Harmonic Vibrato

[The Radium King]

hi folks. heard some smart fellers hang out here and i'm in need of some smartness. i have an amp with a two triode opto-isolator vibrato circuit and a spare triode, and i want to buld a harmonic vibrato circuit ala brown tremolux.

from what i can see in these circuits, they use a two triode oscillator into a split load pi, to create two inverse sawtooth waveforms. they also send the preamp signal into a frequency divider to create a bass and treble signal. each signal is modulated by one of the sawtooth waveforms, so that bass is increasing as treble decreases and visa versa. the two signals are then combined in a tweed mixer and sent off down the amp.

so, i'd like to use my spare triode as the split load pi, and avoid using the tweed mixer (don't have the two addtional triodes). if the two outputs of my split load pi each drive an opto isolator and i use a 2-gang intensity pot, i have to somehow set up my preamp signal so that i can independantly ground either treble or bass signals without attenuating the ungrounded signal. make sense? i figure i can take a 150 pF (or so) cap to one roach to ground the treble in the signal, but how to i get the bass? take an inductor to ground? what size? lemme know if y'all think this is workable.

[The Radium King]

ok, how about sending the vibrato signal into a split load phase inverter to create inverse modulation waves. each wave goes through a led/ldr assembly and an intensity pot (two gang, 5 Mohm) and is used to modulate the tone controls. that is, one ldr grounds the treble portion of the tone circuit between the treble cap and pot, the other ldr grounds the mid and bass. this way your treble will drop in/out inversely to your low ranges. harmonic vibrato like on the old fenders, but with three triodes instead of five.

next, how about using just one triode as an oscillator? early fender circuits only used one. the second triode * appears * to be a cold bias stage that severely and asymmetrically clips the oscillator output. the end result is an on/off waveform. can this not be obtained using a single triode with a zener on the cathode that avelanches such that the output is either on or off? also, would tone not be better if the modulation output was more sinusoidal; ie, a gradual increase/decrease in signal? an ldr is a linear device, so it should work.

if a suitable output can be obtained with a single triode, that leaves the second triode to use as the phase inverter and harmonic vibrato can be achieved with a single tube. better yet, use the second triode as a second oscillator to achieve your inverse modulation waves. add independant speed and intensity controls and you could get all kinds of cool effects, all with a single tube.

[dave////irongater]

Yeah hopefully someone a whole lot smarter than me. I am smart enough to love brownface vibrato!

[Ray Ivers]

Is this the circuit you're referring to (6G4 Super, 6G5 Pro, 6G12 Concert, etc.) ?


If not, could you supply a Tremolux designation? I couldn't find any Tremolux prints with other than the conventional oscillator-only Fender tremolo circuit.

Ray

[The Radium King]

sorry 'bout that, i was referring to the pro series; most of my analysis is of the 6G5-A circuit. the 6G5 circuit you've posted looks similar. from what i can see, an oscillator, a triode being used to create an inverse signal and a tweed mixer circuit.

another bad i made is calling the ldr a linear device. it is, but neon lamp or led driving it is on/off. probably why fender stuck with the two triode vibrato when it went to opto modulation - to more definitively trigger the lamp. they probably started with two triode oscillators when they went to bias modulation - to avoid taking the tube through varying states of bias. i'm wracking my brain on how to make a proportional trem roach; an array of led's with varying forward voltages to get discrete resistance changes in the ldr?

[Ray Ivers]

Well, what I see is basically a LFO modulating a gain stage to which dry signal is sent, and a polarity-inverted LFO waveform modulating another gain stage to which dry signal is also sent. The output of the two antiphase-modulated gain stages are then mixed and output to the PI and power stage.

If I were you, I would use your three available gain stages for the LFO (or LFO inverter) and modulation stages, and an LND150 or similar MOSFET for the LFO inverter (or LFO) - but that's just what I would do, there are surely other solutions as well.

Ray

[mikmike]

harmonic vibrato

You are always going to have the same problem empirically with this circuit because it is almost completely current driven on the low frequencies.
If you do not equate that with the power amp it will sound like ass.
Moreover, and probably more so, any attempts to intercept dry signal changes Z
so much so that a 2nd or at least quasi phase splitter will always be necessary,
and so far original circuit COUPLED with power amp is almost still best version of this vibrato and that will always be why.
I have experimented with "lite" versions of this topology and the only way I have found to get it is some heavy current on the front end and that doesn't always sound good for guitarzzz.

[ETR]

I wonder if you could just make something like a LTP oscillate but use high freq feedback on one side and LF feedback on the other. If it would still pass signal decently enough you could then just use a balance control on the output to mix. Haven't thought it all thru but it's just one triode and no LDRs, could be a low parts count solution. You may have stability issues with the 2 feedbacks tho.

[Merlinb]

i figure i can take a 150 pF (or so) cap to one roach to ground the treble in the signal, but how to i get the bass? take an inductor to ground? what size? lemme know if y'all think this is workable.

Put a low-value roach in series with the cathode bypass cap on one of the gain stages. Put a small (470nF or so?) cap in parallel with this permenantly. Treble freq's are then permentantly bypassed, while the trem signal controls the bass.
Actually, if you do that and you put a cap and roach to ground from the anode (to modulate the treble) then you don't need a phase inverter for the LFO, since with both lamps on you are shunting the highs and boosting the lows, and vice versa! You could bias the oscillator using two LEDs, each of which controls one roach.

[mikmike]

Might be a good idea, but you would have to be careful with tube and supply variations regarding the generator feedback considering it is an oscillator.
Maybe bring lots negfeedback pre frequency splitter ALA Princeton and you may get the brown sound, but your ground/layout and tolerances will be important.
ps: remember how high B+ is on those amplifiers to get it to do what it does with vacumm tubes unless you are considering SS alternatives.