6X4 tube overvoltage behaviour?

[valotus]

Does anyone know how 6X4 rectifier tube reacts in serious overvoltage situation? I have US AO-28 which has been accidentally connected to European mains without transformer. So the normal 340-0-340V secondary voltage has been doubled to 680-0-680V for some time.

Of course preamp power transformer is mostly dead now, but I was wondering possibility that 6X4 had instantly shorted or otherwise killed and therefore protected other parts of the circuit.

[geoffbrown]

Doubling the intended mains voltage would also double the filament voltage, so if the filament is still intact then you've dodged a bullet,however excessive plate voltage could also cause damage to the plate you will just have to try the amp at recommended mains volts and see what happens

[KC9UDX]

Most 6V tubes can run at 13V for a long time. I'd be more concerned about plate values. I agree. Just try it. It either works or doesn't.

[David Anderson]

Find someone with a tube tester and test it for emission and heater-cathode leakage. That's the only real way to know its condition. We don't have magic crystal balls. 6X4s often fail due to heater-cathode leakage before they run out of emission.

This situation is also a good example of why it's a good idea to fuse the power transformer primary of an AO28, especially in situations where it could accidentally get hooked up to European mains.

[Jyvoipabo]

Hello,

I summarize.
Power transformer = ?
6X4 =?
Other tubes =?
HV Capacitors =?

In such a case it is necessary to re-energize all the components gradually.
First the transformer.
Then the transformer with the 6X4
And the transformer + the 6X4 + The tubes 1 by 1.

At the same time, the consumption of the whole must be monitored.

The trick is to insert in the power line a filament lamp (230V / 60 ~ 100W) which will indicate if there is a defect. If the filament only blushes a little, that's normal. If the lamp light brightly there is a problem.

Don't worry a 110V transformer in a country where everything is 230V always ends up burning. This is an opportunity to buy a new one and transform your device into 230V.

[valotus]

I'm actually not worried about condition of 6x4 which I'll throw to trash anyway, but was just wondering if it tends to get shorted or otherwise prevents voltage to go further in this kind of huge overvoltage situation. That would have been saviour to other components after the tube.

Now the fact is that power transformer is fried anyway, but if almost 750VDC has also gone thru 6X4, at least all electrolytics after it are dead too.

I'll get a new transformer and make it fused before running any tests anyway.

[tpappano]

It's possible that the first electrolytic cap after the 6x4 may have protected the components downstream of it by drawing current when its voltage was exceeded. It probably gave it's life in doing so...

[valotus]

This is going interesting. I measured power transformer again without rectifier tube and secondary + heater voltages were ok. Then took a closer look to 6X4 and found out it had short between both plates and heater, not cathode and heater as it usually fails. Put a working 6X4 on the place, waiting it to heat... and all DC voltages were as they should be. I will renew caps and make overhaul anyway, but this far it seems that rectifier tube burned and saved other circuit.

[KC9UDX]

That's very odd. I'm not familiar with the construction of that tube, but to short plate to heater without cathode seems like an incredible feat!

[valotus]

Seems odd to me too, but when I did other forum searches someone else have had same issue with 6X5 tube. Plate-heater were not fully shorted, but compared to working tube, they clearly have connection.