My Blackvibe EF80 Tremolo Circuit

Clb8484

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Missouri
So I think I was successful on my first circuit mod I did myself. I wanted to add a tremolo to my stock Blackvibe Micro EF80 build. I use the tremolo circuit on the GA-19RVT as a starting point, since it is also a cathode biased amp. I pull the B+ for the plate on the tremolo circuit from B+4, but add an additional 100 ohm 1 watt resistor there to drop the voltage to 240 volts (as specified in the GA-19RVT) before it hits the Tremolo Plate Load resistor. Other change is I add a 1M resistor to the output of the tremolo where it connects to the level/depth pot to reduce the voltage being fed back into the power tube grids. Before adding the resistor I was measuring close to 50 volts peak to peak out of the tremolo circuit on my oscilloscope. The 1M resistor drops it to about 19 volts peak to peak, which still seems high to me, but everything works.

I've built and tested this and it works and sounds pretty good to me. Any problems or concerns or suggested revisions from people who are more knowledgeable.

Thanks!

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Clb8484

TDPRI Member
Joined
Dec 3, 2020
Posts
57
Age
37
Location
Missouri
After playing with this more I noticed at higher volumes there is a bit of a rhythmic whooshing going on even with intensity at zero. Not quite as bad as the ticking on some old fender amps, but in that ball park - just quieter. I think with this circuit design it may be unavoidable, but I’ll prob tinker with it some more.

One change I did so was adding a spst switch between the 500k pot and ground. Opening the switch kills the tremolo and kills the whooshing. So at least there isn’t whooshing when I’m not using the tremolo.
 

Clb8484

TDPRI Member
Joined
Dec 3, 2020
Posts
57
Age
37
Location
Missouri
So I’ve been tinkering with this some more trying to reduce the noise. Switching to a shielded cable between the circuit and the tube got rid of the whooshing when intensity is on zero, so that’s goood.

With intensity up to hear the effect, I still get whooshing. I am pretty sure it’s just that the amp has some noise and the tremolo at the power tube grids causes said noise to rhythmically get louder and softer.

I’m wondering if injecting the tremolo voltage earlier in the circuit would help. The vibrochamp Aa764 injects tremolo at the cathode of the second triode of the first preamp tube. Could I do that with my circuit, and would it reduce the whooshing any? I figure there is less noise earlier in the circuit to be boosted/cut by the tremolo.
 




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