Fox's Mercantile wrote:

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Phil Allison wrote:

In 50+ years of working with valve gear, I have never
used one and know no-one here in Sydney who does.

I have a few, but usually just to satisfy my curiosity.

Not long ago, I designed a special power valve tester


An excellent bit of kit sir.


** Nice of you to say so.

FYI:

My colleague Rod took a few liberties when he edited my submitted article.

He added extra circuits of his own, that he had merely simulated and did not build - he has never seen the prototype shown in the pic that I use here.

Mine is very simple, it uses only external PSUs - the 6.3V comes from a variable 5A DC bench supply and negative bias from another 2A one.

There is no "test" button as there is an on/off switch on the front of my 240/240 isolation tranny.

The only known trap is if you hastily hot switch from from pentode back to triode mode - as the latter requires way more negative grid bias. Normally just pops the fuse.

I soon found that most 6BQ5s could be pushed way more than usually seen in pentode mode - with plate and screen voltages of up to 640V and 320V respectively.

So I built a simple output stage that employed a push-pull pair that way and got a remarkable 60W rms of clean power with no sign of red plating !!

Having long ago gone through all my junk box valves, the tester is mainly used for brand new ones, soon as I get them, in case I find a dud or one well out of match. I have even had one or two with the glass broken !!

If I need to return a bad valve, there is way less hassle if I do so the same or next day.

I also added a banana socket to the rear of the box allowing a short top cap lead to be fitted for types like the 6CM5 and 6DQ6B. A few Aussie made guitar amps from the 1960s used them.



..... Phil