Vox '7' Series Preamp as an effects pedal?

[Suproman77]

I've been toying around with the idea of having the solid state preamp section of the Vox 7 Series built into possibly a pedal or a unit that sets on top of the amp. They used the same preamp circuit in all of the amps in this series (715, 730, 760, 7120).



I'm wondering if anyone would have any interest in undertaking a project like this. Might be a cool design...

[R.G.]

I may have already done that. I did a layout for the Vox Supreme/Conqueror/Defiant, which I believe is a deluxe version of this one. It's very close anyway. It's been a while. Let me see if I still have that.

[Suproman77]

Cool, thanks.  That should be pretty close...maybe even better, actually. 

[Electric Warrior]

sweet! the Supreme/Conquerer/Defiant schematic I have is so blurry... and the values I can make out don't match with anything I have identified in the photos of that circuit.

[drewl]

Heh, that's on my "to do" list someday! but as a preamp!
I've rebuilt plenty of the US Thomas Vox solid state amps, and they sound incredible into a tube power amp.
So I'd like to build one of the 7-series, some of my favorite tones were recorded with those amps.

Wasn't there someone on the Plexi Palace site that was making a rack version?
if i search for it I'll post the link.

[Suproman77]

Cool, I see there is some interest in this.  I'm serious about having this built, but I'm not a builder...looking for someone who would want to undertake this project for me.  PM if you'd like to do it!

[R.G.]

I replied to who I think was Suproman in another forum. Here's where I got.

http://geofex.com/FX_images/SCD%20preamp.pdf
I tried inserting as an image but apparently pdfs don't do that.

[Suproman77]

I replied to who I think was Suproman in another forum. Here's where I got.

http://geofex.com/FX_images/SCD%20preamp.pdf
I tried inserting as an image but apparently pdfs don't do that.

Thank you!  Looks great.

What other forum did you reply to?  Not sure where else I might have asked for it. 

[R.G.]

BGC?
Was that you? If not, someone else has the same request there.

The SCD preamp is very close to the 7xxx series. I think they may be buildable on the same PCB.

[Suproman77]

BGC?
Was that you? If not, someone else has the same request there.

The SCD preamp is very close to the 7xxx series. I think they may be buildable on the same PCB.

That's funny.  I don't even know where or what BGC is!  Haha. 

Thanks for the info. 

[R.G.]

OK. There are two of you. Well, three counting me. 

I'm doing a layout for the Supreme/Conqueror/Defiant preamp and also the 7xxx series preamp, brilliant channels only.

[Suproman77]

OK. There are two of you. Well, three counting me. 

I'm doing a layout for the Supreme/Conqueror/Defiant preamp and also the 7xxx series preamp, brilliant channels only.

Cool.  I got your PM and I'll post the 760 preamp schematic here as well.

[Solidhex]

Yo

  Its fun checking out solid state preamps for pedal ideas. Do you guys have a readable copy of the Conqueror etc schematics? Like Electric Warrior says all the copies out there are unreadable! I can tell the circuits seem to be pretty different than all the 7XXX amps even without being able to read the component values...

--Brad

[Solidhex]

So back to this Vox preamp...

  I got a schematic for it. Had to mail away for a hard copy. Tiny and hard to read grrrr. Anyways I see it uses a 35volt supply. Is that really necessary for this little section? Can it be adapted to 18v?

--Brad

[petemoore]

it uses a 35volt supply. Is that really necessary for this little section? Can it be adapted to 18v?
  It shouldn't blow anything up.
  You might need to attend to transistor bias re-sets, the headroom will be lower.

[R.G.]

Status - I now have the second iteration of PCB prototypes for both the Supreme/Conqueror/Defiant and 7-Series preamps back. I'm building one of the sets, and a volunteer "victim" builder is building the other set, to verify the layout and operations. Once this is done there is a group on another forum which will gather up orders for a group buy of PCBs. The preamps are about 1.5" by 5".

The service schematics have errors in them. As a precaution, I simulated the circuits as I went doing the layout. I did this primarily to be sure I had the values guessed right from the very blurry schematics. Using the best possible guesses, the service voltages on the schematics cannot be obtained. I reverse engineered the circuits from the best-guess schematics, doing the reverse-design work to make them come out, as well as comparing to photos of actual PCBs which have been donated to me.

As for the 35/18v question. Sure - you can do anything that produces the correct voltages inside the preamp. For the SCD preamp without tremolo, that's 14V and 16V. You could simply use 18V *regulated* and change the two voltage dropping resistors to get 16V and 14V in the two main preamp sections that use this.  You could also use two different regulators like maybe two LM317Ls to make 16V and 14V regulated, which would probably be even better than the big capacitors used in the original to bypass the voltages.

I would hesitate to try resetting all the biases for 18V. There are some subtleties lurking in there. It's possible, I could probably get there eventually, but I have a full plate most of the time. My best suggestion? Get a wall wart which makes 24Vac at over 100ma, run that into a full wave rectifier and filter to get 34Vdc, then feed that into the existing circuit. Once you have a working preamp, then you can start adapting it for other voltages.

By far the most distinctive feature of the SCD preamp is the mid range boosting. This can be easily obtained with Small Bear's "Wild Mouse" kit.

[FiveseveN]

Please excuse my ignorance but what the hell is this contraption and what is it doing there? Crystal oscillator? But... why?

[teemuk]

In the context of the circuit, it looks like a feed and pickup of a spring reverb tank. 

[R.G.]

Even better than in context, in actual life it *is* the send/receive units for the reverb tank. The English solid state vox amps are the only ones I've ever known to use piezoelectric send/receive transducers for reverb. I once stripped out the piezo system and hacked the circuits to use a standard magnetic drive/pickup system in a Supreme head. The piezo tranducers looked like abused "ceramic" phonograph cartridges.

Oh, yeah, phonographs were those funny black plastic discs with analog data encoded as wiggles in a spiral scratch in the surfaces of the disk... 

[FiveseveN]

Ah, I see, thanks. Are piezo transducers any good for such an application? Don't they have a nasty resonance?