[R-390] 6C4? 6C4W? 6C4WA?

[Tim Shoppa]

> The R390A FAQ says, "Three 6C4WA tubes are used in the R-390A.
> The 6C4WA designation is for the  ruggedized version of the 6C4, a
> medium-mu triode. The 6C4W tube WILL NOT work as a replacement for
> the 6C4WA in the R-390A. The 6C4 will work as a replacement, but
> sometimes not quite as well as the  6C4WA."

My personal experience: I've got bunches of 6C4's, 6C4W's, and 6C4WA's.
I
believe the technical quantification is "metric buttload".

Modulo small changes in IF/RF gain balance, I see zero functional
difference
between them. I'm not going to claim they all have the same noise figure
etc. but the "WILL NOT WORK" quote is entirely off-target.

The CV133 and EC90's are equivalent to 6C4's too.

> The 6C4 is commonly available for $3 NOS. The 6C4WA seems to bring $5
NOS/NIB.

Those are "asking prices". I often see large lots of all three varieties
going
for less than $1 a tube. The military surplused large lots of JAN 6C4
varieties
in the past couple decades and I don't think we're in any shortage of
them.

> Even more surprising, my 1964 edition of the RCA Receiving Tube Manual
> lists the 6C4 as a power triode, used as a LO and a class-C power
amplifier.
> I wonder what the designers were thinking when the chose the 6C4 for
mixer
> service in the R390A? Immunity to overload? Low noise?

The 6C4 is simply a very common "jellybean" triode. If you think of it
as half of a
12AU7A, you are exactly on target. "Power triode" is a bit of an
exaggeration,
it's no 3-500Z, but it has plenty of plate dissipation for receiver and
driver
use.

I have noted that all the mixer circuits in a 390A use identical DC
operation
points. The repitition is almost enchanting. I still want to analyze the
circuit
in terms of conversion efficiency vs AGC voltage, but I don't think that
any single
mixer stage has a whole lot of gain in typical use.

It is vaguely possible that in some RF uses parasitics might affect a
6C4W differently
than a WA. Someone here was kind enough to point out the degeneration
resistor in
the 390A's mixer stages, and I think that this will swamp all such
differences.

Like any technical data source, even the "FAQ" and "Y2K" manuals are
full
of statements that are simply wrong. Don't get too worked up about any
single
one.

Tim.