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

[Flowertime01 at wmconnect.com]

Fellows,

I like Tim's following points
__________________________

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.

...
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.
--------------------

Once you down to differences you cannot hear you can still measure smaller 
differences with test equipment and meters.

Just look at all the factors Tim listed. There is no substitution for placing 
a tube in a circuit and measuring the output. Who knows exactly where the 
resistor values in your receiver circuit will bias a tube. Who knows where a tube 
will fall on its performance curve. No substitution for lots of tube 
substitution in the circuit to find the one tube that best fits the circuit from a 
signal to noise perspective.

Does Tim have a batch of tubes from one source that behave a lot better than 
other tubes? I can belive he sure does. What do we know from all of this. Tim 
has found some good tubes and some not so good tubes. What can we take away 
from this?
Our miliage may vary.

Our choice is to keep buying and testing what we find.

Roger AI4NI