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.

A short note on 710s and 760s (to be continued) | 2017 | Uncategorized