Master Volumes Types

[azatplayer]

i just got tut3 and found the bootstrapped master V in the 800 pages. I implemented it, rather crudely, into my plexi mule ( trying to find the right "plexi" for me). I found it very cool, my feeling is its a very full sounding master V, in fact retains bass real well, leaves you feeling like you got something for nothing...
Anyways, i wanna put it in but take it out, so the obvious path is with a dpdt pot in the master, i can do a 16mm easily with alpha and other brands, but am looking at the cts 24mm spst. Trying to figure out if i can use this pot.
My thinking is that i leave the pot wired with the bootstrapped deal, and have that connected to the PI tubes input fulltime, but leave the original circuit intact, likewise connected fulltime.
Question is... if both the standard PI connection from the treble wiper AND the new MV connection, are both connected to the tube....
...can i have the MV left in the circuit, whilst switching in the original circuits connection?
So, in a nutshell, the tut3 bootstrapped MV is always connected, and the original 1 meg/.02cap is switched in and out by the pot.
Will the 2 inputs affectthe operation of either one? Im gonna try it anyways, for my own satisfaction and learning.
Just watched the Pacific opener. As massive fans of the bands of brothers series, my son and i have been waiting with abated breath for the sequel to begin. I have the dvd set and have watched it 5 times since first seeing it on tele. Was good.
AS an Aussie, its a little bit of an eye opener to just how much we needed help. I know how close we came to speaking Japanese as a first language, but when you stop to think about it, it really strikes home.
So, thanks to my American friends who went out of their way. Who left what they were doing and came to our aid. I for one, am left with nothing but regard and respect and care for those who came.
Simply amazing.

[Ken Moon]

You guys are pretty sharp

Yes, that's a Mojave-style scaling control in the Talon amp.

The idea is that when you increase the bias on the PI, you greatly reduce the PI output signal swing, allowing the PI itself to then be more heavily overdriven (at the same overall volume level), so, in theory, you should retain some of the nice overdriven sound as the volume is turned down.

The problem is that there is DC on the pot, causing a scratchy noise when turned (not very pleasant), plus I didn't think there was any great tone advantage vs a simple pre-PI MV or PPIMV. And, PI overdrive is just different-sounding than power tube overdrive.

Funny enough, though, I've built a couple of Talons for guys who rave about how great the MV works

Myself, after trying VVR, PPIMVs, and I think every other style that's out there, the pre-PI MV sounds best to me in this amp. I found the same thing on a 2204 Marshall I played around with. I think the type of MV that sounds best is very much amp-dependent, as well as ear-dependent.

Of course MVs are for wussies anyways, but that's a whole 'nother thread

[M Fowler]

Mojave-style scaling control or Watt's control by Pritchard amplifiers.

I tried a watts control on my Express there wasn't any noise issues and it worked but I just didn't like the sound I got.

I do think this control works well in lower powered amps.

Attached is the Carlsboro 60TC schematic

[Colossal]

Yes, that's a Mojave-style scaling control in the Talon amp.

The idea is that when you increase the bias on the PI, you greatly reduce the PI output signal swing, allowing the PI itself to then be more heavily overdriven (at the same overall volume level), so, in theory, you should retain some of the nice overdriven sound as the volume is turned down.

I implemented something similar in a 2x6V6 2204 amp except used 25kB with a paralleled 33k. I find it does an excellent job of taking the sound pressure of the cranked amp down while retaining the overall distortion characteristic very nicely. At lower levels it adds quite a bit of compression but in general there is a nice, squishy warmth overall. The major goal was an alternative to the traditional PPIMV which I tried in tandem with a VVR. I like the scaling control method very much. At low levels, the tone begins to get a bit muddy, meaning a reduction in clarity and definition. But it does a great job of cutting the overall sound pressure of the cranked amp. Everyone's mileage will vary of course.

The problem is that there is DC on the pot, causing a scratchy noise when turned (not very pleasant), plus I didn't think there was any great tone advantage vs a simple pre-PI MV or PPIMV. And, PI overdrive is just different-sounding than power tube overdrive.

The scratchiness is not at all bad and for the utility (and great simplicity) of the technique it is not an issue (for me anyway). An optoisolator could be easily implemented to take the place of the pot for a scratch-free adjustment. Merlin outlines this in his book.

I think the type of MV that sounds best is very much amp-dependent, as well as ear-dependent.

Definitely. Although a VVR does a good job of maintaining tone to a point as well as controlling sound pressure, I just couldn't get into the Type II PPIMV I had on my '68 Plexi build. I am adding one as well as a scale control to a 50W 1987 Lead build I am doing currently to work complimentary to one another and for different situations.

Of course MVs are for wussies anyways, but that's a whole 'nother thread

LOL, so true! But Alas, they are a necessary evil sometimes!

[Andy Le Blanc]

Thank you for posting the Carlsboro hadn't see that one yet.

[M Fowler]

Colossal and I have been busy trying new methods of controlling the amp without using MV or VVR, so along comes the two of us looking over the Egnator Rebel watt control and the Carlsboro prime control.

The key method colossal note is 25kB with a paralleled 33k. No scratchiness with that method.


It works but not for all amps especially the Express. I tried higher value pot as well as higher value bypass resistor across the pot. It controls the volume well but the tone isn't that well. I will try this again in a 80w Rocket I am putting together right now.

[Gaz]

The Carlsbro style control sounds just too weird to me, and fairly similar to the PPIMV, getting fizzier as you go down because of breaking down the feedback loop.

Having tried all these MVs like many of you, I favor none at all, or as someone mentioned, simply using 'the right amount of power for the right situation'. The pre-PI MV is my next favorite, but only if there's enough preamp gain. With a Plexi style preamp I've also tried wiring the pre-PI MV with a 220k resistor to ground, so the control could never be turned below the tone-sucking-lifeless point, which was about 1 or 2 o'clock. I was going to call it "Power amp drive." It was actually my favorite solution, but took it out for simplictiy's sake, and because I didn't want Ken Moon to think I was a wimp

[Structo]

Have you guys tried the Lar Mar MV.
It's basically one of Ken's MV designs changed a bit to work better with the Marshall amps.

[David Root]

This is the one I like. It doesn't get fizzy or lose treble at low levels, just lower level. As it's turned down it lowers the grid leak/bias feed resistance. If you use bigger than 250K pots it can get noisy apparently, I can see why.

[eddie25]

I like the lar-mar a lot. I don't particularly like the loss of the NFB at low volumes, but it's worth what you get at gig volumes for sure.

I have a question for you guys though... What is it called when you simply replace the bias feed resistors with a dual 250k using just one side? Essentially just making the bias feeds variable from 0-250k. Seen it on a Marshall and it works real good, but I have never heard of it (or maybe I have). Does it have a name?

[Structo]

You may be referring to crossline MV.

I tried it on my 6V6 amp and didn't care for it.
To little sweep, more like an on and off.

Actually on my amp what worked the best was the bootstrap MV.

You can use the dual pot method or just one 1M pot for a single bootstrap.

[eddie25]

I'm pretty sure it's not the crossline. The one I'm talking about literally just makes the two 220k bias feed resistors variable from 0-250k.

[Gaz]

That's one of the MVs from the Trainwreck Pages, and is the same as the LarMar, which simply adds 'safety resistors' so if the pot goes open there is still bias applied to the tubes. This one does get fizzy when turned low if you are using NFB.

A lot of folks complain about the Crossline in their Marshalls, but those MVs only sound good in cathode biased amps with no NFB.

[eddie25]

It's not one of the 'trainwreck pages' ones either. It doesn't use the whole pot and doesn't keep the bias feed constant like the type 2.

[azatplayer]

I have tried the bootstrapped with single gang and really liked it.
Just completed a plexi build and went with, to start off with, just a basic pre pi MV, controlling the PI feed from the treble wiper.
Only had a 1M linear pot in my drawer, and man im glad! Gees the linear pot makes that master so much better in its sweep. Very usable actually.
Im keen to run 2 masters and have one footswitchable as a boost.