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Old 2nd Jul 2022, 2:54 pm   #28
trobbins
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Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: Faulty valves - Scrap or keep?

There can be shades of grey when it comes to some valves being 'faulty'.

One such example is the 6BM8, which I'd suggest is the most 'pulled' noval power pentode in Australia - in that anyone involved with servicing valve equipment would likely have pulled them, given the variety of radio's and radiograms etc that this valve was used in.

A few years ago I won a box of 83 such pulled valves for I recall $2. As an exercise in frugality I tested each one in a typical cathode biased test circuit for nominal anode voltages, and for grid leakage of the triode as well as the pentode, and then tested good like valves for matched pairs in a PA amp. I ended up with 17 matched pairs, and an additional 34 ok pentodes (the triode grid was leaky). I didn't test the 'ok pentodes' to see how many could be used as matched pairs, but likely to be a few. Based on that batch, my general comment would be that in Australia there are likely quite a few 'faulty' valves that could happily live a useful life.

Furthermore, one option is to use just the pentode portion of the 6BM8 if it is still ok, especially when the valve has been pulled due to a leaky triode input grid. Obviously not for existing applications that actually need a 6BM8, but certainly ok for those just wanting a noval power pentode (with probably more than a 7W anode dissipation limit now).
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