Quote:
Originally Posted by Leon Crampin
This is a common problem with high dissipation output valves mounted in small glass envelopes - B8A, B9A and B7G.
The cause is usually an excessive g1 temperature, giving rise to grid emission with the resulting rise in anode current exacerbated by gas - often released because the valve is just running too hot.
Fortunately, there's a dead simple test for this for those without a valve tester. Assuming autobias with a cathode resistor, simply run the valve in its equipment with a DVM connected between g1 and earth - ie across the grid leak resistor.
A good valve will probably show a small positive voltage, but this must remain practically stable as the valve heats. A poor valve will show an increasing voltage over several minutes of operation (I test for about 15 minutes, unless the valve is so poor that the test places other components at risk). For a 12W pentode, I would expect no more than 100mV positive grid voltage, stable after 3 minutes or so.
I have found EL84s to be very poor in this respect, and ECL86s little better. You can't miniaturise the Watt.
Beam tetrodes are noticeably better than pentodes in this respect - I suspect grid alignment gives higher efficiency and results in a lower g1 temperature. The 6BW6 and the 6CH6 are my devices of choice - unless there's room for an octal or loctal valve.
The above test assumes that, of course, the grid coupling capacitor to the previous stage is above reproach.
Leon.
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You're right, that's a way to catch a valve showing grid-1 current.
Another way is to use a valve tester built for that same purpose.
Have a look at:
https://www.vintage-radio.net/forum/...d.php?t=148050
Rgds,
/Torben