Questions about Pre and Post PI Master Volume in preamp gain type amp.

[psychepool]

I like PPIMV very much.
I think it is a master volume method that allows you to enjoy the crank-up sound most similar to the real thing at the bedroom level by applying additional distortion to a vintage-type amplifier with a small number of tube stages by overdrive the PI.
We can crank up the power tube actually with an attenuator, but there's a lot of tone loss when we get it down to the bedroom level.
Personally, I prefer PPIMV to the attenuator in this situation. I think it has more clearer sound than an attenuator at extremely low levels.
(I'm using the THD Hot Plate. I don't know how about the more expensive attenuators.)

Previously, I made a 35 watt amplifier that shortened the Borgner Ecstasy.
Volume control was not easy to use at home, so I added PPIMV and I was satisfied.
In particular, this mod was more useful because the amp had a Plexi mode.

But this time, I made a Mesa Dual Rectifier-based amplifier.
I also implemented a vintage/modern switch, one of the unique features of Dual Rectifier.
As you know, this is technically just an NFB On/Off switch.
When I lowered the PPIMV, this function hardly worked.
I did a search for this symptom, and I found out that if PPIMV is applied, the signal going out to the output transformer is significantly reduced compared to the signal coming from the front end of the power amplifier.
The ratio of NFB was also reduced that much, so that it almost stopped working.
I haven't thought about it before, and this is the first time I learned about this.

I don't like the tone loss of the attenuator at the bedroom level, so I want to add an additional global master volume in any style.
Here are a few questions to ask.

1. If the master volume is installed in front of the PI to reduce the signal entering the PI, will NFB work normally?
If the reason the NFB does not work well when the volume is reduced with PPIMV is because the NFB signal is weak compared to the PI input/output signal,
If the signal input to the PI itself becomes smaller, will the ratio between the signal and NFB be balanced so that the NFB works well?

2. In this type of amplifier, will there be a lot of difference in the sound if a master volume is added in front of the PI in addition to the master volume of each channel?
Anyway, even if the master volume is set to the maximum, a 500K or 1M resistor connected to the ground in front of the PI is additionally installed.
Will this affect the amp's sound a lot?
And no matter how much distortion is obtained from the preamp, if you hardly drive the PI, will the sound's dynamics drop a lot?

3. Recently, I looked at the circuit diagram of the diesel VH4. In addition to the volume for each channel, the global master volume(pre PI) was additionally attached.
However, unlike ordinary volume pots, pins 2 and 3 are tied and just connected to the ground only without a variable resistor connected in series to the signal line.
What's the difference from the ordinary wiring of pin 3 input, pin 1 ground, and pin 2 out?

20210129_010326.jpg

As I try to make an amplifier that can be used in various places, I always seem to have such unnecessary troubles.
Please advise me.

[pompeiisneaks]

That circuit makes no sense. The direct connection between the top and the wiper means the volume pot does nothing. Basically there shouldn't be a jumper between wiper and input.

The signal path has three routes to follow

1. 1M to ground through pins 1-3 on the pot
2. some resistance through 1-2 (wiper) back into the next stage (what the volume should be doing) that resistance obviously varies depending on the pot level
3. the 0 resistance path through the jumper to the next stage

Electron flow follows the path of least resistance, or 0, if there are other options with varying resistances it equally follows those, but will basically ignore everything else over a 0 resistance path.

I'm guessing it's just an error on the schematic.
That being said, the dumble amps I've built all have a MB like that just before the PI to allow you to send the signal to earth instead of off to the power section. It's a great volume for amp designs that don't incorporate power tube distorion, and dumbles have the tone in the preamp from most of my understanding and the power tubes just work as a clean amplifier, but I don't recall if I've ever dimed mine on non OD mode to see how it sounded

OTOH if you have an amp that has strong power tube drive, which some do, about the only way to keep the tone and cut down the volume is a power attenuator. Of course you're correct that attenuating down to bedroom levels always introduces tone suck.

~Phil

[mhuss]

That does seem odd, but I have seen applications where outputs of preamp stages (particularly high gain ones) were shorted to ground to avoid leakage when another preamp path was being used. Also, it might just be an error in the schematic you're working from -- there's certainly a lot of errors in generic internet information.

[psychepool]

Thanks for the answer. Anyway, if I attach the master volume, I think I should attach it in the usual way using all three pins.

In fact, the focus of my question was item 2.
Even with a high-gain amplifier that does most of the distortion and tone shaping in the pre-amplifier, when the Pre PI MV is attached, the PI does not drive enough at the bedroom level.

It wasn't difficult to add a volume pod, so I tried it myself.
With PPIMV installed, I compared it with PPIMV by attaching a 1M pot in front of the PI.
In conclusion, there was no significant difference in sound between the two at the bedroom level. Even the Pre PI MV felt a little cleaner.
As desired, NFB switches and Presence were operate well and had many advantages over PPIMV.
Unfortunately, PPIMV was more advantageous to adjust to the bedroom level.
Compared to PPIMV, Pre PI MV has a little discomfort because the sound is quite loud even if it is slightly raised from the volume setting 0. However, it is an acceptable level of discomfort.

I decided to use Pre PI MV instead of PPIMV. Thanks for your help.

[roberto]

With high gain amps, MVPPI alone is generally speaking not a good option: you have too much output from the preamp, and the PI works out of its useful range.
You can apply both MV, pre and post PI, and obtain better results by properly setting both. Just consider not to blast the preamp if you add power amp distortion, or you will just get muddy sounds.

Lower the gain, set the pre and post MV, and start from there.
Please note that also presence and depth will be less effective.

[teemuk]

3. I see nothing invalid in the scheme, although it differs from the most usual scheme of wiring pot as volume control. Yet, it's still a basic resistive divider: In one extreme the signal takeoff point is between 220k (series resistor) and 1M (least attenuation), in the other extreme between 220k and 0 ohms (full attenuation). Between these extremes the resistive division is between 220k and whatever is the resistance of the potentiometer wired as -variable resistor-. If you think about it, there's really no significant difference to usual scheme, except that overall tapering of the division is different. Perhaps one providing more sensitivity, because the designer chose this particular setup over the basic one.

[roberto]

That does seem odd, but I have seen applications where outputs of preamp stages (particularly high gain ones) were shorted to ground to avoid leakage when another preamp path was being used.

Why to you define it odd? It is a standard in most if not all high gain multi channel (pre)amps.

[mhuss]

Why to you define it odd? It is a standard in most if not all high gain multi channel (pre)amps.

Well, sure, using relays or Vactrols. More unusual to do it with a volume pot.

[teemuk]

I don't quite follow the logic... You dial volume pot to "0" and the wiper is essentially grounded and thus the signal path too. Both schemes do this; the usual method of volume pot wiring, and the schematic example's series resistor to a pot wired as variable resistor (which isn't overly different from the usual volume pot wiring except for retaining a fixed input series resistance and changing the overall tapering of the resistive division).

What does any of this has to do with channel switching automatically muting the signal path by shorting the unused channel's signal path to ground?