Tremolooooooooo......

[gary sanders]

Im building a Vibro Champ and have little chassis room so if I could drop a pot that would be great.I want to use a mini switch for the "Intensity" control for the tremolo and switch between two resistors to replicate 50% and 100% of the control.How would I put these resistors/switch in place of a pot?

[Structo]

Looks like it is just wired as a variable resistor so a 25K and perhaps one around 12K would work.

Better yet, hang a pot off of the circuit board and experiment with different settings.
Decide what you like the most then measure the pot value where it's set and replace it with a resistor.
Then use a On-On switch to switch between the values you like.

[Firestorm]

You could also use a concentric pot and fit two pots in the space of one. Knobs for concentrics are a little pricy though and the pots won't come in the values you want so you have to cannibalize pots of the right value and swap the carbon tracks into the concentric. PITA (but I do it sometimes).

[ChrisM]

Looks like it is just wired as a variable resistor so a 25K and perhaps one around 12K would work.

Better yet, hang a pot off of the circuit board and experiment with different settings.
Decide what you like the most then measure the pot value where it's set and replace it with a resistor.
Then use a On-On switch to switch between the values you like.

It's actually wired a variable voltage divider (none of the lugs are jumpered). So you can't just sub resistors on a switch.

[tubeswell]

Looks like it is just wired as a variable resistor so a 25K and perhaps one around 12K would work.

Better yet, hang a pot off of the circuit board and experiment with different settings.
Decide what you like the most then measure the pot value where it's set and replace it with a resistor.
Then use a On-On switch to switch between the values you like.

It's actually wired a variable voltage divider (none of the lugs are jumpered). So you can't just sub resistors on a switch.

And its an audio pot so you need to think about the non-linear differences in resistance that you get at the different rotations of the pot

[gary sanders]

Looks like it is just wired as a variable resistor so a 25K and perhaps one around 12K would work.

Better yet, hang a pot off of the circuit board and experiment with different settings.
Decide what you like the most then measure the pot value where it's set and replace it with a resistor.
Then use a On-On switch to switch between the values you like.

It's actually wired a variable voltage divider (none of the lugs are jumpered). So you can't just sub resistors on a switch.

thats what I was wondering about,and I had an old Gretsch 6151 that only had a speed control and it was set to the hard tremolo effect and all you got to do was control the speed,id be good with that.maybe could compare the schematics and figure out what the intensity is controled by.Oscillators are like another language to me for now

[Phil_S]

Agree that it is a voltage divider.

Check out how a pot measures; let's assume the 25K pot. From outer lug to outer lug, it always measures 25K, no matter what the sweep, because the difference is found at the center lug - the wiper. Between the wiper and either outer lug, the sweep is from 0-25K. On one side "off" is zero, and on the other side "10" is zero. In this case, it's a reverse audio pot, so take note that it's the opposite of usual, but it doesn't change the basics.

I think, to mimic the pot on a switch, you will need to consider that the 50% position is actually 12.5K-0-12.5K.

To mimic the 100% position, you've got to make it 0 ohms on one side of the divider and 25K on the other side. I think this works as shown. I didn't have the symbol for a DPST swtich, so I used two SPST, but it works. On gives you 100%, with 25K on one side of the divider and 0 on the other. Off gives you the 50% position with 12.5K on either side of the divider. You should verify that the 100% position is 25K on the ground side. I may have got this wrong. Someone else will have to opine on this, as my brain just isn't working right on this particular detail.

I realized just now that it is an audio pot, so the 50% position isn't actually 12.5-0-12.5K. You'll have to figure that out and adjust the resistor values accordingly. At least you'll get something going here.

[gary sanders]

Makes sense,wil try that.Thanks Phil!

I would do the dual pot but schematic calls for a 3M speed and a 25k rate and I dont think anyone has ever made a dual close to that

[Phil_S]

I found a 25KA pot in my supplies that meters at 28K. It divides, as best I can tell, at 50% rotation, 24K/4K.

I think I would make R1 in the range of 3K-5K. That should give you a fairly good approximation of 100% position. I think I'd try a pair of 22K for R2 and R3. Given the 85%/15% split, I don't think you will be able to get that close to the 50% position. As I drew this out, using a pair of 22K on the ground side will give you a split of 22K/4K (about 50%) or 11K/0, which is somewhere else on the rotation of the pot and may not be enough cathode resistance to trigger decent oscillation. Trial and error will probably get you in the ball park after a while.

Since the 50% position is only at about 15% resistance, you may want to use one fixed 22K on the ground side and only switch the 4K in and out of the circuit. This should be a reasonably close approximation of the audio pot rotation at 50/100%.

I'd like to know how it turns out.

[gary sanders]

Will post results,waiting on a transformer to get here then I will be ready for a test drive

[martin manning]

Or maybe like this? I'm assuming a 20% taper RA pot, so the resistance to ground would be 80% of max at 50% rotation.

[Firestorm]

Makes sense,wil try that.Thanks Phil!

I would do the dual pot but schematic calls for a 3M speed and a 25k rate and I dont think anyone has ever made a dual close to that

Right. But what I've done is buy this onehttp://www.musiciansfriend.com/accessor ... 2000000000
and then swap in the 3M and 25K. You can do it fairly easily since it's a CTS pot.

But it sounds like you have a good alternate plan.