welcome Vox 4series 7series




“When we finished ”Revolver”, we realised that we had found a new British sound almost by accident.”

  – Paul McCartney, 1966



The life & times of the rare Vox 7 & 4 – series amplifiers, 1966-68.

(**Blog Index, & Gallery links now all active).   Further entries added as website unfolds.  Either scroll down ↓  to see just the most recent posts on history / amps / memories (click on  ‘more..‘  to expand full post),  or see blog post INDEX above for the whole list & links to individual posts.


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A late Vox 730 from Bavaria

A late Vox 730 from Bavaria

Vox 730 serial number 3089. Sold by “Piano Werner”, Straubing, an outlet that still exists. Further pictures posted here.

Also to note in relation to the post on an AC50 and 760 cab below, a 715 – serial number 2040 – supplied by Servaas, The Hague. Other pictures of amp and cab are here.

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The unnumbered 710 schematic

The unnumbered 710 schematic

This schematic is closest to the actual build of the amps. For further comment, see the two posts below.  Surviving 71os are registered on this page.

Click for a large downloadable version

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Vox UL710 (continued)

Some more details on the 710, picking up from the immediately below.


Further anomalies in the schematics

i) In the unnumbered schematic the 100K resistor that appears in all production amps, standing between the input sockets and the grid of the ECC83 that drives the reverb, is absent. It is however registered as R24 in schematic OS/112.

ii) Conversely, OS/112 envisages two cathode capacitors – C9 and C15.  These are not registered in the unnumbered schematic, and do not appear in the amps as built.

Thanks again to Tuomo for the info.


Serial number 1047


A 710 amp (number unknown) put through its paces

The dates and so on mentioned in the preamble need a little revising, but fine playing.

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A short note on 710s and 760s (to be continued)


Notes on the vibrato circuit (updated)

Info on the 710 thanks to Tuomo Grohn.  The earliest of the two schematics that have so far come to light (ie. the one without the number) seems to reflect best how the amps were actually built.   OS/112, however, does register elements that were introduced as the production of the various batches proceeded – resistor R36, for instance, in the reverb circuit -not found in serial number 1005 (registered here), but present in serial number 1036.

The dates of apparently new elements noted on the later schematics must be treated with caution though.   The schematics are not designs for amps.  They were mostly drawn up well after the event.  Triumph built amps in short runs, introducing changes along the way.

It is always best is to take the amps themselves as the primary starting point. The schematics may be perfect for one, but not for another. Differences are simply differences. There is no mystery, no conspiracy, no *desire* to misrepresent reality.

As mentioned, for the 710, the unnumbered schematic is the closest to the batches of amps that Triumph actually built.  The vibrato circuit is OFF until switched on via the footswitch.

However, a good number of amps have a jumper across pins 4 and 5 of the DIN socket, which ensures that the vibrato is always on (ie. cannot be switched off by the footswitch).

In OS/112 a new design of the vibrato circuit/footswitch is envisaged – vibrato ON until switched off via the footswitch

“Left one is from un-numbered schematic and presents the actual built circuits in all amps which I have seen (my own and looking at the pictures in your pages) and the right one is from OS/112.

On left circuit the vibrato is always OFF until you switch it on with the footswitch and on right circuit vibrato is always ON until you switch it off with the footswitch.”



Info thanks to Steve Walsh.  Going by serial number 1004, early 760s do not have the negative feedback circuit that is noted as an addition on OS/115 (power section).   Again, do not take the date on this schematic as indisputable fact.   The change was evidently brought in during the course of production.

Pictures show that serial numbers 1041 and 1063 – pictured on this page – incorporate the feedback circuit, a purple wire running from the board adjoining the ECC83 valve socket and tying off on the 15ohm terminal of the impedance selector.  When info on datable components of these amps comes in it may be possible to judge better when Triumph made the change.

Serial number 1004 has other early features too – these will be noted soon.

The schematic for the power section of the 460/760 – OS/115 – is given below; so the schematic for the 7-series treble preamp – OS/117.

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760 cab and AC50 sold by Servaas, The Hague

A 760 cab and AC50 sold by Servaas, The Hague

An old picture, but perfectly legible.  The cab denuded of its front covering (scrim and grille cloth); the metal badge originally fixed by Servaas to the cloth transferred to the baffle.  Servaas was an important outlet for Vox up to c. 1968.  For conventional AC50 and AC100 sets with Servaas badges, see this page, entry for 3rd Dec.  At least one other pairing of an AC50 with a UL760 cab in Holland is recorded.

Fully solid state Vox amps with Servaas badges have yet to come to light – it is possible that the shop sold that new range, but nothing definitive so far.

Whether the cab above came to the shop without a 760 amp is unknown.  When Vox decided to wrap up the 4 and 7-series range of amps, other uses for the cabs were found – so again, perfectly possible.

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Vox UL460 and UL760 power section – OS/115

Vox UL460 and UL760 power section – OS/115

Click for a larger downloadable image

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A 4120 / 7120 schematic with notes

A 4120 / 7120 schematic – OS/116 (adjusted) – with notes

updated 5th December

Click for a larger downloadable image

Worked up in 2010. It is not known at present which amp was tested and measured.  Note that the sheet has been digitally altered to accord with the build of the amp – it is stated that in the original schematic prepared by JMI, R92 is 20K ohms/10W;  R100 is 5K ohms/10W; and R109-R112 are 330ohms.  See below, this is not entirely correct.

Beware of the bias voltage mentioned – this could be too low for modern valves.  It should also be said that plate voltages may differ from amp to amp.


Above, a detail from OS/116, unmodified. Click for a larger much sharper image.  R92 is not 20K ohms/10W as the tester noted on the modified sheet, but 10K, which is the value he encountered in the amp in front of him.

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Vox 7 series preamp schematic – OS/117 – for the 715, 730, 760 and 7120

Vox 7 series treble preamp schematic – OS/117 – for the 715, 730, 760 and 7120

Click for a larger downloadable image

Comments to follow.

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The Vox UL730 power section schematic – OS/114

The Vox UL730 power section schematic – OS/114

Click for a larger downloadable image

Above, OS/114 for the Vox UL430 and Vox UL730 power section, dated 28th February 1966.  Note that a small batch of pre-production amps – made at much the same time as the Beatles’ small-box 7120s – were built differently.  At least one of these early 730s survives (currently in the UK).

OS/114 indicates that the negative feedback line should only be for UL430 bass amps (FOR BASS MODEL ONLY).  As mentioned in the post below, UL760s had no feedback circuit to begin with, but at some point mid production one was subsequently added.

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At least one more Vox UL430

A Vox UL430 chassis on the work bench.  Note the small ding at the lower left corner.  This amp does not seem to be among the 430s already registered on this page.  The amp is included in the corrected total known to date a couple of posts below.


Possibly the same amp photographed more recently, though the detail is too small to tell for certain.

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