Vox UL4120 power amp voltages

[Robert M. Martinelli]

Hi all,
I'm on the verge of selling my beloved VOX UL4120, so I've re-tubed and thoroughly tested/verified it. Knowing there's not much "tech info" about these amps, I thought it would have been good to share my findings here. I then took one of the 4120/7120 power amp schematics, "cleaned it" some, added my findings/measurements and highlighted some differences I found. The amp's been tested at 220VAC mains (instead of 225VAC) so the voltages are somewhat (-2%) low, but I think the given values are more than enough to state if a particular specimen's voltages are in the ballpark or not. I have set the bias at a rather "conservative" level (KT88s quartets aren't cheap...), but it seems enough to keep the XO distortion away.

I know there aren't many of these babies left around, but I hope someone will find this useful sooner or later.

Hope this helps

Best regards

Bob

[Robert M. Martinelli]

Sold (to a very special and very knowledgeable person, that I know will take good care of the amp).

Anyway, I Hope the above schematic will be useful to someone someday!

Cheers

Bob

[vintagekiki]

Looking for Vox series 4 preamplifier schematic.

Thanks in advance.

[pdf64]

What an unusual monster! A choke input B+ filter, power pentode phase splitter. Well done in getting the documentation improved.

[Robert M. Martinelli]

What an unusual monster! A choke input B+ filter, power pentode phase splitter. Well done in getting the documentation improved.

Thanks Peter, it's good to know that my efforts are appreciated....these amps are so rare, the design so unusual and there's so little to be found about them that I thought it good to take some measurements and share them while the "beast" still was on my bench...(now it's gone, I found a good new home and a very proud new owner that I know will treat it good, but I miss it already.... ).

Cheers

Bob