bootstrap MV

[dazzler]

hi. could anyone describe how to hook up this master/components used? Do those who have tried it feel its an improved MV for a trainwreck or a marshall?
thanks

[Stanz]

As I understand it, the bootstrapped MV is a 1MA duel pot which replaces the two 1M resistors in the PI. I have not done this (yet), but my understanding is that it is even better than a PPIMV.

Check out PowerScaling.com and look up the 5100 project (Marshall plexi-PCM800). They have put in a bootstrapped MV as an attenuator for the power amp input when power scaled.

[txbluesboy]

I did this to my Express and think it is a great improvement.

[dazzler]

ahh i think ive seen similar to this on metroamp. the pot value may be different though. i recall its grounded on the turret where the bias resistors used to join. are any resistors used on the 1meg dual pot?

[txbluesboy]

I added a 100k ohm resistor from the wiper to the phase inverter grids.

[billyz]

Could Someone please send me a Schematic of the Bootstrapped MV ? For some reason the Power Scaling site won't let me register and view the forum.

thanks

[dazzler]

As I understand it, the bootstrapped MV is a 1MA duel pot which replaces the two 1M resistors in the PI. I have not done this (yet), but my understanding is that it is even better than a PPIMV.

Check out PowerScaling.com and look up the 5100 project (Marshall plexi-PCM800). They have put in a bootstrapped MV as an attenuator for the power amp input when power scaled.

isnt it the 220k resistors in the PI the pot replaces?

[billyz]

I was finally able to register with Powerscaling.com. Apparently, they don't like Mozilla. IE7 worked .

[Stanz]

Here is an example of the two approaches.

[skyboltone]

I did this to my Express and think it is a great improvement.

This envolves taking the PI grid wires half way across the chassis and back. Did you use shielded wire? Do you have 12AX7s in V1-V3? What happens to your harmoics when you use it? Is it stable? You got pics?

Honestly I don't understand the attraction to MV with these amps. If you're not clipping the power tubes it ain't a TW.

Dana H (UR-12) has several folks working with his scaling board to scale the entire amp. I don't know if anybody's finished the job satisfactorily yet but it sure seems like a sounder approach to me.

YMMV
Dan

[Ron Worley]

+111 on Dan's point of view..... pushing the power tubes is the secret sauce, IMHO.....

Ron

[txbluesboy]

I did use shielded wire to the pots. My Express Was really driving the output tubes hard with the volume just barely turned up. I Powerscaled the output section on my amp when I installed this ("Improved Bootstrap") master volume. As I reduce the power on the output tubes they distort more easily. Turning down this Master volume with the output tube power reduced allows me to get the same amount of overdrive but at a reduced volume. With everything all the way up it works like a regular express ( This may or may not be true as I have never really heard a real Express, But it sounds really good to me! ). But between the original volume control, the bootstrapped master volume, and the output power tube control, I am able to go from very clean to either, preamp clipping, or output tube clipping, or both, like a regular Express but at reduced volume. The amp is assembled in its cabinet right now but if I get some time Ill pull it out and take some pics.

[dazzler]

a pic or diagram would be great, i cant read schems to figure out what goes where .

[Fischerman]

Honestly I don't understand the attraction to MV with these amps. If you're not clipping the power tubes it ain't a TW.
Dana H (UR-12) has several folks working with his scaling board to scale the entire amp. I don't know if anybody's finished the job satisfactorily yet but it sure seems like a sounder approach to me.

Dan, With all due respect I think you might want to read up on power scaling. With power scaling you could set the amp (just by turning the MV and Scale controls) to have the power tubes clip even earlier than a regular amp...or later...or at the same point. If you turn down the scale knob and leave the MV up full then the power tubes are clipping more...that's why you need the MV with power scaling so you can adjust the power tube drive as you decrease the power tube headroom. It's your choice on how much power tube clipping you get. That's if you scale the power tubes and not the whole amp...if you scale the entire amp then you're sort of 'locked in' to one gain structure...which might be very close to stock for much of the scale range but never exact unless you're at full voltage (i.e. Scale full up).

[skyboltone]

Dan, With all due respect I think you might want to read up on power scaling. <snip>

Thanks txbluesboy and Fisherman. Well, my instincts are satisfied by Tex's explanation. I just couldn't imagine running all over the place with the PI grids without going into oscillation. And as far as reading up, I just did. Thanks. What you're saying makes good sense to me Fisherman, in order to add more control if that's what you want. As long as we don't lose that delicious harmonic structure by overcomplicating things. The RichMod PPIMVs that I've put in amps will not do the job in a TW amp in my opinion. In a Marshall circuit it's kinda like "even bad breath is better than no breath at all". I've not done the bootstrap mod. I'll give it a try..........but not on a wreck

Dan