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

[Shoppa, Tim]

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"

I do not see where you measured "correct current". My gut feeling would be to suspect that the current regulator is working properly but that the voltages are off for some other reason.

If the current is correct but there's 15V across the BFO and PTO tubes, this is not a failure of the regulator. This is the regulator doing its job in the face of not-quite-in-spec BFO and PTO filaments or some poor contacts in the filament circuit. I write "not quite in spec" because filament voltage vs current is not generally something you use in the same breath as a NBS calibrated standard. Oops, that's NIST now, sorry Roy :-). If a 6BA6 drew 260mA at 6.3V instead of 300mA I'm sure that it would be regarded as acceptable. Note that poor contact on the tube socket filament pins can also increase voltage drop; I've seen some pretty cruddy looking sockets in 390A's.

AC current and AC volts are important to measure properly. There's peak readings vs RMS readings. I assert that RMS numbers are the correct things to measure here. Makes sure you are using a true RMS instrument, and that it correctly reads RMS voltage as well as RMS current.

One failure mode for a ballast tube is that one but not all of the thin iron wires has broken. This will reduce the current.

Tim N3QE