VOX: UL Series Preamp

I quite like some of the guitar sounds from The Beatles Revolver album, and after looking at the Vox Supreme, I thought I might have a play with the UL series preamp.

I'm also waiting for the build docs to come out for the Aclam Dr Robert on pedalpcb as a comparison.  It's also a UL730-based pedal - I believe it's the normal channel, and they have also modelled the power amp section.  Keen to see how they have done that.

VIBRATO CHANNEL

Everything below relates to the vibrato channel on the amp, which also happens to be the channel with distortion/fuzz, which is what I'm interested in.

FUZZ / DISTORTION

Strangely there's no control to dial in the fuzz/distortion circuit on the amp itself - it was turned on by a foot switch, and you got what you got.  

The switch increases the gain of T1 & T2 (using T3 and T4 as switches), and at the same time, it ties the OA200 silicon diodes to ground or V+ to create fuzz/distortion.  I'm really not sure which way the diodes connect, as I don't know what's happening with the foot switch.  The schematic is not clear in this regard.  Either way, it all looks about the same regarding wave shapes and frequency response.  

Bypassing the distortion leaves you with a nice clean signal, not unlike the normal channel.

BOOST SWITCH

There's also a boost switch on the front control panel of the amp, but it doesn't appear to do a great deal at the output, maybe a bit of a top boost.

LAYOUT

For some reason, the mid-range control is immediately after the clipping section but before the volume control and gain recovery section (T5 & T6).  The volume control feeds the gain recovery section, with bass and treble controls at the end of the signal chain.  I'm sure they had their reasons at JMI.   This is not a master volume amp either - one volume control per channel, and that's it.

VOX UL SERIES PREAMP SCHEMATIC

VOX UL SERIES PREAMP LTSPICE

This is with a 200mv sine wave at 440Hz, with the distortion/fuzz switched on.  LTspice file is here if that's of any interest.  You may need to change transistors if you don't have the BCs in your library, but the rest should be fine.

Collector T2

Post diode clipping

Output - bass, mid and treble all set to 50%

Output - bass, mid, and treble all set to 50%   Diodes removed from the ground - which I don't believe is part of the normal switching, but I might have to give this a try.  Looks like a rather large square wave.  i.e. a solid fuzz...  

The tone controls are pretty interactive - bass / mid sweep

Mid / treble  sweep