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

[Tim Shoppa]

eldim at att.net wrote:
> I think that's called "SHAKE n BAKE"!  But just in case there is
> a serious debate on this, then I'll make this suggestion.
> I'd be happy to prepare a mixed variety of 6C4 type and manufacturers
> for anyone who is interested in running comparison tests".  
> Of course, there are other factors to take into consideration 
> besides the tubes suffix.  Such as Manufacturer and Date of 
> Manufacture, etc.  Let me know if I can be of assistance in this endeavor.

All the 6C4's are used as mixers.

Triode mixer performance is strongly dependent on:

1. Local oscillator injection strength
2. Gain of the triode
3. The way the gain changes depending on LO strength

Most of these come in a non-linear way - for example for marginal
LO injection a small 20% change in LO strength can correspond to
a 40% (or sometimes more) change in conversion efficiency.

For weak-signal work, also important is number 4: noise figure.

So potentially 3 of the factors depend on the tube.
One of the factors depends on LO strength.

Also: In both my RF decks, many of the carbon comp resistors
had drifted way off in value in many stages, including the mixer.
The 2.2K 1/2W units were the worst, and note that each and every
mixer stage has TWO of these. These being off directly ties into
factors (2) and (3) above. I did not do any before/after measurements
but after replacing the resistors and realigning I know the deck
was way way way more "peppy" in performance.

And one obvious difference I've noticed between 6C4's: Their height!
Sylvania 6C4W's are the shortest, almost as small as the
6AK5W's elsewhere in the radio. Non-JAN 6C4's are the largest, in
my stash I have both GE and RCA's that are tall. I also have some Brit
CV-133's that are tall too. I have not noticed any obvious difference
in performance in real on-the-air listening with the different sizes/
brands. Someone who lived in a low QRN/QRM environment might have
a different conclusion though.

Even though I claim no detailed measurements, if I pull the
RF Amp tube and go through different 6C4 varieties in different
sockets, I can hear differences in noise. For somebody who 
lived on a remote Pacific island I could see these differences
being important and obvious in daily use.

Difference between tall and short tubes: The short 6C4W's are
Sylvanias, and have the same basic construction on the bottom of
the tube as the taller 6C4WA's from Sylvania, but on top the tall
6C4WA's are much more elaborate, with an extra insulator and a
substantially more elaborate getter structure. The consumer
6C4's and the CV-133's have their base triode structure much
more elongated.

As for date codes: Most of my JAN 6C4's are July 1945 National Unions.
Most of my 6C4W's are Sylvania's with 6-62 date codes, and my 6C4WA's
are 7-62 Sylvanias. Note that I am NOT CLAIMING that I have two
months worth of Sylvania's tube plant production in my basement. No sir!
I will deny any claims that I have all those tubes, and I am STICKING
TO MY STORY!

OK, I think I set the record for most attention to triode brands
EVER.

Tim.