The AM Forum

Well, they've been baking for some time now and there's still some kind of flaky stuff stuck on the glass. I've only run the fils to heat things up but perhaps that's not enough.

No, I'm not using the Valiant as the next step :-/

I've been thinking of using a 25 Watt bulb and 120 VAC as the source to test the Fil/Plate circuit. With this arrangement, even a worst case scenario would simply cause the bulb to light to full brilliance.

Thoughts?

Those flakes will never go away. :-[ 

They are plate coating material, dislodged by someone applying full voltage before the tubes were hot enough. :P

They may still work, but their lifespan will be shorter. ???

If you would like to run some (I do!), PM me and I'll see that you get some good ones. :D

73 Dennis

The 25W bulb should be fine for peak voltage testing. To subject it to current then a larger lamp would help.

The peak test voltage would be equal to the peak secondary voltage. A variac could be used and an AC voltmeter across the variac output (before the lamp) could be calibrated to correspond to the peak transformer voltage.

Instead of a lamp, a resistor of known value would let you calculate the peak current better. A scope and shunt might also be used for more precise measurements.
If considering using the tube, you should test it with current and see if it still behaves. See if it will go up to its peak current rating. Still no guarantee how long it may last.

A lot of bother since 866 are somewhat plentiful, but if you really want to know, do the experiments!

If you want 3B28s I have them to trade for good 866's but they don't glow as nicely.. so take up the offer of the 866s!

Indeed. This is quite a fraternity this amateur radio thing...