[R-390] 3TF7 replacement

[Craig]

Good Morning All,

For the purist, plug that puppy in (3FT7)!The belief is the 3FT7 regulator
tube was included in the design to control variations in supply voltage to
the receiver; such as a R-390/A installed in the rear of a deuce & a-half
towing a generator thru swamps, over hill & dale, etc. Highly recommended
for those who own a deuce & a-half!

On the other hand, the voltage to my abode is quite stable. I'll spend the
money on good whiskey rather than a 3FT7.

Your money, time, and radio; YMMV

Regards,
Craig


I believe in keeping my R390A as original as possible, but there are
caveats. Before shouting "Hallelulya, I've got the original" and plugging in
your new(?) 3TF7, have you thought what it might actually be doing?

I decided to check out my two 3FT7s. One of them was not too bad, but the
other definitely overloaded the heaters, in my opinion.

The test needs only a variable PSU, an accurate voltmeter, and an accurate
ammeter.
This last can be a digital voltmeter looking across an accurate resistance,
the current is the important thing in an iron-hydrogen barretter like the
3TF7.

One reached the "stable level" at about 9.0v, and kept it there up to 20V.
No further, I didn't know when the filament might pop! So the stable level
current was between
0.310 and 0.312A for a range of 9 to 20V, not too bad.    

The other reached 0.3A at 6.5V and continued to climb up to 0.37A at 11V
where it stayed up to 20V. Not healthy, IMHO!

I suspect that as these were originally hydrogen filled, over the years the
gas has become diluted with air, which gives a different cooling effect to
the iron wire. Or something.

Paul Galpin
ZS2PG

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