Vox AC30, vintage

Owner decided to power up on full 240V mains after being in an English garage for many years. Worked perfectly before "storage" and well for a few minutes and then arcing and sparking crackling noise so he switched off.

Insects? Now carbonised?

Gareth.

No smoke or burning smell. Next thing to try is variac powering again but on the 110V Tx setting, then IR thermometer to see where that current is going

No smoke or burning smell. Next thing to try is variac powering again but on the 110V Tx setting, then IR thermometer to see where that current is going

It'll be going through the fried spider.

Gareth.

Take a good look at the voltage selector - maybe something is bad there.

.... Phil

Valve rectifiers or solid state ?

The OP mentioned a 5V heater winding.

... Phil

Tried on 115V selector setting at about 20% (UK) mains and current draw went up to about what it was with 50% of UK mains on 245V setting. IR thermo showed the Tx was warming up nicely even at 20% of 240V main, current too low on variac ammeter to measure but .2 to .4 A ac. I wonder how hot this Tx got , presumably in hours of use, stinking the place out with tar smell (noticeable at just 10 deg C over ambient), before it was stored away?

Anyway trying a bit of the tar on DVM-R, squashing the probes together over a bit of it, perhaps about 0.2mm thick, could get a few megohm. Trying on a megger over a 1mm or 2, easily got 500M to 1G. So has anyone ever tried removing one of these Tx and baking in a low oven , outdoors presumably, to drain liquid tar out, then replace it with what ?. I'll disconnect all the wires and see if there is a pair of points I can measure with a Megger, if any of the team think its worth trying in an oven or other ideas? Conductive resistor path inside would explain the linear characteristic of the excess current , but primary , secondary? With Tx removed I can check inter winding leakage and to frame,screen etc With Tx removed, would it be worth removing the outer cooked paper and underlying cloth to get a closer look at the windings, perhaps tar is just leeched from the cloth originally (for anti-damp purposes and not holding windings in place? Where to research Tx construction of the 1960s? The owner said that while it was working properly last week and the start of the arcing/sparking noise , before switching off,there was a puff of what he called steam rather than smoke, before switching off. Perhaps related to damp or fried insect and nothing to do with the tar resistor problem. I've not had anywhere near full HT on this amp.

Valve rectifier, not SS, BTW

If paper burns at 451 deg F, what temp for near to charring, the voltage & current ratings are just about readable with the right light on the Tx, as black print on now almost black grade of brown paper, colour maybe partly due to tar vapour of course

For instance, wood, from which paper is made, burns from between 900F and 1 ,200F, and can be much higher if oxygen is forced into the fire. Paper is m ostly cellulose without the volatiles, so burns at a lower temperature, aga in unless oxygen is added.

Peter Wieck Melrose Park, PA

Hang on a bit , a quote on submarine cable manufacture "Two layers of coal tar-impregnated jute over the armor wires complete the cable structure."

Not much burns underwater. Jute resists seawater better than most other natural fibers. Coal tar, while vicious stuff (and one of the ingredients for Greek Fire), also does rather well under water, unlike asphalt.

Would this be the specification used by the Great Eastern? That is about the correct vintage...

Now, without reference to the NET, what is significant about the Great Eastern? There are masny things, but exclude cable-laying for this answer.

Peter Wieck Melrose Park, PA

The Tx came out easy enough, as did the outer layer of paper, after warming with hot air, so cannot have been that cooked. Frame and wires to each other showed nothing to 30M on DVM-R or to 20G on megger. Enamel of outer 6.6V,6A winding looks fine, but blacker paper under that layer. No sign of any tar, looks as though it is beeswax? , looks like whatever the yellowish wax that was covered on those notoriously leaky waxed paper caps, its that which has carbonised to black tar-like material probably, and flowed out of the Tx. No cloth seen in lower layer either, seems paper and wax. Over about 2mm of ome of the wax/tar on the inside of the removed paper, got a Megger reading of 5G. I'm assuming a distributed "carbon resistor " over a large area (to distribute .5KW of heat) rather than shorted turns as at no point has there been any of that nasty magnetisation buzz of shorted turns. How to determine if the "resistor" is on the primary, nearest core so hottest, or the HT secondary winding?, unlikely the high amp windings. Nothing to loose placing the Tx on a tray in a low oven, but which orientation, as in normal use or the other way up and hope enough wax flows out. It would be nice to take some before and after ohms reading of something , if anyone has any ideas.? Leaves the possibility of near enough solid carbon at the main hotspot, that will not budge at all. You'd think after decades at this game you'd have seen all possible fault scenarios.

I just thought , take R and L readings with a RLC meter, for the sections of primary (assuming same gauge of wire through the length) and