This 1961/64 (copper panel) Vox AC30 chassis was brought to me with the
request to sort out "a burnt-out power transformer". It has had a fair few component changes in the past but this had been done quite well and didn't need any further attention.
The GZ34 had been replaced with a plug-in silicone rectifier mod. As the amp just kept blowing the 3amp mains
fuse, the owner just assumed it was the power transformer. In actual fact, one of the silicone diodes had gone short and was quite simply the cause of
this. The power transformer was fine.
One modification that had been done however was to put individual cathode resistors and by-pass capacitors on all four of the EL84 output tubes (see
photo below). A pointless issue really as this does not change/improve performance in any way. I have simply removed them and put it all back to
original; one resistor and cap for all four tubes.
After changing the 12 volt pilot lamp for the correct 6 volt one and fitting a new GZ34 rectifier, plus two very tired EL84's, it all came back to life and tested perfect throughout at 31 watts RMS too.
This AC30, though most certainly a 1961/1964 chassis (copper panel and only one stage gain on the brilliant channel), nevertheless has at some point had a cabinet change. My guess is that by the early seventies the original case was probably "well roadied!" and the original owner had approached Vox for a new case. (?) Hence all the tell-tail signs: the black air vents on the wooden chassis base, the plastic air vents on the top (normally brass at this age), all the wooden base and both back's tee-nuts are metric thread, not BA; and the most obvious sign would be the black speaker fret which is normally tan on this model!......Still a great old amp though.
- John
