Moving Tremolo Drive on Hoffman AB763 single input

[RoadShow]

Good Afternoon,

I'd like some advice on moving the tremolo drive on the Hoffman AB763 single input.  I had disconnected it to resolve an issue, and also added dual bias to the design.  Also, when I originally built it I had added a MV.

My thought is to add it per some other AB763s such as the Tremolux where it's introduced before the 220k mixing resistor.

Will "AB763 PS - BA ver 2 - dual bias w-Trem.pdf" work as drawn?  Should I add a 220k in front of the 1M MV?

I'm not much of a tremolo guy, so if I can't hook it back up there's no sleep lost...

Thanks for looking...

[stratomaster]

You have a fundamental misunderstanding of what's happening with these tremolo circuits.

The Hoffman circuit users a tremolo oscillator to adjust the bias point of the output section.  This is literally fluctuating the output power.

The tremolux uses a similar oscillator to drive a light bulb that flashes onto a light dependent resistor that shunts signal to ground in time with the oscillations.

You can't take one circuit and just transplant it over to the others' insertion point.

You'll need to replicate the complete tremolux circuit on the Hoffman board to get it to behave like the tremolux.

[RoadShow]

You have a fundamental misunderstanding of what's happening with these tremolo circuits.

The Hoffman circuit users a tremolo oscillator to adjust the bias point of the output section.  This is literally fluctuating the output power.

The tremolux uses a similar oscillator to drive a light bulb that flashes onto a light dependent resistor that shunts signal to ground in time with the oscillations.

You can't take one circuit and just transplant it over to the others' insertion point.

You'll need to replicate the complete tremolux circuit on the Hoffman board to get it to behave like the tremolux.

Yes indeed, WTH was I thinking 

I've redrawn the schematic with the tremolux version, attached... hope I didn't miss anything 

So, to my original question, do you think I need to include the 220k mixing resistor, can I just tap in to the top of the 1M pot, or what do you think

Thanks for looking...

[stratomaster]

You don't need the mixing resistor, but you might want to include a resistive voltage divider there.  The originals have a non trivial amount of attenuation at the passive mixers. A voltage divider is made by the two mixers and the plate resistor to HT of the opposite channel. 

Since you have a pre phase inverter master acting as a voltage divider, you probably won't notice a difference unless you're at full volume.

I'd say build it and see how you like it. You'll have a bit more signal on tap than a traditional Fender at full volume, but you're also in control of that with the MV.  If you find it gets too hairy and out of control, you can always add a resistor before the input leg to limit the upper end signal.

Finally, the driver stage of the tremolo circuit benefits from a lower value cathode cap.  The later versions used 5μF (4.7μF is the nearest standard value), but I just use an 8μF MOD cap as they've been cheap, easy to get, and reliable enough so far.  This will help with the tracking with speed knob changes.  I find sometimes I lose the oscillation entirely if I move the knob too fast unless that cap is reduced.

A see you're also hinting at LED biasing the oscillator. That is a good move.  I have also taken to using large cathode caps there as an alternative to the LED when restoring the originals. A 250μF cap in the cathode network performs very similarly to an LED provided they bias the triode at the same point.

[RoadShow]

You don't need the mixing resistor, but you might want to include a resistive voltage divider there.  The originals have a non trivial amount of attenuation at the passive mixers. A voltage divider is made by the two mixers and the plate resistor to HT of the opposite channel. 

.
.

A see you're also hinting at LED biasing the oscillator. That is a good move.  I have also taken to using large cathode caps there as an alternative to the LED when restoring the originals. A 250μF cap in the cathode network performs very similarly to an LED provided they bias the triode at the same point.

Thanks for the feedback and suggestive tips.  I'll get the LDRS from AES, breadboard it and tie it in.  Once I'm sure I'll transfer it to the turret board (I already laid out the change) It'll be a couple of weeks before I know where it's at.

Thanks again...

[SEL49]

You may be interested in this...

I modified a Hoffman single channel AB763 to use the Trem-O-Nator tremolo circuit. Sounds much smoother than that choppy neon roach. Here's a link to more info about the Trem-O-Nator as well as sound clips. You may really be interested in the demo by "purpletele" since he built the single channel AB763 exactly like the attached pdf.

https://sluckeyamps.com/trem/trem.htm