11-29-2016, 06:16 PM
I like easy questions.
> Still don't know why the grid leak voltage drops out when I turn the tuning cap to the low end of the dial.
Because there isn't enough feedback to keep the osc running. This can be caused the tube not having enough gain (weak or voltages too low), Bad coil, or out of tolerance micas/ tuning cap shorted. Just for grins check the heater voltage on the 6J8 (needs to be 6.0vac or more).
> The oscillator can't be running with no bias can it?
The control grid needs or will be negative when it is oscillating. Not oscillating the grid will be at 0 or positive using a high Z meter, no VOM.
For a drop in replacement try a 12A8. None the the 6J8's 6v friends have a 150ma heater there all 300ma. Electrically the 6A8/12A8 has more gain than the 'J8 which is good.
> Still don't know why the grid leak voltage drops out when I turn the tuning cap to the low end of the dial.
Because there isn't enough feedback to keep the osc running. This can be caused the tube not having enough gain (weak or voltages too low), Bad coil, or out of tolerance micas/ tuning cap shorted. Just for grins check the heater voltage on the 6J8 (needs to be 6.0vac or more).
> The oscillator can't be running with no bias can it?
The control grid needs or will be negative when it is oscillating. Not oscillating the grid will be at 0 or positive using a high Z meter, no VOM.
For a drop in replacement try a 12A8. None the the 6J8's 6v friends have a 150ma heater there all 300ma. Electrically the 6A8/12A8 has more gain than the 'J8 which is good.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got it!
Terry
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