Solid state tremolo oscillator for tube amp

[st]

New forum member; introduction is here: http://el34world.com/Forum/index.php?topic=22897.0

My most recent project is the conversion of a Hammond AO43 amp into a guitar amp, basically a Fender Princeton preamp/PI into a push-pull, cathode biased EL84 power amp. It also has reverb (mostly from the stock schematic, i.e. driven through a cap by a paralleled 12bh7 with plate choke).

I now plan on adding a solid state, IC-based trem, preferrably one that works by bias-shifting a preamp tube. I have NOT been able to find a lot of info on such a thing, here or elsewhere.

Weber had an add-on kit that did just that in the past. I had a hard time finding info or a schematic, but I did [file attached].

I also found this blog post (schem included) of someone who reportedly did one: http://abrammorphew.com/notes/2013/05/19/solid-state-tremolo-for-tube-amps/

So my questions:
- Why is it a bad idea?
- forseeable problems? Things to consider?
- What kind of voltage swing am I after to wiggle a 12ax7 cathode bias enough to have a decent effect?
- Both use half a filament supply (3.15VAC) and a voltage doubler to power the circuit; why not use a two-wave rectifier to gain an additional volt or so?

[shooter]

good wiring, grounding and shielding, when you move small signals with longer wires you invite unwanted guests.
2-3vac should be enough to smack any early stage preamp tube.  Not sure about IF there is a bypass cap on your selected cathode, what "issues" that might have, gut says you don't want a bypass cap

I'd build what has worked for others, then get creative

[st]

Thanks. 2-3VAC should be easy enough with a dual opamp.

I'd build what has worked for others, then get creative

That's the thing: can you name a single example? Preamp cathode wiggle trems are relatively rare (18 Watt marshall is one of them) in their owner right. Solid state much more so? The blog post i referred tot is the only build report/example i have ever seen...

[sluckey]

Preamp cathode wiggle trems are relatively rare (18 Watt marshall is one of them) in their owner right. Solid state much more so?

Look at the Valco, Supro, Silvertone schematics for some examples of preamp cathode wiggle trems. I know the Supro S6424 sounds good...

     http://sluckeyamps.com/supro/supro.pdf


Sunn has a SS tremolo circuit that drove an opto-coupler (incandescent bulb/LDR) that was put in the preamp circuit. The opto-coupler is obsolete but there are several workarounds. Here's mine...

     http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf

Maybe these circuits can give you some ideas about how to adapt your circuit to a tube amp. Another idea is to put a passive FX loop in your amp and plug in a trem pedal.

I've done several conversions with the AO-43 amps. Have you considered just adding a tube based tremolo?

[st]

Look at the Valco, Supro, Silvertone schematics for some examples of preamp cathode wiggle trems. I know the Supro S6424 sounds good...

Thanks for the Valco/Supro reference. I will definitely look into those for inspiration. At first sight at least your  Supro appears to confirm that the cathode wiggle will work satisfactorily even if Rk is bypassed by a big cap (as it is in my Fender input stage).

Another idea is to put a passive FX loop in your amp and plug in a trem pedal.

I've done several conversions with the AO-43 amps. Have you considered just adding a tube based tremolo?

I already added a tube just for signal amplification. Problem now is: amp is done, dead quiet, and I don't want to add new stuff that potentially messes this up. More importantly: the idea of an SS lo-v osc has been keeping me for a while. It's going to get built somewhere in the next few weeks anyway (and I will report here), but I was just after some more ideas, warnings, etc.