[R-390] Ballast Tubes

[Bob Camp]

Hi

 From what's been said here they are still available for something in 
the $30 to $60 range. Not quite a price that would encourage me to grab 
a couple dozen.

One modification that has not been mentioned as part of this thread on 
ballast tubes is probably the oldest of the batch. Grab a plug that 
looks like a tube base and wire a resistor to it. The value needs to be 
right to get the filaments to run right but that's about all there is 
to it. It pulls no more power than the ballast tube and it's a totally 
reversible mod. When the bottom drops out of the ballast tube market 
you can plug one of those two dollar ballast tubes in there and nobody 
will ever know what you did.

Somehow I doubt that ballast tubes will get cheap again unless there is 
a Chinese factory we know nothing about making them by the ton. They 
are not terribly high tech so it is a possibility. There may be a long 
forgotten warehouse in South East Asia with a few hundred thousand of 
them sitting on the shelves - stranger things have happened.

If it was my radio I think I would do one of the re-wire mods to 
eliminate the beast. The filaments would be un-regulated but there 
would be less heat and no additional stuff inside the cabinet.

The function of the ballast tube in the radio is questionable. With 
modern wall voltages the original ballast is running at best on the 
edge and at worst over the edge of it's ratings. It's not doing a real 
good job of stabilizing the filament voltages on a radio plugged into 
120 to 125 VAC. Fortunately for all of us the filament voltage has a 
pretty small impact on the tubes in the radio. It's my belief that the 
problem comes in on the low voltage end of the equation. If you try a 
radio on 100 VAC then the ballast is probably a good idea.  Don't see 
much of that coming out of the wall outlets around here ....

	Take Care!

		Bob Camp
		KB8TQ





On Jul 2, 2004, at 4:17 PM, Forrest Myers wrote:

> Hello All,
>  Came into the shack and found my r-390a dead. A quick check found the
> ballast tube was shot. I've seen a lot going by on ballast tubes these 
> past
> few days but am interested in getting a real ballast tube, if I can 
> afford
> it, back into the radio.
>  Anyone have a source for a 3TF7?
>  If I must, I'll put in a mod to get around the 3TF7 but would rather 
> not.
>
> Thanks,
> Forrest Myers
> AG4ND
>
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