Maybe this is a dumb idea, but is it possible?

[Tyrannocaster]

Some of you know that I like to do Danny Gatton-style (or really, to me it is Jeff Beck-style, since that's where I learned it) tone control rolls. (Example here: https://www.box.com/s/a281b15438d32f08d6eb ). Anyway, what I want to know is this: could it be possible to create a "tremolo" circuit in the amp that wiggles the tone control instead of the output stage bias? Obviously, I'm talking about something totally different from a normal tremolo and I have no idea how this could be done, which is why I'm posting this as a question. But I was listening to some stuff I recorded the other day and there was one section where I just wiggled the tone control like a tremolo rather than with my phrasing and I thought "That would make a nice effect!".

The tone control rolls wiggle a pot physically and I can't see any way you could do that in an amp. But what I don't know would fill many, many books. So do you need ICs and newer technology to pull it off? Nevermind, if that's the case, I guess.

Yeah, I know, another wild hair idea...

[Ed_Chambley]

Most people use Auto Wah pedals to extreme.  The problem with this technology is you don't have the control.  I have a midi expression pedal you can set in the processor where the swing is minimum.  The Rocktron Voodoo Valve will do this, but here you have to be running stereo and the processor to one cabinet and your amp to the other or you end up with a processed sound.  that is the only way I have ever been able to figure out how to do tone rolls and have control.

[tubenit]

1st answer is ask Sluckey.  He knows amps great and has some experience with the "Maggie" amps which has a true pitch shifting vibrato.

2nd answer is that Physconoodler did an amazing vibrato circuit in his amps that had a phase like feature to it.  Maybe he would share what he did.

3rd answer is you can modify a wah pedal to have much lower gain and get that effect and of course control it with your foot.

with respect, Tubenit

[thelonious]

The tone control rolls wiggle a pot physically and I can't see any way you could do that in an amp.

You can't wiggle a pot with a tremolo circuit, but you could "wiggle" an optoisolator or photocell with one. I'm sure there are more elegant ways to do this, but as a starting point - what if you started with an opto trem circuit like the one from an AA763 Tremolux (see schem from the library at http://www.el34world.com/charts/Schematics/files/fender/TREMOLUX_AA763.pdf) and made the photocell do something else?

It looks to me like that circuit works by varying the bottom resistor of a voltage divider, shorting more or less signal to ground as the tremolo pulses the lamp. If you added a small cap (120p? 500p? 1n?) between the trem circuit and where it connects between the preamp stages, it would only act on high frequencies, leaving the lows untouched, and you'd have a "tremolo filter". Maybe. Anyone want to comment on whether that would work or not?

Edit: Maybe something like the attached

[HotBluePlates]

EDIT: Thelonious beat me to the post.

If you only need (even as a first step for development) a fixed rate of tone-wiggle, yes.

Why not take a tremolo system, using a vactrol optoisolator, and use the resistance element in series with a cap to ground? You keep the same oscillator circuit, but couple the optoisolator to a (maybe grid) signal wire through a cap. The cap value is probably something you'd need to experimentally determine.

You might say, "but this sounds like a blackface trem, which has a choppy sound." Yes, but the blackface uses a neon bulb; an incandescent bulb or LED in the optoisolator should smoothly go up/down rather than on/off. You could make this setup do on/off (if desired) by using a sqaure wave instead of a sine or triangle wave to control the light source.

Hell, there might be a good way to drive both this auto-tone circuit and your trem from the same oscillator. But that probably depends on the specific optoisolator used, and how much current is being controlled by the oscillator circuit.

[Tyrannocaster]

Thelonious and HBP, it sounds like you've grasped what I'm talking about. It's not auto-wah, at least as I understand the term (and as it is used in my J-Station, where I never use it because I think it sounds awful), it's just a fixed rate with pass wiggle whose rate you can set.

I am very interested in this. I hope I can understand the explanations; I often feel as if HBP is speaking an entirely different language, which isn't far from the truth given my lack of understanding of this subject.

[fdesalvo]

Awesome idea - love to hear what it would sound like. 

Pardon the pun, but I believe I just had my own light bulb moment - incandescent and LED based trems are smoother because of the slower decay of the light as it turns off.  You can see that happening in you own living room by throwing the switch.   :faceslap   

[HotBluePlates]

... incandescent and LED based trems are smoother because of the slower decay of the light as it turns off.  ...

And on, too.

Neon doesn't turn on until the voltage across the bulb exceeds a firing threshold. Then it's full-on, even if the voltage across the bulb drops slightly. Once the voltage across the bulb drops far for turn-off, neon extinguishes the light very quickly.

The same could be true of LED, but you don't work them that way. LEDs need a certain minimum voltage across them; but if you feed them more and less current, the brightness will go up and down.

And with incandescent, they're not instant on-instant off. But mostly, what is of use is there is some range of middle-ground between off and on.

It's that middle ground that allows something other than the sharp stutter-trem of neon optoisolators.

[TIMBO]

Hey Tyranno, I'm really going out on a limb here but better people than I have come up with some really unique ideas when it comes to amps and effects. Neil Young was UNIQUE and came up with his "WHIZZER".

I call this the "WIGALATOR" (thunderous applause)  Laughing at myself hope you are.
Back to the topic.

The effect is made by the TONE POT turned back and forward fast or slow to create the "WIGGLE". NICE clip by the way 
A tone pot could be attached to a small motor and with some simple veriable speed electronics (easy to find at the local electronics store)that would move the pot control back and forward.

This could be a separate stand alone unit that you have between guitar and amp, but i think you might need some more amplification in the device to boost the signal, may be a tube???  I think the LIMB is starting to CRACK. Thanks

[PRR]

Some of Fender's oldest tremolos split the signal to Bass and Treble, then alternate between.

[Tyrannocaster]

Thanks, Timbo; glad you liked it.

I know about Young's Deluxe but a mechanical solution doesn't seem like the right way to do this, given how many times it would have to wiggle that knob during a typical usage. (Let's see: three minute song, 6 hz trem, 3x60x6= 1080 wiggles. It would almost certainly go out of adjustment and require constant tinkering, which Mr. Young can pay a tech to do and I can not.  But I thought his amp mostly changed settings like a person would, not like a tremolo might.

I like Thelonious' idea, which seems feasible at least. Anybody have a Blackface already open who would be willing to try it? Seems like a lot to ask someone to open up an amp just for this. I know how much work it is to pull the chassis in my Twin and until I hear more I don't think I would try it myself.

I wasn't aware that there are LED and incandescent trems out there; I thought the optical ones all used neon. That IS what Fender uses in roaches, isn't it? Can someone name names?

PRR, I love the sound of those old two-tube trems (I owned a black Tolex Vibrasonic for years - it must have been a transitional model) but I must admit they are intimidating to me, who has had trouble getting even single tube circuits to work. Heck, I've had trouble with single triode circuits. That they also use up to two and a half tubes means there's some serious real estate inside the map dedicated to the tremolo. No wonder they went to the roach, from a cost standpoint.

[sluckey]

Sunn used an incandescent trem in a widely available plugin module. Unfortunately the module is now extinct. I replaced mine with a Vactrol VTL5C1 which is a LED/LDR optoisolater.

http://www.el34world.com/charts/Schematics/files/sunn/sunn_sceptre.pdf

http://home.comcast.net/~seluckey/amps/sunn/sunn_sceptre_1971.pdf

[HotBluePlates]

... a Vactrol VTL5C1 which is a LED/LDR optoisolater. ...

And available here at Hoffman's.

[Tyrannocaster]

Can those be used in circuits that call for neon bulbs? Doug has a lot of cool little parts on that page.

[sluckey]

I haven't tried this but several others have, including phsyconoodler, I think. If this works as advertised, then you should be able to use the VTL5C1 also...
=============================

1.  Cut open the trem bug.
2.  Remove the neon lamp.
3.  Replace the neon lamp with a red LED. (See notes.)
4.  Tape up the modified trem bug.
 
Note 1: The LED is polar, so it'll only work one way. Before you tape up the trem bug, warm up the amp and see whether the LED blinks. If it doesn't, reverse its leads.

Note 2: You should pick an LED that's about the same size as the photocell(s) and frosted so it diffuses the light to cover the cell(s). I spec'd red because the red LEDs seemed to be brighter than the yellow and green LEDs that came in the same blister pack I bought from Radio Shack.

This fix works because the LED is a current-sensitive device and the trem circuit contains a 100K resistor to limit the current to an appropriate value for the LED. The ticking is gone because there's no sudden large voltage change across the LED (as there was across the neon lamp) to get coupled through the photocells into the sensitive audio circuitry.

[HotBluePlates]

Note 2: You should pick an LED that's ... frosted so it diffuses the light ...

You can turn a clear LED into a frosted LED by sanding.

I did this, except with a file, when I needed to shape some LEDs recently.

[Tyrannocaster]

Cool; I have a '70 Dual Showman Reverb that could use a new roach. I have put that info in my scrapbook.

[sluckey]

You don't actually have to destroy the roach to see if this idea works. Just unsolder the neon bulb wires and connect a LED across the eyelets. If it does not flash, reverse the connections.