The Little Voxes: The AC10 and AC15

Just a couple of little issues with these amps:

First, about the AC15/AC10. There is a weak spot on these old amps. The EZ81 rectifier valve runs a 6.3v filament, giving cheapskate manufacturers the chance to run it off the same filament supply as all the other valves, rather than having an extra 5v supply as required by most rectifiers. However there is a good reason why rectifier valves should have separate filament supplies – they do have a tendency to short the high-voltage supply to the filaments, and if they’re not on a separate supply this can cause damage throughout the amp (I have seen two examples of this, lovely old power transformers burned out, ugh). So if you have one of these old amps, get a really good unused EZ81 and fit it. Most of the early 60s AC10/AC15s I see still have the original Mullard, but even Mullards aren’t immortal. Best replacement is of course… an NOS Mullard.

And in an unconnected development… The outstanding good point of an AC10/AC15 is that it gives you Vox power amp overdrive at lower volume levels. A neat and kind-of-fairly non-invasive mod for an old AC30 is to separate the four EL84 outputs into two pairs, each on its own cathode resistor, and make one pair liftable from earth, giving you a switchable AC15 output stage in your AC30. Bigger output transformer and bigger speakers than an AC15, sure, but in a stage situation it is for many people just what they need to get their optimum sound without drowning out the band, and it does sound good (…or get an attenuator. The Weber MASS is the best I’ve heard).