Question about Log-Antilog pot from Dual Ganged Linear pot practical use

[kagliostro]

Here you can find documentation about how to transform a Linear pot in a Log or Antilog pot simply adding a resistor between wiper and one lug

http://sound.westhost.com/pots.htm

http://www.geofex.com/article_folders/potsecrets/potscret.htm

This is the schematic of the FXLoop of the Engl Fireball



The question is:

In this circuit where the input of the signal in the pot (I mean the dual ganged pot connected to the grid of the recovery tube) is from the wiper instead that from a lug, is the trick of the resistor still functioning properly ?

Thanks

K

[PRR]

I think that is a dual-Linear pot.

If audio-tapered, the sound would be very weak at 50:50 mix.

If you doubt me... simplify to the core circuit, test it with DC. A 1.5V flashlight battery is handy. I bet, with linear pots, the "grid" voltage is 1.3V at all pot settings.

[kagliostro]

That is very interesting

Merlin in his book discuss about the Fireball FXLoop saying that the original requires a Log+Antilog dual ganged pot but as this are like unobtainium he say a linear pot must/can be used

As I remembered the trick about the use of resistors as to obtain an "akin" Log pot I was wondering about the fact that the input of the signal in this FXLoop go to the pot through the wiper instead of one of the lug and this put on me the doubt about the efficiency of the trick with this particular architecture

Your explanation is illuminating

Many Thanks PRR

Franco

[HotBluePlates]

... Log+Antilog dual ganged pot but as this are like unobtainium ...

I've heard the same statement. And that log-antilog pots were used as Balance controls in a stereo.

Merlin in his book discuss about the Fireball FXLoop saying that the original requires a Log+Antilog dual ganged pot ...

I wonder about this, as Engl is a modern company, and the Fireball is currently in production. So if it uses an unobtainable pot, how do they make the amp?

[kagliostro]

Ciao HotBluePlates

last year I dismantled some broken recent SS HiFi Amp

on those you can find a large sort of unobtainium pots, like four ganged pots, dual staked pots with pots of different values, slide pots acted remotely via a special mechanic that has a knob to be turn on the front panel instead that the usual runner

this kind of things are like unobtainium for mere mortals, to get one of this special parts is near impossible if you are not an authorized service center

so, may be the Engl Fireball is in production and may be it uses one of this special Log/Antilog pots, but you can switch the seven labors without even succeed in finding one of it and if you find one I think the price will be inarrivable

Franco

p.s.: I explain that I have been using the term unobtainium, Merlin was more faint on this thing

[PRR]

OK, I had more coffee.

The standard Audio taper, in standard connection, gives 10% of signal at 50% of rotation.

If you connect it "upside down", it gives 90% of signal at 50% rotation.

Two such pots, opposite taper, give 90% of signal in center, shifting to 100%/0% at either end.

I had a Tascam mixer which used this technique for "pan pot", so a mono input could be shifted to Left or Right or center.

It is debatable whether the "ideal" response for a pan-pot (mono to L+R) is 90% in center. But this is not the place for that argument.

This "Fireball" circuit isn't a pan-pot. And it seems to be carefully designed to NOT need special tapers.

But again: do not trust me. Get a dual-Linear, two resistors, and a battery, try it.

[kagliostro]

I confirm further that your explanations are very sensible (I hope this is the correct term and you can understand me in the right way)

and I believe that there are very, very high probabilities (if not certainty) that what you say is exactly what is the reality

Thanks again PRR

K

[sluckey]

K, you do see how that dual pot works in that circuit, right? It needs to be a linear pot so that when the pot is at half rotation you have equal amounts of dry signal and effects return signal.

[kagliostro]

Ciao Steve

Yes, I see and I agree, to say that was a log/antilog pot was Mr. Merlin and I followed him

K