The tale of two Vox Cambridge 30 SS amps

[bluesky6]

So I managed to repair the blown TDA2050 on the single speaker Vox Cambridge 30 Reverb that I picked up as a non-working orphan.

One thing I did was to put both this and my Vox Cambridge 30 Reverb Twin (the 2 speaker version) side by side and compared the tone. This was using the same guitar (Squier CV 50's VB) and same guitar cable.

The difference, with all the knobs set to the same positions, running in the overdrive channel, was remarkable.

The Twin was bright, trebly with lots of string separation. The non-Twin was darker, louder and more bass/lower-midrange. It was like I was playing through the neck pick of the tele...!

I also tested with an external cab to eliminate the 2-vs-1 speaker possibility and found the same difference.

Since the non-Twin had a 5-pin opto isolator swapped in, I tweaked VR11 to match the overdrive volumes between the two amps. But the difference was still there.

As a further data point, the Twin serial number is in the 1xxx range, while the non-Twin was 2xxx.

Has anyone else seen this kind of differences?

The tube on the non-Twin is the original Sovtek 12AX7WA. I'll check the Twin later.

[Jazz P Bass]

As far as I can tell from the schematics that I have, they are two different amps.
Circuit wise.

Nothing earth shattering in the differences.
But there are subtle changes here & there.

[bluesky6]

Thanks for the schematics. I see that there are minor capacitor/resistor differences.

I was a little misled by Will Page's "Vox Cambridge 30 Repair" webpage where he implied that only early serial numbers (less than #600) had the tonal and component differences.

The good news here is that I've another project on my plate: to tweak the non-Twin to match the tone of the Twin!

[bluesky6]

Well, it looks like Will Page may not be wrong after all.

I stripped the non-Twin and went over it with a magnifying glass. The add-on capacitors and new value resistors were already on the board. Some jack-a** replaced the opto in the past but left a white residue of what he probably assumed was flux remover. The add-on capacitors were soldered on post wave-soldering/post-production and the solder job was not as good as I would have liked.

So some re-soldering was needed in addition to the cleaning.

After some thinking, I also replaced the LEDs in the drive channel feedback circuit. Rationale: the drive channel was sounding a little wonky (part of the tone difference with the Twin) and one of the add-on capacitors was soldered onto a leg of one of the LEDs. So there was a possibility of a bad LED. Or at least a poorly soldered on one.

In any case, my time was free and I have a whole bag full of high-intensity 3mm LEDs from my Vox Pathfinder project.

End result: the two Vox Cambridge amps now have a much more similar tone and behavior with the volume and tone controls.

For future reference, the o-ring size for the 12AX7 shield is 15/16". I bought a set of 2 from the local hardware store for a "mere" $0.79.

[Jazz P Bass]

Way to go!