Vox AC80/100 no. 254
AC100 serial number 254, conforming to the "AC80/100 Amplifier" schematic: thin-edged box, brown grille cloth, green Woden transformers, cathode biased, its chassis made by Westrex for Vox in the summer of 1964. For related amps, see the pages here.
A general introduction to the amp can be found here. Below, a series of details kindly taken for me in the US.
The changes were carefully made and made as far as possible to be reversible: extra capacitance in the power amp, along with replacement cathode resistors and capacitors. In the preamp, a hum balance pot and some higher than specified filter caps.
The cathode resistors were paralleled - 350R and 681R = around 237R (as measured) per valve, which is much too low. The schematic specifies 270R, quite low enough as it is.