Boot-strapped MV

[grtamp]

Hi ,does anyone tryed the Boost-strapped master volume on JCM800 ?

I heared that the 1M dual pot work better!! right?

Thanks

[xtian]

I like the stock MV on my 1983 JCM 800 2204. You're talking about adding another one, or replacing the existing one?

[grtamp]

I want add.

[pablogt]

I tried it in a jcm800 preamp with EL84 pa.... Works very well, there is no treble loss at lower levels, unlike with the traditional mv.

Pablo

[grtamp]

thanks all!

[Colossal]

I tried it in a jcm800 preamp with EL84 pa.... Works very well, there is no treble loss at lower levels, unlike with the traditional mv.

Pablo

Pablo, did you vary just one or both of the PI's 1M grid leaks with your implementation of the BS'd master volume? With the PPIMV you can vary the ratio of preamp distortion to power amp (and lower volume of course). How would you compare the bootstrapped sound to the PPIMV? With the PPIMV you can load down the PI to get a bit more crunch as well as moderate NFB effects (which keep the amp from opening up too much). You'd be cold biasing the PI with the bootstrap version but leaving the power amp wide open without the PPIMV...just wondering about the sonic differences between these approaches.

Thanks,
Dave

[Structo]

This is what I came up with for a bootstrapped Master Volume.

In a Marshall style amp, the Lar-Mar or Master Volume Type 2 is considered by many to be the best master volume on Marshalls.

[pablogt]

Actually, because of my lack of experience and theoretical knowledge, in that amp I tried six different master volumes, many of them simultaneously. I tried

Traditional pre-PI MV: Lost tone when dialed down. The worst of them all

One side boostrapped mv: Could not dial volume all the way down. (later on I read in Merlin's book on preamps that I could have prevented this from happening by using a bigger pot)

Two pi sides bootstrapped mv: (tandem pot) most transparent to tone, does not add crunch when dialed down.

Cross line PPIMV: simplest to implement (like a cut control without the cap), some artifacts perceptible on distortion at lower levels

LAR-MAR PPIMV: (tandem pot). better than crossline

"scale" control (variable bias for pi): This is just changing the bias resistor for a 50k rev audio pot and a 10k resistor in series.

I tried all these mvs in conjunction to power scaling the amp's pa. The conclusions were that both PPIMV types and the scale control had he same problem: when dialing down you had unwanted crunch. In the case of PPIMV, because of the decreased NFB (Dave, I think the amp opens up with decreased NFB, not the other way round). The scale control decreases the PI's headroom, it could make a good pseudo-power scaling control for a dirty channel (at the cost of a pot)

If what you want is more crunch at lower levels, PPIMV and scale control will get you that, but I don't quite like the sound of it. For amps with NFB I prefer bootstrapped mv with dual pot. For amps without NFB, I use PPIMV with dual pot (that's what I use for Rockets)

Pablo[/list]

[Structo]

Another alternative would be the VVR but not sure if that applicaiton would work on the Marshall 50.

Hall has them for fixed bias and cathode biased amps for 50w or less.

http://www.hallamplification.com/main.html?src=%2F#2,2

[Gaz]

Doesn't the bootstrapped MV lower drive to the PI as well break down the feedback loop? It was my understanding it was only beneficial in amps that used a pre-PI master, but even then power amp controls do nothing at lower volumes.

[pablogt]

Yes, with bootstrapped MV you also get reduced NFB, that goes with the current at the speaker outputs, but I guess since PI's drive is also reduced, you don't get the fizziness you get with PPIMV.

Quite frankly, at one point I could not re conciliate all the stuff I read about MVs from different sources. That's the reason why I tried them all in the same amp and let my ears be the judge.

That amp originally had switchable NFB (it was a Ceriatone 36W) so you could also test the effect of the different mvs with or without NFB.

On the JTM45, I would try first the cross-line PPIMV just to see how it works for you, and if you really need the traditional MV too. You just need a pot wired as a variable resistor and two alligator clips. Clip it to both PA grids and there your are! If everything sounds better with the traditional MV maxed, then you don't need it. If it gets too fizzy for your taste, you probably need the bootstrapped MV.

On the VVR, I would not consider it an alternative MV. In that type of amp I would not scale the whole amp, only the PA, and you need an MV to compensate for the reduced PA headroom when scaled down. All my amps are PA scaled now, since PA distortion is part of the tone I like and you cannot get it at reasonable volume levels without scaling or attenuating.

Pablo

[Darkbluemurder]

Doesn't the bootstrapped MV lower drive to the PI as well break down the feedback loop? It was my understanding it was only beneficial in amps that used a pre-PI master, but even then power amp controls do nothing at lower volumes.

Pursuant to KOC TUT 3 no because it is not in the feedback loop.