Vibro Champ Reverb Amp?

[Seiche]

So I've stumbled upon the Champ Reverb Amp Threads, one of them on offsetguitars, one here.

I would like to build a vibro champ with reverb. My strategy is as follows:
- take princeton reverb preamp stage including reverb
- add champ output stage with single 6V6
- take vibro champ LFO and add it to the second triode of the preamp just before the reverb link

From what I understand so far the main challenge will be getting the correct voltages to ensure good operation of each individual part (e.g. PR preamp has lower voltages than the (vibro) champs preamp, but the reverb input has a higher voltage of 400V in the princeton so the voltage might be taken from a different B+, or the power supply redesigned).

I have a attached a mockup schematic cut together out of the two schematics

Schematic_VCR.png

[martin manning]

Isn't what you are proposing pretty much a Princeton Reverb with a SE power amp replacing the PP?

[Seiche]

Yes. But the tremolo on the princeton is power amp bias modulation vs preamp bias modulation on the vibro champ.

[martin manning]

That'd be covered under "pretty much," I think. It could be done either way.

[Seiche]

I'm actually asking if this would work with the reverb or not, because I'm not sure

[martin manning]

The problem I see is that the Champ power stage only wants about 350V on the plate, and 300 on the screen. That's going to leave the reverb driver and tremolo oscillator with less than 300V to work with if you power them from the screen node. Perhaps a branched power supply would work.

[Seiche]

The standalone reverb seems to have voltages below 300v as well

[didit]

Those voltages at screen really ought to be adequate. I'd be more concerned about clean decoupling across preamp, tremolo, and reverb. Martin's suggest helps with that, and requires only one extra tie-point and an extra resistor plus filter electrolytic. The tremolo signal perhaps should get injected into final preamp V3B, thus avoiding putting it through the reverb circuit. Both these changes are easily tested experimentally with minor wiring modifications once you have it built. Learn by trying?

Best .. Ian

[Seiche]

The tremolo signal perhaps should get injected into final preamp V3B, thus avoiding putting it through the reverb circuit.

What's the advantage of this? It would be closer to the princeton in that way, but it feels more intuitive to have trem before reverb (similar to a pedal going into the amp).

The other thing I've pondered is the dry signal will not be pure vibro champ with that 3M3/10pf and extra gain stage. Maybe the 3M and extra gain stage should be in the wet path only, similar to the champ reverb on offsetguitars.

[didit]

The tremolo signal perhaps should get injected into final preamp V3B, thus avoiding putting it through the reverb circuit.

What's the advantage of this? It would be closer to the princeton in that way, but it feels more intuitive to have trem before reverb (similar to a pedal going into the amp).

It avoids putting tremolo through the reverb circuit, which is how all Fender and most other topologies are arranged. Try it both ways? See whether you appreciate the effect of your reverb being pulse pounded. And actually it will be precisely stock Vibrochamp. Analysis worked from power out back toward the input is more accurate to understanding generally, and certainly in this case.

Best .. Ian

[Craig B]

I built a similar amp (but "high power") with good success--eventually. I got a bit of unwanted distortion when using the VibroChamp trem circuit at maximum intensity. So I used the Vactrol-based trem developed by Sluckey and 2deaf. http://el34world.com/Forum/index.php?topic=24334.0 Their circuit partially grounds out the signal but does it more gradually and smoothly than the fender roach. I found that in my hot chassis the intensity diminished the longer I played. So I switched to a solid-state plug in rectifier to cool the chassis. That worked well and I enjoy the amp a lot. It uses a PT with dual secondaries. The high voltage readings are in parenthesis on my schematic:

Sumatra 10 Vibrochamp Vactrol 10-02-2019.gif

[Seiche]

And actually it will precisely stock Vibrochamp. Analysis worked from power out back toward the input is more accurate to understanding generally, and certainly in this case.

Hi Ian, I don't understand this sentence.

[didit]

And actually it will be precisely stock Vibrochamp. Analysis worked from power out back toward the input is more accurate to understanding generally, and certainly in this case.

Hi Ian, I don't understand this sentence.

Apologies, perhaps my words were a bit too terse. Also left a critical verb "be" out (which I've edited to fix). I mean that the last preamp triode is correct location for injecting tremolo per the Vibrochamp design and my recommendation. It is often better when comparing circuits to work through the signal chain in reverse starting with output power stage. As I interpreted what you had written it seemed clear you had worked forward from the input jack.

Looking at the schematic posted by Craig, that seems correct to me, perhaps other than his higher power output. I believe one could tame the tremolo without resorting to vactrol though. Particularly appreciate the use of silicon rectification.

Best .. Ian

[Seiche]

Ah gotcha

[jjman]

Trem before vs after reverb is important and a personal choice. Experimenting with pedals could be fruitful.