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The Vox UL4120

Vox 4120s and 7120s have highly unusual power sections - the rectified voltage travels first to the huge choke, and from there to the filter caps (two 200uf/350v electrolytics in series). Normally (in other amps) first in line after the rectifiers is a filter cap, or a filter cap/choke/filter cap network. The topology of the 4120/7120 is known as "choke input filter". The object was to provide as smooth and stable a voltage feed as possible. These are perhaps the only production guitar amps to have been designed in this way.


The main power valves (KT88s): 582v on the plates and screens; bias voltage around -80v. Unloaded, that is with KT88s removed, but the driver valves still in place, there is around 640v on plates and screens.

The schematic is OS/116, dated 17th March 1966. Note that Vox schematics were drawn up for repairmen. They are not blueprints for the assembly line. In the case of the 4120, 470ohm screen resistors are sometimes found in place of the 330ohm in OS/116. Other differences occur too. This should not be surprising. Triumph amps regularly deviate from the "formal" schematic.

Make sure that the bias supply filter cap, 32uf / 350v (C69 on the schematic), is in good shape and of the stipulated (or higher) voltage.

Despite what is sometimes said (the "received wisdom"), the bass channel is great for guitar. The only really unusable sounds come with bass full on, treble full off - something that one would not really do with other amps in any case.


Amps pictured below are registered in two sections: (1) by serial number if known; and (2) lower down the page, serial numbers unknown.


Serial numbers begin at 200 or 201. Note that they are not a reliable guide to relative date, ie. amp no. 230 could be later than amp. no. 240.

Click on the thumbnails below for larger versions of the image.

SERIAL NUMBERS KNOWN

Serial number 213

Serial number 221

Currently in the UK. Pot codes "GN" and "HN" = July and August 1966.

Further pics



no. 221 with two original closed-back 4120 cabs. The cabs are now in Austria.

Serial number 252

Sold on ebay in 2014. Fitted with EL34s instead of KT88s.

SERIAL NUMBERS UNKNOWN

Shown with a repro cab and trolley. Surviving 4120 and 7120 cabs .

Currently in the USA.

Serial number unknown

Currently in the USA. Thanks to Mark for the pictures.

Pictures of the amp on ebay, UK, 2014.

Currently in the UK. Fitted with EL34s instead of KT88s.

Currently in the UK. Paired with a 2 x 18" Super Foundation Bass cab.

Currently in Italy. A full bass stack - two 460 closed-back cabs - but no trolley.