Faulty valves - Scrap or keep?

[Richardgr]

I suppose it depends a little on the criteria to decide that a valve is kaput. I had a radio that used a 2A3 output tube, that played excellent music from blue tooth input, but when I tested the 2A3 (lovely old original Arcturus) it was just 10% emission. I had another even worse 2A3 in another set which still worked enough to listen to the set.

However I have recently discovered a couple of 12AX7's that are zero emission on one side only, so they will go into my 'dead' box with '1/2' inscribed indelibly. Maybe I breadboard something that just needs a single triode.

I was also excited to find some Mullard XF1 branded EL34's in recent auction lot, but one was poor, and the other zero emission again. They are in the dead box, but there are people out there who collect such things.

Which begs the question. How does a tube get to zero emission? It must be bad for a long time, or is it a symptom of another failure, e.g. a coupling capacitor? Could the grid be melted due to having positive DC on it?

I have now recapped the amp with the bad 12AX7s, but I really must invest in a reasonable capacitor tester.

[G.Castle]

I do tend toward testing in circuit, I've seen a lot of apparently near dead valves soldier on, seemingly at full performance in less demanding circuitry such as a radio. A valve tester is a useful tool but can condemn a perfectly serviceable component.

Valves that are second hand and have been stored for decades can appear dead. One trick I've learned is not to give up hope, connect cathode to grid via a milliamp meter and power the heater. It's remarkable how they often wake up after a few hours and become useable once more. A totally dead example may look OK but have often just gone to air...usually easy to tell, they get unusually hot with just the heater running.

Greg.

[Tim]

I sometimes keep older ones for the bases etc. I have made adaptors and “ converted” electrically similar valves to a different base

[McMurdo]

Since my mountain of unmarked valves has grown, I've began keeping all the broken valves that have intact type numbers. It might help me to identify the unknown ones by holding them side by side! Unfortunately it doesn't work with metallised valves but there you go

[60136 Alcazar]

Originally Posted by Richardgr

... , but I really must invest in a reasonable capacitor tester.

Decades ago I constructed a variable PSU from scrapbox items for this purpose which allows me to check capacitor leakage at full working voltage - it uses an EF80 as a regulator, and provides a continuously variable output from 100v to 500v with cathode emission limiting to about 14mA, thus making it also suitable for reforming electrolytics.

As far as old dud valves go, I once donated a failed Mazda AC/VP1 with the glass envelope removed and glass edges rounded off to the physics department of my former secondary school as an example of a valve electrode structure, it being big enough to see all the elements clearly.