Help Build A Valve Reactor Pedal

[TomJS]

Hey, I am very curious if you'd be able to make a Valve Reactor pedal that you'd put in the effects loop. Since I quite like my VOX AC30VR I thought it would be a fun project to be able to add such a small power amp tube to any solid state amplifier. I do have a vox ac30VR schematic in the attachment is the power section which includes the valve reactor tube. But my question to you guys is is it possible to create a pedal with a valve reactor tube and if so what part of the schematic is part of the valve reactor circuit and what part of the schematic is part of the solid state power amp circuit? For more information on the Vox Valve reactor circuit visit this thread https://www.seymourduncan.com/forum/sho ... actor-quot

Thanks and regards,

Tom

[nworbetan]

I googled up a legible schematic with the preamp still attached.

https://music-electronics-forum.com/sho ... 222&page=2

AC30VR SCHEMATIC.PDF

I do want to give them a little bit of credit for using discrete transistors instead of opamps for most of the preamp. On the other hand, the "overdrive" channel uses clipping diodes instead of overdriving the 12AX7. One step forward, two steps back, or something like that.

While there is a preamp tube on the "power amp" page of the schematic, there's also a pair of TL072 opamps in the signal path after the 12AX7 and calling that pair of TL072 part of the "power amp" circuit feels like a bit of a stretch to me. I personally would say that the power amp is the LM3875TF chip and the dozen or so components supporting it, and about 90% of the AC30VR is preamp. I don't work in a marketing department though.

So I guess the good news if you're really interested in turning this amp into a pedal is that the so-called "Valve Reactor" part of the circuit (aka the pair of TL072 and associated parts) does look like it would be possible to keep intact if you replace U4 with a voltage amp instead of a power amp. There's some negative feedback into U7 that would need to be adjusted though, and I can't say how big of a difference on the overall sound that's going to make. It might be vital.

I'm sure rebuilding it as a pedal is possible, but I think a pragmatic first step would be adding a line out circuit after U5 and a dummy load to an AC30VR. If using a resistive load doesn't kill the "magic" then maybe it's worth putting the effort into building a pedal from scratch.

[martin manning]

A better plan might be to construct a tube preamp pedal that replicates a more conventional design using a couple of 12AX7 triodes running at high voltage. It's been done before, as described in this recent post: https://ampgarage.com/forum/viewtopic.p ... 0&p=393261

[Tony Bones]

While there is a preamp tube on the "power amp" page of the schematic, there's also a pair of TL072 opamps in the signal path after the 12AX7 and calling that pair of TL072 part of the "power amp" circuit feels like a bit of a stretch to me. I personally would say that the power amp is the LM3875TF chip and the dozen or so components supporting it, and about 90% of the AC30VR is preamp. I don't work in a marketing department though.

I understand what you're thinking. OTOH, all of the opamps after the ECC83 are inside a NFB loop that includes the LM3875, so I can also see how it might be argued that they're part of the output stage.

I any case, if I wanted to put the ECC83 part of the circuit into a pedal, I wouldn't worry too much about the details of the circuit surrounding the tube. The two transistors before the tube make up a phase splitter that drives the two sections in push-pull and IC1A is a difference amp that converts the PP back to SE. You could copy the existing circuit or spin the same function in any other way that you want. The ECC83 itself runs with a plate supply of 30V with 47k plate resistors and a shared variable 2k2 cathode resistor with bypass capacitor. That's it. Not sure where to adjust the cathode bias. Is it a trim pot? A front panel knob?

EDIT: The schematic I was looking at is the service manual linked to in the music-electronics-forum thread. The schematic on page 4 shows that the cathode resistor is 2.2k variable. The component diagram on page 6 shows that it's a trimpot. It's really not good form running current through a pot, even if it is only a few microamps.