[R-390] R390 progress

[Laird Tom N]

Here is a clip from my archives from Dr. Jerry K0CQ

Tom Laird WC9M
Moline, IL.
==============

I did find full data sheets on the
'J and the 'K. There was a difference in standard characteristics, at
least in the GE brand the J's heater cathode voltage was limited to 90
volts but was rated at 120 volts in the K. The other ratings including
capacitances were identical.

In typical operation the Gm of the J is half that of the K, but with the
plate voltage so low the J's curves look more like a triode's curves,
because they don't get up to the constant current regions of the normal
pentode. There are more curves shown for the J than for the K but they
are miles from overlapping so are nearly impossible to compare.

Since the input C and the maximum voltage ratings and dissipations are
the same, I stick by my prior conclusion. There could be a difference in
cathode material maybe to encourage electron emission at the lower plate
voltage of the typical operation of the J, but if it was significantly
different they couldn't have the same maximum plate voltage, so I stick
by my conclusion that the J is a K tested at 28 volts and the K is a J
tested at 120 volts on the plate. And the plate test voltage is selected
at the end of the production line according to which tube they need at
the moment. I suppose these days I should say "was!"

There were some Western Electrics, 404 family if I remember correctly
that fit the same sockets with higher gain and lower noise that might be
interesting for RF stages. I know the Western Electric tubes definitely
worked better in old 2m FM radios. Wouldn't be any benefit in
oscillators, but might make the higher frequency bands a bit livelier in
the RF stage.

Probably in a 1938 or 1939 (maybe later) issue of the IRE Proceedings of
the Electron Devices group, there's an article about the J and K. Maybe
something in the MIT RAD lab books on components since the K was used
heavily in WW2 radar IF strips. Maybe something in the IF amp book too.

Jerry, K0CQ

-----Original Message-----

I pulled the two oscillator tubes, 6AJ5's according to the manual and
markings on the radio. But one of mine was a 6AK5. Both tested ok and
the 6AJ5 is apparently the same tube as the 6AK5 designed for low plate
voltage. They indicate about the same gm when tested according to the
Hickok settings for 6AJ5,  which puts low voltage on with my tester.
Does anyone know if the 6AK5 performs reasonably well in place of the
6AJ5 in a 390?