Redplating again.

[richard]

I tried swapping the output tubes in my Roadhouse around and thought I had identified the problem as one of the tubes. However, when I put the new tubes in one of them redplated. I've read that once a tube has redplated it will continue to redplate. Is this correct ? I only ran the new tubes until I could see one of them start to redplate and immediately switched it off.

[jpfamps]

As long as the valve has not been red plating for very long it should be fine.

The issue with red plating is that adsorbed gas can be released from the internal structures into the envelope.

Gas in the envelope causes grid current which can result in thermal runaway in the power valves.

It looks to me most likely that you have a poor contact with grid pin and the socket, and you may need to retension the pins in the socket.

[richard]

Thanks jpfamps. How do I go about retensioning the pins ?

[jpfamps]

Firstly remember you will be inserting a tool into a part of the amp which may have high voltages on it, so you will need to ensure that the filter caps are drained before working on the amp.

Measure the voltages on the valve pins with your multimeter to check it's OK to proceed.

I assume your amp has the "prong" style pins.

To retention them you need to remove the amp chassis and bend the pins in slightly.

You will need a tool that can slide in behind the pins and allows you to bend the pin inwards.

A jewelers screw driver is a good option (yes I know you shouldn't use screw drivers for this!).

You need to be careful that you don't snap the prongs off as this will require a change of valve socket.

If this fails, then you will probably need a trip to a tech.

[richard]

jpf - thanks a million. You were exactly right. Re-tensioned all the pins, gave the amp a good 10 mins to warm up and no redplating. Your advice is very much appreciated.

[jpfamps]

That's good news.

Well done!