Master Volume pot

[martin manning]

Here's the build thread: https://ampgarage.com/forum/viewtopic.php?f=1&t=17915
A couple of copies have been done by others with success.

[ViperDoc]

The only dual 250kL pot I've found so far for sale was a 8mm shaft, but I want a 3/8" shaft to match my other Alpha or CTS pots.

I understand this preference. And I know you know this, but I've found a good shim when fitting 8mm shaft pots into 3/8-inch holes is to wrap the base of the shaft with a fitted strand of 22 gauge, stripped solid-core wire. Just wrap the stripped wire around the shaft so it crosses itself, cut on both sides of the cross, Adapt with pliers, place the wire on the shaft base and lock in place with an inner locking and an outer flat washer. Centered.

[martin manning]

Alpha brand 250k dual audio pot with 3/8” bushing https://www.amplifiedparts.com/products ... 38-bushing
Tubes and More/Antique Electronic have a listing too; same company, I think.

[Ledvedder5150]

Alpha brand 250k dual audio pot with 3/8” bushing https://www.amplifiedparts.com/products ... 38-bushing
Tubes and More/Antique Electronic have a listing too; same company, I think.

Thanks! I was looking for linear taper. But I already had a few dual audio taper pots on hand, so I started making the change last night.

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[martin manning]

I was looking for linear taper.

That source has linear taper too, but w/ 8mm bushing, for future reference.

[Ledvedder5150]

I was looking for linear taper.

That source has linear taper too, but w/ 8mm bushing, for future reference.

Did you make some changes to the PI also to work better with the ppimv?

[martin manning]

I don't think so. Values are in the layout, though.

[jazzbass]

Hi folks,

but no one has thought of a simple 1Mega potentiometer as in the Dumble projects?

A hug.

Franco

[dorrisant]

Hi folks,

but no one has thought of a simple 1Mega potentiometer as in the Dumble projects?

A hug.

Franco

In higher gain amps you get some of the distortion from the PI itself. If you use the single-gang volume pot prior to the PI you will loose this. A dual pot with sections for each of the PI outputs preserves this character/distortion by placing the attenuation after the treatment of the PI triodes.

Most Dumble power amps tend to be a PA, not how they are frequency-tuned, but as regards distortion... They cleanly reproduce the magic that happens in the preamp. In this case, attenuation prior to the PI has the desired effect and is simpler to install. Although, I will say that I firmly believe the Dumble PI needs to be properly tuned for maximum harmonics. Yet more magic!

[jazzbass]

Hi folks,

but no one has thought of a simple 1Mega potentiometer as in the Dumble projects?

A hug.

Franco

In higher gain amps you get some of the distortion from the PI itself. If you use the single-gang volume pot prior to the PI you will loose this. A dual pot with sections for each of the PI outputs preserves this character/distortion by placing the attenuation after the treatment of the PI triodes.

Most Dumble power amps tend to be a PA, not how they are frequency-tuned, but as regards distortion... They cleanly reproduce the magic that happens in the preamp. In this case, attenuation prior to the PI has the desired effect and is simpler to install. Although, I will say that I firmly believe the Dumble PI needs to be properly tuned for maximum harmonics. Yet more magic!

Hi Dorrisant,

thanks for the extremely logical contribution. My reasoning had multiple options:
First of all with the replacement of .002/150K with .001uF/56k cap/resistor after the 2nd stage the amplifier is now more manageable in my opinion and then I was thinking of using outputs/inputs from the preamplifier to be able to use a Dumblator or perhaps a reverb/echo.
In Dumble projects these are placed after the master volume and before the PI.
After your reasoning I have many doubts.
I built this Express ten years ago but I hardly used it because even the clean sounds were too dirty. My main instrument is a Stratocaster whose volume could never exceed half to have some headroom, while the volume of the amplifier could not exceed the quarter turn because then the Express was pure distortion with no perceptible increase in volume.

Thanks Franco

[M Fowler]

I have been using 250KL dual pots in my PPIVMV builds and no reported problems. These are larger 3/8" long thread Pots so no trouble with face plates thickness problems.

Mark

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[Mattbedrock]

I built this ‘Wreck Express a number of years ago, two others as well that I sold or gave as gifts.



While I love how it sounds, it never worked well for me in a live band setting. I tried an attenuator with it, but it just sounded squashed to me. It is just a cheap resistive one. But unattenuated, it was either way too loud or turned down to the point where I might as well have been playing through a transistor radio.

On another forum, the purists pooh poohed the idea of a PPIMV and said it would ruin the amp. But it’s pretty useless to me for gigging right now as it is. I have a bag full A250k dual alpha’s, so it costs me nothing to try it.



I installed the pot in the ext speaker hole, so no chassis drilling. Used shielded wire coming off the PI and 2.2M safety resistors. It only took me a few minutes to install.



The amp is running through a slant 2x12 cab with 25w greenbacks. I have the volume at noon, where the amp really sings. With no master volume, I measured the volume at about 6 feet (where I’d likely be standing on stage) and it was averaging a scorching 109 DB. Uncomfortable. With the master at half, the volume dropped to a much more comfortable 99db. At about 25%, it was a little fizzy but a very moderate 94db.

I’ll need to try it live to really know how this will work out, but I’m hopeful I can dial in a usable volume this way.

[dorrisant]

Many here would "Pooh Pooh" on the other forum. This has been the most widely used attenuation for an Express by a long shot.