Vox ac30

 Plenty of problems on this one. This was a very early model, and normal, not top boost. The Bright channel was very low output, the vib/trem didn’t do anything, both of which faults were reported by the owner. The fact that the output stage was horribly assimetric, wasn’t. The vib/trem switch had fallen to bits, and a ground had been removed from the oscillator section. And the mix arrangement on the normal channel had an ill advised modification. Apart from that, it was fine(?)

           In an AC 30, each pair of inputs has its own half of an ecc83, these being double triode valves. The last half is used to introduce the modulation (vib/trem). All the resistors at this time(early 60′s) were solid carbon, and solid carbon resistors are noisy, and comparatively unreliable. If you want to stay completely original, you have to use solid carbon. But that gives me a problem, because I warranty everything I do, and solid carbon resistors are a fault waiting to happen. So if you want to put it in a museum, either leave it not working, or stick with original components; or uprate if you want to use it.

             The fault in the bright channel was simple; coupling cap from the anode of V1 had gone open circuit. After sorting the vib/trem switch, it still didn’t work, and it turned out that the preset for mod depth (mounted on the tag strip, this was on the top one, but they’re on the bottom one on later models) wasn’t there at all, and the ladder network that provides the positive feedback for the oscillator, had no link to ground. That done, it worked. This has no footswitch, and, that being the case, you have to short the two 1 meg resistors in the ladder network to ground, or it will be effectively switched off.

           Assimetry in the output stage means that the top half of the signal is amplified more or less than the bottom. Someitmes it sounds like a loss of power, and sometimes it’s an irritating kind of distortion. The cause can be anything from screen grid resistors to phase splitter faults. Or even output transformer, if you’re really unlucky. This had a problem with the two coupling caps that supply the grids of the output valves with signal from the anodes of the phase splitter.

                  A word about the biasing of the output stage of an ac 30; which didn’t alter significantly over a lot of years. There is no bias adjustment. It relies on a 47ohm (or 50 ohm later on) which is bypassed with a big cap. The voltage across this is the bias voltage and anything between about 9 and 12 volts is ok. If it goes less than this, it’s trouble, and can be the bypass cap, or valve faults, or leaking coupling caps. It can also be tracking valve bases, and even open circuit cathode resistor.

                  Many faults that come up on the vib/trem can keep you busy for a long time; the whole thing has more feedback loops than you can shake a stick at, and everything affects everything else. Ouch.