Vox '7' Series Preamp as an effects pedal?

[R.G.]

Ah, I see, thanks.

Are piezo transducers any good for such an application?

In my humble opinion, no. I was modestly astonished to see them when I first worked on the Supreme. I did not like the sound. But clearly they can be made to work.

Don't they have a nasty resonance?

Depends on the transducer. Obviously, they can be made to reproduce audio kinda OK, or they would not have been used for phono pickups. But they are somewhat difficult to work with.

[R.G.]

I'll post the 760 preamp schematic here as well.

Just to illustrate, I got a chance to look at this schemo today.

If you look , the tremolo shows the LED being driven from two AC128s in darlington configuration.

Only it shows the AC128 as an NPN.  Either the actual circuit uses NPNs and the "AC128" is bogus, or the actual circuit uses AC128s and the schematic is rearranged to "help" you. 

[skumberg]

Hello!

I wonder if anybody ever made the 7 series or the defiant into an preamp? Perhaps there is a project available?

I heard a clip of a pedal based on the 7 series and it sounded very good. The "lemon drop" from lumpy tone shop. I'd rather try to build something myself though.

[R.G.]

I wonder if anybody ever made the 7 series or the defiant into an preamp? Perhaps there is a project available?

Yep. You want to be a guinea pig to test out the toner tranfer?

[skumberg]

Thanks R.G.! I was hoping you had something drawn up. 

I was collecting info on this a year ago and started a vero project for a "Defiant box". But it never got past Q2 since all I have is blury schematics and a few photos. Then I saw your thread at the Beatles forum and hoped for a project. I realise you can't work on these DIY things all the time although most of us probaly would like that. So I waited, but now the time had come to ask. And yes I would love to be the pig man.

I think I'm the most interested in the Supreme defiant conqueror preamp if you got a layout for that one. Otherwise I'd be happy to try the 7 series preamp.

[R.G.]

As it happens, I just got the yet newest version of the SCD preamp back as PCBs. I had enough in the toy fund for one run of two prototypes. Since I had to pay for that one, that one is available at my cost, $20. You can have the toner for the SCD or the 7Series for free. All's you gotta do is build one, then figure out how I screwed up the layouts and what I left out and got wrong in the documentation. 

The tremolo circuit was the biggest problem with the first version of the SCD, so I did a re-lay of just the preamp, nothing else. The SCD has MRB and distortion, but no tremolo (which was in the normal, not distortion channel anyway). The 7-series had distortion and tremo, but no MRB.

So - what'll it be pardner? 

[skumberg]

I'm tempted to try an etch for the SCD preamp first since I got some pnp blue and chemicals at home. If it fails I'll buy your pcb, thanks for offering it.

Yes it's a deal. I'll do the build and post the results here with pictures. I've sent you a pm with my mail adress.

[Gus]

Just drew an LT spice sim of the normal channel.  One can add the other parts. Voltages are from the sim.  Stock LT spice download transistor models

R25 is a 25k mid potentiometer wired as rheostat
R32 is a 1K log treble potentiometer wired as a rheostat
R30 and R31 are one 220k bass potentiometer

R27 and R28 are a 220k volume control


The voltages look OK.  I don't think you need the stock transistors.  Looks to be forgiving of transistor numbers used Q4 should be the higher collector current transistor.

Looks like 18VDC should work with a little adjustment of some resistor values, jumper R33 and adjust R20 and R10 values might be all that is needed.

[R.G.]

I'm tempted to try an etch for the SCD preamp first since I got some pnp blue and chemicals at home.

Works for me. Watch your mail.

[Gus]

I like the reverb transducer drive circuit.  It reminds me of a Fender drive circuit.  Anyone measure the primary inductance, resistance and the core size of the SE transformer?  One might be able to wind a nice gapped reverb transformer and use a stock accutronics reverb tank and a different recovery circuit.

I also like the transistor gain switching used.

R.G. does the sim match the real circuit?

[R.G.]

I like the reverb transducer drive circuit.  It reminds me of a Fender drive circuit.  Anyone measure the primary inductance, resistance and the core size of the SE transformer?  One might be able to wind a nice gapped reverb transformer and use a stock accutronics reverb tank and a different recovery circuit.

I once converted a Supreme from the stock piezo drive/recovery to a magnetic drive tank. I didn't rewind the transformer, I just opened the high voltage secondary and used the primary inductance as a load for the driver, and then fed the voltage through a capacitor to the input of the tank. I had to goose up the gain of the recovery stage to make up for the lower recovery voltage. But it worked well. As much as I've messed with transformers, I still avoid rewinding them where I can.

I also like the transistor gain switching used.

Yep. It was one of the first soft-switching systems in guitar amps.  In this, it shares a lot with the soft switching system in the final version of the Thomas (USA) Vox amps. They did a lot of effects switching with JFETs. The USA Vox amps are roundly maligned, but I like them. They have some very interesting technical features and IMHO sound a lot better than they are given credit for. The Supreme/Conqueror/Defiant series shares certain circuit subsections with the Buckingham/Royal Guardsman/Beatle line; they obviously had some overlaps in design.

R.G. does the sim match the real circuit?

Pretty much. I did the distortion channel and tremo. The circuit operation seemed to match the simulations I did pretty well.

[Gus]

R.G.

If I understand you post correctly you used a choke loaded / parafeed circuit to the magnetic reverb tank.  One can buy chokes, hammond makes some inexpensive ones that might be fun to use
155C might be good enough.  I don't think the transformer capacitance will matter with a guitar frequency range
http://www.hammondmfg.com/153.htm

One needs to be careful with the cap value used to couple the collector/choke to transducer because of the  RLC RL

[R.G.]

Yep. What you said.