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Re: Harmonic Tremolo in a pedal?

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9/9/2000 6:38 PM

Re: Harmonic Tremolo in a pedal?

Corwin,
With the the cap and resistor values in the Fender Brownface oscillator circuit, dropping the pot values to lower ranges will cut down on the speed range. You can tweak the R + C values up or down, to try and recover some of the slow or fast speed,but ultimately you'll be affecting the width of speed ranges available by using the lower value pot. You'll ultimately come to a point that the circuit won't oscillate at all if you go too low with the pot value.

I'm using 4+ meg pots that I pulled from old audio gear; a lot of the old Magnavox stuff used CTS audio pots in the 3-5 meg range. Not RA, but they'll work.

As for the 10 meg pot, see KOC's mods in TUT2; he has it set up so you can use a 1 meg pot for the depth control. Use a lower value resistor off the CF, though. I believe he indicates using a 10 meg prior to the .02 blocking cap to the pot. I found 1 meg here makes the circuit come alive; the other value damps the LFO signal too much.

You might also try subbing other oscillator networks, like from the Ampegs or Magnatones. They use lower value pots and different value caps and resistors in the oscillator circuit. I haven't actually checked the available low frequency spread in any of these circuits, just seems by ear that the Fender will allow dialing in slightly faster or slower speeds than the others.

For a very complete discussion, I believe Randall A. has an exhaustive discussion on design parameters. let me know if you need the link.
cya.
Jon

9/14/2000 11:34 PM

I used a 3 meg ra pot for rate and built a 10M Ra "pot" from single pole 12 position rotary switch.

I simulated the taper using 820K, 1M, and 2.2M resistors between each of the switch contacts. The first and last contact point are essentially the outer two lugs of the potentiometer and the center contact is akin to the wiper. I obviously don't get the infinite resolution that a continuous pot would have but eleven positions seems to be fine enough. (The most ccw position is off) It might be different if it was a rate control, but since it's for depth I haven't yet said, "I wish I could get right in between these two settings."

I had also found a 10meg s-taper pot in an electronics surplus shop, but didn't like the taper so I went back to the switch.

hope this might help some

steveR

9/10/2000 9:35 AM
Fopoman	How did you power the "brown era" pedal? I've wanted to build a pedal with some sort of fender trem. thanks Fopoman

9/10/2000 9:56 AM
Fopoman	Jon, do you happen to have schematics for the tremelo? thanks again, Fopoman

9/10/2000 1:34 PM

hi,
I'm using small Chicago Standard channel mount trannies for all my tube footpedals. The ones I have measure 2.25 X 2 X 2 inches or so. There's an unequal CT on the secondary, with rectification and twiddling the R-C values, I usually run between 200-300 plate volts on the dual triodes or pentode/triodes.

I do the design work with a regulated variable DC supply to see if the plate voltage affects the behavior of the effect. On the trems, anything 160-220 plate volts has worked fine for me. My tube wha was real finicky. These trannies will handle 4 12AX7's.

The main consideration has been the footprint of the enclosure, proper placement of components to minimize hum, and shielding. Nowadays, I'm welding up my own steel boxes (done about 10 to prototype different circuits)as nothing commercially available suits my needs. This gives me the option of setting all my threaded studs prior to finish "body work and paint", so no screws are visible in the finished product.

I have been advised that my setup is not up to the various consumer product codes as I use detachable power cables. Since they're solely for my use, I'm not going to bother with the AC wallwart and multiple voltage multiplier circuits that would be necessary for a commercial offering.

As for schematics, I'd suggest picking up KOC's TOT, TUT and TUT 2. I found that by using his free-standing reverb wet/dry splitter/mixer as a guide, I was able to sub in the Fender circuit as he's presented it, with minimal changes in component values. You should really experiment here; for me the learning process was invaluable.

I'm sure these things can be done with small signal transistors, to get around some of the problems I've noted above. Instead of going in that direction, I find myself way more interested in exploring what can be done effect-wise with tubes....
Cya,
Jon

9/11/2000 5:02 PM

I have built a couple of tube pedals also, and Jon is right, the packaging is the toughest part.

I use Hammond 1456 slope front enclosures, which are not ideal, but readily available. I add an internal divider, another piece of sheet metal with three breaks, for more mounting surfaces.

I have found that if you use the proper 3 prong AC power cord there is usually a ground loop with an amp, formed by the ground prongs of the two power cords. This can be addressed by transformer coupling out of the pedal.

If you want to attempt this, I recommend making very detailed layout drawings or making a cardboard mockup of your enclosure to ensure all the components fit. It requires very tight packaging to get the pedal down to reasonable size, much tighter than a typical amp.

9/11/2000 7:36 PM

Hi Scott,
One further suggestion for layout: don't mount anything permanently at first (velcro?) or plan on trashing your first few enclosures (it took me a long time to work out a general layout scheme that works reasonably well) with tons of holes till you find the best orientation for components to minimize hum, oscillation, etc. I dry-fit everything at first (can't draw anything to scale).)

The good news is that tube effects when done right tend to enhance "tube feel". I even like my solid state amps better with them...
cya,
Jon

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