Adjustable NFB to counteract effects of PPIMV?

[pjd3]

Hi all,

Came across a thread somewhere on the topic of PPIMV and NFB. The explanation was that as the PPI MV is used to attenuate, that the amount of NFB will increase to the point of being too much and bring bad sonic artifacts to the tone, in particular the high frequencies. The recommended solution for that was to have an adjustable NFB there to compensate for the use of PPIMV.

I just got my PEC 100KA pot and ready for that installation so, should this be the case, its not a big job just to add a rheostat wired pot to the 22K ohm negative feedback resistor that Sluckey called out in his 6V6 plexi schematic that I'm about 3/4 of the way through building now.

Did I understand all of this right? Please correct me if I'm wrong!

thanks eveyone!
PJD3 - Phil

[sluckey]

As you attenuate (turn down) the grid signal the NFB signal will also decrease.

[pjd3]

Thanks Sluckey,

After reading a little more I came to understand the dynamic (I think).
With the preamp volumes cranked and the PPIMV attenuated alot the PI would be seeing a large signal from the preamp but only a small signal from the OT output. So that would account for the raspy sizzly sound that gets described - a relatively very small amount of effective feedback.

So I'm imaging you could either strap a "reostat" in parallel with your NFB resistor to lower it or say, choose the lowest NFB resistor you would want to have, and place a "reostat" in series with that. It seems easier to think through the series pot arrangement but, unless theres any reason to choose one over the other, I'll sit down with some math and see how either could work based on reasonable resistance ranges.

I also understand that there is a limit to how much NFB you can apply before the tube or loop will shut down. Ha, I don't know how to account for that so, will need to research that a bit.

Thanks!
PJD3 - Phil

[roberto]

The explanation was that as the PPI MV is used to attenuate, that the amount of NFB will increase to the point of being too much

It's the opposite. When you reduce the MVPPI you reduce the NFB as well. Same happens if you saturate the PI with MVPPI fully up.
I've a variable NFB on my amps to balance the NFB to anyone's tastes, and cab, and volume, and...

[pjd3]

Thanks Roberto, I did catch myself on that original misunderstanding after I read through more thoroughly, thought about it and looked at the schematic more closely. And I have been interested in having an adjustable NFB anyways so, I suspect its a good idea for all the reasons you outlined. I also just build and use Rob Robinette's single channel Vibroverb which is so clean, I'm thinking it may benefit quite a bit on occasions if it had some kind of adjustable NFB. It is a great pedal platform however, that it can be that clean.

That's why I need a Marshally amp!

Thank you. Now, just to figure out the best configuration and resistance range for implementing the adjustable NFB. I've already seen many takes on it and they sound reasonable.

best,
PJD3

[roberto]

Share a schematic and we'll help you.

[Xander8280]

Did some math just on the ratio of preamp to PI output gain. The PPIMV changes the open loop gain of the power amp and the only way to get similar power amp response with low open loop gain would mean crazy NFB. I believe it is unrealistic and would require cascaded long tail pairs where the preamp-sided LTP could use a PPIMV but the second LTP would retain the feedback loop.

The "second LTP" could just be some high voltage FET's or BJT's to keep things easy.


As I went through the math I found that the feedback gain would have to scale in reverse of the PPIMV attenuation; if you got -10dB from the PPIMV you'd need 10dB gain on the NFB. Common sense is super low open loop gain means NFB is pretty much out of question (instability).


I wish there was a trick. You could always have the smallest NFB resistor your amp can handle without going berserk for the low PPIMV settings and then something larger for higher PPIMV settings.

[roberto]

There's no need to have a constant feedback at every volume.
Part of the sound of the PPIMV is due to the loss of NFB, exactly as when you overdrive the PI and power amp.

[Roe]

adjustable NFB is great even without ppimv. Try 22k in series with a 250k pot @16ohms or the half resistance @ 4ohms

[roberto]

It is indeed, and in all my amps I have controllable presence, depth and feedback.
But not to counteract the effect of the PPIMV.

[pjd3]

Thanks for all the informative replys, and bravo to the one who showed the math to do your own calculations. Very generous, thanks.

I will have to sit down with a few pots of coffee sometime and mentally work through the calculations that arrive at the results. but for now, how about this:

With this being a Plexi with 6V6 output (sluckeys 6V6 plexi), what would you think may be the lowest value NFB resistor that might be used without running into any instabilities? Or, the lowest value NFB resistor that would still be practical? The amp has a 22K ohm resistor as shown in the schematic. I'm really just looking for a reasonable starting point. I can adjust as required

Thank you,

PJD3

[pjd3]

This comment I found in a thread:

"The NFB resistor value has to be at least twice of the resistor's value below the slope R to ground, otherwise the poweramp will oscillate"

Would the "slope R to ground" possibly be referring to the value of the presence pot in Sluckeys 6V6 which is a 5K linear pot?

If that were the case then could shoot for say 10-15K as the lowest NFB resistor value.

Am I even in the building here?

Thanks,

PJD3

[Roe]

This comment I found in a thread:

"The NFB resistor value has to be at least twice of the resistor's value below the slope R to ground, otherwise the poweramp will oscillate"

Would the "slope R to ground" possibly be referring to the value of the presence pot in Sluckeys 6V6 which is a 5K linear pot?

If that were the case then could shoot for say 10-15K as the lowest NFB resistor value.

Am I even in the building here?

Thanks,

PJD3

yes 15k/5k is about as low as you can go. I'm using 22k/5k. note that the nfb affects the presence controll, which becomes more effective with increased negative feedback

[pjd3]

Thank Roe, that's what I'll do then, at least for starters.

Its actually a 33K NFB resistor thats in there now. I'll series up the 50KL pot with it and have the switch put another 33K Ohm resistor in parallel with the NFB resistor thats already there. That will give just a tad above 15K ohm as my lowest NFB resistor along with the 5KL presence pot thats already there. (with switch in the "high" range)

And when I stop being a goofball, I'll put a 15K resistor on the board, order a 220K pot to put in series with it, and have how it should be to begin with!

Thanks so much!
Phil PJD3