[sdiy] Varistors in tremolo circuits: worth it? Merits of various tremolo methods?

[anthony]

I have a nice stash of many varistors that I've collected from countless old 
power supplies & surge protectors & such.

I noticed that Magnatone used varistors in at least one of their amps - I 
think it was for the tremolo circuit. How does the sound quality compare to 
other tremolo circuits? Is it worth the hassle of figuring out what the 
specs of each of my varistors is and trying it out? I have quite a few 
SNR130K20's.

This made me wonder about a comparison of all of the methods used for 
tremolo circuits in tube amps: how do they compare? Which is better? Or is 
it more of a Beethoven vs. Bach comparison?

I notice that most amps that have tremolo are push-pull amps and the LFO 
modulates the bias on the power tubes.

Single-ended amps with tremolo do it by varying the cathode bias of the last 
gain stage before the power tube.

There is definitely room for a big difference in the way the tremolo could 
sound between these methods.

And then there is the lamp/LDR method that seems to be a lot less common in 
tube amps, although many great tremolos have been made based on LDR's. I 
think both neon and incandescent lamps have been used. I would guess that 
neon would yield a nice and choppy tremolo, but wouldn't be reliable over 
time. Same with the incandescent lamp really.

And THEN I started thinking about tube amps with tremolo AND reverb and how 
the reverb sound is modulated along with the dry signal just the same, but I 
would think that it would be better to have the tremolo bang on the reverb. 
My studies of Hammond Organ schematics have revealed their clever way of 
doing this: they just take the output from the main amp, use a light bulb to 
reduce the volume and drive the reverb tank straight from that - no reverb 
transformer. The reverb output goes to a separate amp and maybe even its own 
speaker. But it made me think you could make a groovy little(ish) 2x10 combo 
amp with a tremolo/dry sound in one speaker and another amp with the reverb 
sound, taken from the secondary output of the 1st amp's output transformer 
straight into the reverb tank (with attenuation of course). You could 
probably do it with one speaker by mixing the reverb back into the power 
tube.

Lastly, I want to make a dual clone of a Gibson GA-5T Crestline (have the 
6AQ5A's and the 6X4's...). I want to make the tremolo(s) work independent, 
in unison (just one LFO) and then panning. One thing I don't think I've seen 
a schematic for is a tube circuit that shifts the phase of an LFO signal 180 
degrees. Pretty much the same way you'd do it with solid state parts?

SpongeBob SquarePants,
Anthony