Single 1M vs dual 1M bootsrapped master?

[iknowjohnny]

I noticed you can do a bootstrapped master either by replacing the input side 1M grid resistor of a LTPI with a pot or BOTH 1M grid resistors with a dual pot. with a typical JCM800 style amp with NFB, what are the benefits or disadvantages of each?

[teemuk]

They are pretty much the same. The one with the dual pot seems like a great way to hinder AC bypass of the common grid amp.... not that it really has such a drastic effect on overall operation. But yeah, I'd consider that a disadvantage.

IMO, both are great in modifying open loop gain of the amp and ensuring your amp will NEVER have consistent response throughout the volume dial. Frequency response and damping characteristics of your amp will be different at each different master volume setting.

I'd can hardly call that anything else but disadvantage.

[John_P_WI]

They both work well, it depends on the "sound" that you are chasing. The single pot alleviates the tonal change that you would find in the JCM 800 conventional master. The dual pot is an improvement on the single, in fact Kevin O'Connor calls this implementation the "Improved bootstrapped master". I have used the dual pot in power scaled amps which required two separate channel masters and switched via relays. The bootstrapped masters work the best for amps that generate their distortion in the preamp and use a clean power amp as you will lose the grit of the PI being overdriven that is possible to achieve with the PPI MVs (Lar Mar).

You may have some problems obtaining bedroom levels with the bootstrapped masters - but anyone with bootstrapping knowledge can overcome this.

[iknowjohnny]

Thanks. I DO use it mostly at home so it does need to get pretty low in volume. How would i go about working around the low volume issue, and does it affect both dual and single?

Still not sure what the difference between the dual and single tho. "Improved" doesn't explain how they differ.

[John_P_WI]

It is possible to use a much smaller resistance valued pot (less than half or even 1/4.).

To me, the dual pot was more "transparent" with the least effect on tone. Make sure you twist your wires and keep them short.

[iknowjohnny]

Thanks. How about this tho....

You may have some problems obtaining bedroom levels with the bootstrapped masters - but anyone with bootstrapping knowledge can overcome this.

Do you know what the issue is and a way around it? If it's a matter of bad tone then a BS isn't going to work for me since i use it mostly at low volumes.

[John_P_WI]

Use a smaller resistance pot or a 470k resistor or smaller strapped across the outer pot lugs (parallel making the pot a smaller value).

[iknowjohnny]

So it's a smaller value pot that is the secret to making it work at lower volumes? Because if thats it I could use a 500k or whatever and have a switch to parallel a very small resistor and brink it down to 50k or some such tiny value for low volume use.

[John_P_WI]

Yes, I would not go below 250k though.

[Tone Lover]

Thanks. I DO use it mostly at home so it does need to get pretty low in volume. How would i go about working around the low volume issue, and does it affect both dual and single?

Still not sure what the difference between the dual and single tho. "Improved" doesn't explain how they differ.

My friend having been down this very road I understand your pain.
First i built a 2 x 6v6 amp still wasnt quite enough. Now Im working on a 2 watt hoping that will do the trick.
All of the master volumes and VVR,s at least to my ear do not sound well at bedroom or household volumes. so thats how I ended up so far.
I actually use the 6v6 version to jam with the band It sounds really nice cranked and if I need more volume I add more speakers. I might as well sell my 50 watt It doesnt get played much anymore.
Bill
PS. I know this doesnt answer your question but you would have ended up were Im at eventually

[renshen1957]

They both work well, it depends on the "sound" that you are chasing. The single pot alleviates the tonal change that you would find in the JCM 800 conventional master. The dual pot is an improvement on the single, in fact Kevin O'Connor calls this implementation the "Improved bootstrapped master". I have used the dual pot in power scaled amps which required two separate channel masters and switched via relays. The bootstrapped masters work the best for amps that generate their distortion in the preamp and use a clean power amp as you will lose the grit of the PI being overdriven that is possible to achieve with the PPI MVs (Lar Mar).

You may have some problems obtaining bedroom levels with the bootstrapped masters - but anyone with bootstrapping knowledge can overcome this.

Hi John,

That is the best summary of the bootstrapped master volumes I've read to date and coincides with my experience. Always good to see your and Gingertube's posts; I miss powerscaling.com forum.

Best regards,

Steve

[ampdoc1]

I've recommended this solution several times, but I don't think anyone here has ever actually tried it. Years ago I wanted the same thing, lower volume without changing the tone. I finally tried an idea I had, which was to put a variable resistor (in my case with a pair of 6L6s, a 2 OHM 25W) in line with the output tube heaters, thereby reducing only the amount of output in the power tubes,. and not affecting any other voltages/sound in the rest of the amp. It worked very well,...I could get ABSOLUTELY the same sound/tone down to the point where the speakers could not react normally.

If I was doing it today, I'd use a VVR to regulate the heater voltage, instead of a resistor. Either solution does require a few seconds for the voltages to change as you turn it up or down, exactly as the VVR works in "normal" use.

This sounds like it is too simple to work, but you'd be amazed!!

[jazbo8]

I've recommended this solution several times, but I don't think anyone here has ever actually tried it. Years ago I wanted the same thing, lower volume without changing the tone. I finally tried an idea I had, which was to put a variable resistor (in my case with a pair of 6L6s, a 2 OHM 25W) in line with the output tube heaters, thereby reducing only the amount of output in the power tubes,. and not affecting any other voltages/sound in the rest of the amp. It worked very well,...I could get ABSOLUTELY the same sound/tone down to the point where the speakers could not react normally.

I'm not sure how low you are running them... but if it is too low (say >10%), the tube life could be significantly shorten because the cathodes may become “poisoned.” A cold filament acts as a getter; which attracts contaminants. When contaminants attach to the surface of the emitter, the affected area of the emitter is rendered inactive, causing a loss of emission.

So it would be safer to VVR the B+ not the filament and that's why it is the preferred practice.

[rp]

OP, guys, you are amp builders. All in one amps are for non DYIers with limited funds. Build yourselves a little amp for bedroom volumes. I strongly favor 1X 6V6 amps over 1X El84 amps. Probably too loud for a true bedroom amp but IMO the 5F1 is the best at giving real wide open rich dimensional tube amp sound at modest levels. If you want something tamer that for me works, if not as a bedroom amp then as a late night amp, try a 5C1 w/ a good Edcor OT. If that's still too dynamic for your late night adventures and you want a compressed even distorted sound at low volume buy a little ss practice amp. It'll be as good or better than any MV or VVR squashed down big amp and lower your electric bill too. As for MVs and VVRs or putting a pedal in front of a little tube amp to squash it down even more, again might as well just use a little ss amp. If anyone knows how to take all the dynamics and volume out of a tube amp and still have it sound great at bedroom levels I'm ready to build it.

[martin manning]

Good point rp! How many great studio recordings have been made using cranked-up little amps- Champ, Pignose, etc.? I recall a recent Jeff Beck interview where he said the Champ's 8" speaker was the source of a lot of good harmonics. Bedroom/recording amp construction would make a very good topic for discussion and projects.