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

[David Drew]

I could use guidance from the group regarding which version of the 6C4 is best to use in the R-390A.

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."

Now, in the Y2K R-390A Technical Reference, there are many references to the 6C4. There is one 
reference to the JAN-6C4W in the parts list. And there are NO references to the 6C4WA.

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

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?

I also looked in Bud Tomer's 1960 "Getting the Most Out of Vacuum Tubes." He mentions the "W" suffix 
for ruggedization, but doesn't say that a W-type would be better in other ways. There isn't much 
vibration in my shack!

The Army's "Basic Theory and Application of Electron Tubes" wasn't any help, either.

In the end, I believe in the collective wisdom and experience of the FAQ. Still, for me, part of the 
fun of using these wonderful old receivers is learning more about them. Why is the WA the best, and 
the W, the worst?

Regards,
Dave - K3DX