A good point and one that I hadn't considered. It dawns on me that the
old single PA tube transceivers were protected in this way (like the
Swan Cygnet transceivers), whereas the dual PA tube transceivers were
not. When I rewired my Swan 500c from the old two 6LQ6's in series
configuration to running 31LQ6's in parallel, maybe I should have added
current limiting resistors in there. Although I suppose that the
majority of the damage to my sweep tube PA's is not likely to be coming
from cold-starting the filaments.
-Adam AJ9D
On 12/19/2011 11:37 AM, John Rehwinkel wrote:
Power each tube's filament in parallel from the 5v supply, in series with a
voltage dropping resistor. Since the filament will draw a fixed amount of
current, one can easily calculate the value of the resistor needed to lower the
5v supply to the supply voltage appropriate for the filament. This has the
disadvantage of wasting a lot of power in the resistor, however.
It also has the advantage to soft-starting the filaments - cold filaments have
low resistance and draw heavier current until
they warm up. The series resistors limit this current surge.
- John
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