Home-brew pot question.

[DummyLoad]

no, not that kind of POT, sheesh!   

looking through some older gibson amps, i stumbled across the GA-88S - a stereo 17W amp that looks appealing and i thought i'd try to clone it. one problem though, the tone stack uses a weird-ass 2MRA tapped pot. i'm assuming the tap is 50% for arguments sake since i don't have one, nor can i hope to gain access to other gibson amp models that uses that part to measure. my question is in the graphic attached. are the two circuits relatively equivalent? if yes, would changing the ratio of the 6M shunt(s) move the taper point. i'm guessing yes, just hoping the gurus will chime in and set me straight.

http://www.el34world.com/charts/Schematics/files/gibson/GA-88S.pdf

much thanks in advance...

--DL

 

[tubeswell]

Hmmm... interesting amp circuit!  Its not just that the tap is 'somewhere' 'in the middle' of the original pot, but that as the wiper is moved, it changes the resistance 'in front of' the tap where the cap is. But your idea might be something that sounds good anyhow, so...

[kagliostro]

Some days ago I was thinking to select an adequate (mechanically) pot, put on the back of the carbon track a little piece of adhesive copper (to be used as the additional pin) and connect (at the right resistance point) the carbon track with the added pin with silver paint

here

http://www.guitargear.net.au/discussion/index.php/topic,33782.0.html

someone give me the council to use a rotary switch and a bunch of resistor to build a "tapped pot" equivalent circuit

K

[PRR]

If you use Grow-Lights, tap them before the meter; otherwise the spike in electric use will bring the sheriff to your door. (Putting away my pliers.)

> are the two circuits relatively equivalent?

The way Gibson used it: not at all. (But why ask? You could mock-up that tone control and shunted pot about as fast as you typed/sketched that.)

What kagliostro said. Remember car window de-icer conductive paint, or Radio Shed sells a conductive pen which ought to be "perfect" in a 150K-2Meg circuit. Or the 12-way switch.

[jjasilli]

Also, I don't think conductive tape or foil will work, because I think it conducts only on the top, not on the bottom adhesive side; and it may block the swipe of the wiper.  I also don't see how a wire lead could be properly soldered to a make-shift trace. 

Another possibility might be to place a voltage divider between the input lug > wiper of the pot.  I think this might make the voltage divider tap more interactive with the sweep of the wiper (not sure  ); but it will alter the taper of the pot.  In that case a linear taper pot might be used (if available), with the added voltage divider doing double duty to mimic an audio taper.

[kagliostro]

Also, I don't think conductive tape or foil will work, because I think it conducts only on the top, not on the bottom adhesive side; and it may block the swipe of the wiper.  I also don't see how a wire lead could be properly soldered to a make-shift trace.

Only to clarify which was my idea

the piece of adhesive copper must be put on the opposite side of the support where is the carbon track, not on the carbon track directly, this new "pin" is to be connected to the carbon resistive track using silver paint and the wire coming from the circuit is to be soldered to the added copper  "pin"

however I think that the rotary switch + resistor solution will be a good and easily feasible one as to recreate the original tapped pot in the right way

K

[xm52]

You can build a stepped attenuator like the trim-1 (http://glass-ware.stores.yahoo.net/trstat.html).

It might be a good idea to figure out why they wanted a 2MRA pot in there. Maybe a 1MRA will do. Sometimes it comes down to a very small difference  and only when the tone control is on full. Something that you might never do with this amp.

[tubeswell]

Or you could use 3MRA (with a home-built tap on the pot's wiper trace) and change the rest of the tone stack values proportionally (Cap values go down by a 1/3, resistor values go up by 50% - if I've worked that out correctly)

[kagliostro]

I was thinking to mount the resistor directly on the rotary switch

like here

http://www.platenspeler.com/diy/preamps/gainchooser/uk_gainchooser_2.php

http://www.platenspeler.com/background/attenuator/uk_attenuator_1.php

or this



as you need a mono control, the construction will be less complicated

K

[DummyLoad]

Thanks for all the replies. I breadboard the ckt. and it does not behave at all like a tapped pot. It does work but likely not like the original ckt. I had thought of the stepped tapped attenuator but was hoping to avoid the expense. The reverse audio 3 meg fender pot is likely the path I'll take. I don't think modifying a part will yield positive and lasting results. I captured some simulation graphs of the frequency domain and I'll post the results when I get home.

--DL