Back to SURVIVING AMPS index page
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
Old pictures - lacking handles at this point. Note the distinctive scuff on the front panel though and the replacement indicator lamps.
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 235
Currently in the UK. Has its original GEC KT88s. Used for some years as a PA amp. Thanks to Tim for the pictures.
Serial number 252
Sold on ebay in 2014. Fitted with EL34s instead of KT88s.
SERIAL NUMBERS UNKNOWN
(ii)
Serial number unknown
Currently in the USA. Thanks to Mark for the pictures. Also registered on this page.
Pictures of the amp on ebay, UK, 2014.
(iii)
Currently in the UK. Fitted with EL34s instead of KT88s.
(iv)
Currently in the UK (Wales). Paired with a 2 x 18" Super Foundation Bass cab.
Still in the UK, but currently (October 2017) for sale on Reverb, so too (separately) the SFB cab. In good electronic condition. Serviced by Noble in Cardiff at some point in its history. Noble seems to have seen a good number of Voxes in the 60s and 70s.
(v)
Currently in Italy. A full bass stack - two 460 closed-back cabs - but no trolley.