pot, (223k and 225k). I am using 6V6s, I doubt a 100k dual gang would be much difference, didn't need shielded wire.


Now I'm thinking that these various MV's are only good on a 100 watt amps, not a 22 watt amp. Not a bedroom player, and we have pedals to get distortion. The MV on my Allen Old Flame is very different and works fantastic, a 40 watt amp. Uses a dual 1MA pot, four .1 caps, keeps the 220k grids. The Old flame, (SR) master starts working at 9 and gradually cuts the volume each number, this Fontelli doesn't start cutting till it's down to 3 then cuts very fast, off at 1.
Normal...?

[kagliostro]

It is strange that a PPIMV that isn't so good on paper

(the way the pot are inserted to the circuit, with the wiper connected to the PI, is prone to give an excess of load to the PI instead of the standard connection with the wiper connected to the grid of the power tubes, where the load to the PI is constant)

is claimed to have so good performance




Franco

[shooter]

never done one, but have changed the grid R's LOTS.  there seems to be a "medium range" sweet spot, tube dependent also, messing with a KT88's grid leak can take the "heart" out of the tube.  An EL84 seems way happier over a bigger range. 

I would think the PI type is a big factor, from my last build, you really don't want a cathodyne to distort, so a MV in front or after would do the same thing to the signal
Reading Merlin, he says even a long tail can be as ugly as the cathodyne.

[dude]

Right shooter!


The more I play this amp with the PPIMV Fronetti mod, the more I kike it. I said I didn't get much change till down to 3, but I was wrong, there subtle differences. I also have the pre-MV in this amp so depending on the amount of signal I get depends on the setting, mixing is fun. More I play the better I like it, but at a gig or jam I'd have it wide open.

[sds1]

My recent test-drive of Frondelli MV matches your experience. Very transparent, a very tone-friendly MV but seems to do nothing until way low on the dial as you say. But if you look on the scope the output waveform as you dial down the MV you'll see the signal reducing proportionally and as expected. So it's a ear hole thing then I reckon?

[shooter]

But if you look on the scope

I've looked at the harmonic content also, so ya, an ear hole thing, or mind over matter 

[dude]

Finding the Frondelli PPIMV with a pre-PI Master, does have subtle changes and does not take any touch sensitivity away. Having the pre-MV around half way and the PPPIMV about halfway, there's a little more distortion at a lower volume, just enough and is still very touch sensitive. I tried I switch the wiper leads, like the LaMar but like this Frontelli a little better.
Bottom line for me, IMHO, having both MV's makes the amp more versatile.