“You’ve done this one already” you might say. True, but there are a lot out there and they keep turning up at my workshop. This particular one was another result of a ‘GAS attack’ of Marc’s. He does suffer, does that gentleman. It did throw up a few issues that might be of use to those unwary enough to delve into these things. It also broached the subject of just how much we should pay for…well, anything, really.
So, for a start what do I mean by that? Well, the bits inside an amplifier cost the manufacturer 77 pence. (Approximately.) The folks who put it together are mostly robots and their oil bill for the century is about the same. Or it’s made in China or somewhere else they eat rice and where 38.5 amplifiers = 1 bowl of rice. So that is another 77pence. (Approximately.) That works out to 2 pence per amplifier. Even given the benefit of any doubt that might be lurking, the amp cost a bit less than a quid to produce. (Approximately.)
Now we get to the really expensive bit. For every bloke/robot in the shed actually knocking these things out there are at least 174 blokes in designer sunglasses selling ‘em. It’s called ‘Marketing’ and that’s where our money goes. So we buy things to be persuaded to buy things.
This is the underside of the power section of the Vox AC30. Pretty much original but the mains transformer has been swapped in the past.
This was after I got hold of it. I often use solid copper ground rails where they once relied on the metalwork of the chassis. The contact to the frame deteriorates, and you can clean them up as much as you like in a year or two they’ll be back causing you problems again.
The output valve bases were replaced with lacquered ceramic bases. They hardly ever track and the gold contacts are very robust. Anyway that’s all pretty specific, but the generality interest comes with actually switching on an old amp that has been in retirement for a long time that has a valve rectifier. Don’t do it.
Here’s what really needs to be done. Take the rectifier valve out. Solder a couple of diodes (say 1N4007) to the valve base. The sides with the silver band are connected together and soldered to the HT output (the single wire that goes to the first capacitor) and the other sides go each to the transformer connections. Then put a capacitance meter across that first capacitor. An ohm meter reading is better than nothing. This one was a short. Had I switched that on, the rectifier valve would have blown and it could possibly have taken out the mains transformer. Best to be cautious. As it happened the rectifier valve also had a short, so that would have been a total disaster. You can check out the whole of the amp leaving in the two diodes. If anything nasty happens, your 20pence diodes will survive, and your nice new £15 worth of GZ34 probably wouldn’t.