[R-390] Ballast Tubes (no, not the dead horse!)

[Drew P.]

Paul Galpin wrote:

"I recently got two 390As, one working, one not. Both were fitted with 3TF7
ballasts. Hooray, original, great! I thought.

Then I measured the voltages, 25 Vac in, 15 Vac out. Correct tubes in BFO
and PTO.

So at the correct current, they are not dropping the specified voltage.

No problem, I put in the 42R resistor.   

What I want to know is - is this a common failure mode? If so, why?

Paul Galpin
ZS2PG

The current is likely not correct either.  This is a reasonably common failure mode, though not as common as the filament going open.  Or is it?  Most of us do not notice a somewhat out-of-spec current because the radio still operates.  Open filament lets us know quickly by virtue of the resultant deafening silence.

Failure to maintain correct current when operated within ratings is likely due to leakage of the backfill gas mixture of hydrogen and helium.

But need you take my word for it?  Certainly not!  Goto r-390a.net, references, Pearls of Wisdom, Ballast Tubes.  There you will find Wei-i Li's tireless labors of collection and distillation of the wisdom and lore pertaining to this topic as has passed through this forum for over 10 years.  You will learn of construction, history, failure modes, and every possible ballast substitution scheme known to mankind.  Fascinating reading, thanks Mr. Li!

Drew