[R-390] 3TF7 replacement

[Paul Galpin]

Hi guys!

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