Tremolo Help (Cathode Biased Amp)

[clbraddock]

I built the tremolo shown here recently. I basically grafted the tremolo from a GA-19RVT onto my Rob Robinette Blackvibe Micro EF80. The tremolo itself works and sounds good, but there is more "whooshing" than I would like. The amp has a lot of carbon comp in it and is not the quietest amp on earth to start with. I think the tremolo is boosting and cutting the background noise, thus the whooshing. With intensity on 0 there is no whooshing, so it is not a problem with "ticking" or with the wiring proximity.

My question is if I can reduce the whooshing by moving where I insert the tremolo voltage earlier in the circuit. I'm looking at the schematic of the vibrochamp AA764 and there the tremolo is inserted into the cathode of the cathode biased second preamp triode. Could I just disconnect my trem output from the powertube grid leak (and reconnect grid leak to ground) and then connect it to the cathode pin of the second (or first?) preamp triode? Or would I need to make other changes? Or would moving it earlier like that not make a difference to the whooshing? 

Also open to any other suggestions.

Thanks!

[sluckey]

Is there a ceiling fan in the room?

[shooter]

with no input signal and trem knobs around 5, is the whooshing in time with the trem?

[clbraddock]

with no input signal and trem knobs around 5, is the whooshing in time with the trem?

Yes, it is in time with the tremolo. Thats why I think it is the trem boosting and cutting the amplification of the amps inherent noise. My thinking is that moving the tremolo earlier in the circuit there would be less noise to be boosted/cut. I could be wrong about that though.

[tubeswell]

Vibrochamp trem is less whooshy  - trem is injected at preamp cathode, so the trem driver (or buffer) has to be designed to shift pre-amp bias voltage into cutoff (which is less than required to drive output tube into cutoff)

[clbraddock]

The tremolo on max intensity puts out about 19VAC peak to peak. I’m guessing that is too much to insert into a preamp cathode (although I thought it would be to much for the power tube grid leak too and it works fine there) Do I need to drop the voltage more before I try connecting it to the preamp cathode?

Also, you mention the tremolo buffer/driver (which I know is in most fender tremolos) but I am using a single triode, so no buffer/cathode follower. Would that be a problem?

[tubeswell]

... single triode, so no buffer/cathode follower. Would that be a problem?

No. But you may need to experiment with the level shifting of the LFO 'signal output' to ensure it's right-sized for the pre-amp cathode biasing. This may take a bit of fidgeting. The thing you want to achieve is a signal swing output that's big enough to drive the preamp bias into cold enough territory that it cuts the tube current off (when the LFO pulses are peaking)


However, a MOSFET source follower is a cheap and easy way of getting a buffer without needing another tube stage

[pdf64]

Maybe it’s just too much modulation being applied?

How about if the cathode is bypassed? It may not make any difference 

Moving the modulation to earlier in the signal path seems more likely to cause thumping 

[shooter]

any chance you can disconnect the MV for testing?

[sluckey]

Most trem circuits use a smoothing cap on the trem signal.

Example... https://el34world.com/charts/Schematics/files/Fender/Fender_vibroverb_6g16_schem.pdf

I would experiment by connecting a .1µF cap across the INT pot outer lugs. Easy to do. Might help.

[Bieworm]

About preamp or powertube bias tremolo.. the amps I build with preamp tube bias tremolo all whoosh to some degree. The ones I built with power tube bias tremolo never do...
YMMV...

[clbraddock]

Most trem circuits use a smoothing cap on the trem signal.

Example... https://el34world.com/charts/Schematics/files/Fender/Fender_vibroverb_6g16_schem.pdf

I would experiment by connecting a .1µF cap across the INT pot outer lugs. Easy to do. Might help.

This seems like the easiest thing to try. I will try that and report back.

[clbraddock]

Finally got around to adding the .1 uf cap across the intensity pot and I think that fixed it. There is still a very very faint whooshing, but it is so quiet now I don’t think it could realistically be made quieter.