A simple "master volume" mod -- adjust PI bias

[xtian]

If you replace the 470R cathode resistor of a LTP PI with a variable resistance of 470R to ~50K, you get a nice, reasonably transparent "master volume" control. Does not allow for power tube distortion, of course, but this mod is sounding good on my JTM45-style build, allowing me to get good distortion out of it at low volumes. Control pot is a little scratchy due to DC voltage, but not bad at all.

Now you don't need to watch this guy's 15-minute video. You're welcome.

[xtian]

as implemented:

IMG_1036.jpeg

[Colossal]

Yes, it's a useful mod and works. It's in Merlin's first book, a version was done in some Carlsboro amps if I recall correctly, and used in Mojave Ampworks amps as well, as a reverse audio pot in series with the PI bias resistor. I implemented it on a 6V6 Plexi some 16 years ago now. It took that amp from earsplitting to a warm distortion.

[Lauri]

Limiter control in Carlsbro amps does the same thing. Connecting the pot how it's in Carlsbro amps should be less scratchy because there's a lot less current going through it.

[maxkracht]

Didn't know I was keeping so many secrets...

[sluckey]

Several Hoffman members were playing with that idea about 20 years ago. We were calling it "power dampening". I just used a 6 position rotary switch. I never permanently installed in any of my amps. The larger resistors made the amp sound kinda strangled and thin to me. I lost interest after a week and never looked back, although the topic would reappear every few years.

Da Geezer’s Bypass Cap attenuator was another interesting idea for cathode biased output tubes. It was very effective at controlling output power but the idea just didn't catch on.

You can see both of these schemes on page 7 of my amp scrapbook.

[psb962]

...

Da Geezer’s Bypass Cap attenuator was another interesting idea for cathode biased output tubes. It was very effective at controlling output power but the idea just didn't catch on.

You can see both of these schemes on page 7 of my amp scrapbook.

The bypass capacitor attenuation methods you guys discussed way back were very interesting. The conclusion was that if you implement separate cathode bias resistors for the power tubes of a push pull amp, then remove the bypass caps or link them through a resistor, you can cut a lot of power without losing tone. I immediately got to work in LTspice and found that on my 5e3 model it worked perfectly, with a 270 ohm resistor linking the cap negative terminals giving half power, and 820 ohm giving quarter power. Just linking them (and not grounding them) gave full power. I was just about to add it to the list of mods I want to try on my 5e3 when I noticed a potential gotcha: the virtual ground of the heaters is commonly wired to the power tube common cathode resistor to reduce hum (it sure does) but if you implement separate cathode resistors to play power games you shouldn't keep the heater virtual ground wired to either cathode as they are no longer a constant DC value. Do you agree?

[sluckey]

I agree.

You'll need to just connect the heater center tap to ground. I have been happy with the 5E3 filaments referenced to ground. Or use two resistors and one filter cap to make a voltage divider connected to the B+ rail and reference your heater CT to that.

[lonote]

Or use two resistors and one filter cap to make a voltage divider connected to the B+ rail and reference your heater CT to that.

I just used that set-up for the first time, on the amp I am finishing up, borrowed directly from one of your power supply drawings.

Works great & I plan to make that a regular practice/feature from here on out.

[LOUDthud]

Wasn't there something like this in the Duncan Convertible amps ? The Master in those amps worked remarkably well.

[Reeltarded]

If you replace the 470R cathode resistor of a LTP PI with a variable resistance of 470R to ~50K, you get a nice, reasonably transparent "master volume" control. Does not allow for power tube distortion, of course, but this mod is sounding good on my JTM45-style build, allowing me to get good distortion out of it at low volumes. Control pot is a little scratchy due to DC voltage, but not bad at all.

Now you don't need to watch this guy's 15-minute video. You're welcome.

I think at least 50 people on Metro did this very early on and everyone decided it made their amps sound and feel dull after undoing and redoing it a few times, because maybe it isn't.. ok.. yea it is.. wait.. do I remember how.. nope it's dull.. one more time.

I still think PPIMV with backwards soldered Vox cut is the best for everyone.

[M Fowler]

Colossal Dave

I remember clearly our working on my amp a 50w 6L6 Rocket Reverb using that watts control in the PI like the Carlsboro 60 amp. Still have the amp and still love the tones.

Mark