Tremolo in small SE amps ?

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JiM
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Post by JiM »

Hi folks,
Besides all the classic mods for all these 5W class A tube amps (you know : AX84, Epiphone Valve Jr, Harley Benton GA5, Blackheart, Ibanez Valbee, Peavey Nanovalve, etc.), i was wondering how i could add some vintage tube tremolo circuit.
(briggs, you're not the only one with a hole to fill in a Valve Jr. :wink: )

Most of the tremolo amps are in fact in push-pull configuration, and the class AB bias point is modulated by the LFO.
I've eventually found 4 ways to get tremolo in a single-ended configuration :
  • Modulation of the Cathode bias of a preamp tube. This requires a double triode, one for LFO and one as cathode-follower. Examples are the Fender VibroChamp and the Supro S6616.
  • Differential amp modulation. Requires only one triode as LFO, sharing its cathode resistor with a preamp tube. I seems to me that Intensity adjustments affects the overall bias of the stage ? Examples : Marshall 18W and Kalamazoo-2
  • Master-volume like. Also one triode as LFO, mixed with the signal before the power tube. Examples : Gibson GA1-RT aka Maestro M1-RVT (note the clever use of triode-pentode tubes)
  • Some also apply LFO to the screen grid of a preamp pentode (Vox AC-4), but it's not an option for me as i have triodes.
What are the pro and con of each circuit ?
Which one sounds better ? I've read enthousiast reviews of the VibroChamp, but none of the Gibson or Kalamazoo.
Which one is easier to tweak ? I'll have to read again the tutorial from Aiken Amps : http://www.aikenamps.com/PhaseShiftOscillators.html
It looks like the simplest designs only have a Speed control, is it possible to add Intensity also ?

I'm working on a Harley Benton GA5, and i have a handful of NOS Russian 6N3P. 8)
I guess the power transformer will not allow for more than one additionnal tube (the 6N3P needs 350mA@6.3V), and my plan is to use one triode for tremolo and one for some kind of booster/second channel, or as EQ recovery. Or you may have better suggestions ?
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KippeKiller
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Post by KippeKiller »

Hello,

i'd prefer the differential amp tremolo, with a 4k7 cathode-resistor it could probably also hearable affect the upper harmonics of that stage.

Intensity-control could be done via a 10k Pot wired as variable resistor on top of the 4k7 resistor, possibly.
As far as i've thought through it one would be able to change the bias of the oscillator by turning the 10k-pot while the tube in the signal path still sees the 4k7 to ground, perhaps one needs a bigger pot to get the oscillator into cutoff (? right spelling?).

Regards Mathias
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Post by RLBJR65 »

The VibroChamp is a bit different than the common Phase Shift Oscillator. The way it's drawn on the factory schematic sucks so I redrew it.
http://sites.google.com/site/diyjunkie/ ... edirects=0
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JiM
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Dallas Rangemaster (w/ noisy OC75, negative ground)
SubCaster tube booster (w/ NoS russian tube, PtP)
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Post by JiM »

Thanks !
I see it as a "standard" phase-shift oscillator, into a cathode-follower. I guess a low-impedance output is required when modulating the cathode of a preamp tube.
More about cathode-follower : http://www.freewebs.com/valvewizard1/dccf.htm

About the 1M resistor that does not go to ground, it's for a faster start of the oscillator, as explained here : http://www.freewebs.com/valvewizard1/trem1.html
I like the LED-biasing thing at the bottom of the page, it looks like both easy and cool.

More math about tube LFOs : http://www.aikenamps.com/PhaseShiftOscillators.html

As i expect the power transformer to allow me only one more tube, and as i'd like to add also a cathode-follower in (but the signal path) to drive a tonestack (sort of), it got me thinking ...
Maybe it's silly, but what about stacking these ? Like a cascode, or a SRPP circuit ? Then you could have both low-impedance output and amplitude modulation with one dual-triode.
Obviously i should do the math first, and the "INTENSITY?" pot looks wrong, but in the meantime what do you think of this quick sketch ?
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srpp-tremolo.png
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RLBJR65
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Post by RLBJR65 »

Thanks for the links!
:P just looking at all that math gives me a headache :lol:

Your design Looks interesting, I don't know enough to say it will work for sure, however it looks to me like the tone stack would need to conect at pin 3 as well. Then just make the intensity pot a variable resistor.
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