Simulating a 10 meg pot with a rotary switch

[Paul P]

The tremelo circuit I'm using requires a 4 meg reverse-audio pot for the speed
control and a 10 meg reverse-audio pot for the intensity control :

http://music-electronics-forum.com/attachment.php?attachmentid=1988&stc=1&d=1205288758

I've bought these pots from Weber but am not terribly impressed with their
quality.

I'd like to simulate these pots using a rotary switch for a range selection and a
1 meg reverse-audio pot for fine adjustment. This works well for the speed
control, which is wired as a rheostat :

http://music-electronics-forum.com/attachment.php?attachmentid=1989&stc=1&d=1205288758

(leaving aside the fact that the resistors could be adjust to better reflect a
reverse log taper)

I have not been able to come up with a similar circuit for the intensity
control since it's wired as a potentiometer :

http://music-electronics-forum.com/attachment.php?attachmentid=1990&stc=1&d=1205288758

I could use just the rotary switch as a stepped attenuator but I'd like to use
a pot for fine adjustment within a step.

Is there any way to do this ?

Paul P

,

[d95err]

I think your overcomplicating things a bit. For the speed control, try a 3M RA pot. These are available from various sources and not too expensive. The basic oscillator is the same as in every other Fender amp, and they all use 3M pots. If you *really* need 4M, use a 3M pot and add a switch to insert an 1M series resistor.

For the depth control, I think fine tuning is not really necessary. Lots of old amps had no depth control at all for their tremolos, others (e.g. Vox) had just two preset depths. So with a 10 position rotary switch, you'll probably get all the control you need. However - don't use 10 x 1M resistors. That would give you the same sweep as a linear pot. That would result in probably just one or two usable switch positions.

To get the correct reverse log taper, use a series of increasing size resistors, e.g. 100k + 180k + 330k + 560k + ... that make up a total of about 10Meg. The total resistance value should not be that important. Anywhere between 5M -15M is probably OK.

[Paul P]

Thanks for the reply d95err. I just got the tremolo circuit up and running
and I can now agree that you don't need all that many different intensities
and that a fine tuning pot is unnecessary. I used a bunch of 1meg resistors
because I had them and I'll have to order ones to give me a better spread,
but it's pretty obvious that there's a lot of change at one end and little at
the other. Still, I should be able to use what I've got to figure out better
values.

Paul P

[Don Moose]

Still, I should be able to use what I've got to figure out better values.

Combinations of series and parallel? It eats up a lot of resistors, but if you have a bucketfull anyway ...

One thing I remember futzing with was a spreadsheet to calculate 20 or so values on pots of various fake tapers. I'll see if I can find that and post or forward it.

Hope this helps!