tubes age 177
Subject: [R-390] 6C4 versus "W" and "WA"
Since I needed to pull my R-390A for some other work, I decided to duplicate
Tim Shoppa's findings on the performance of 6C4 versus W and WA. After 3-4
hours of tube swapping, hot fingers and careful measurements, I see similar
results to Tim's. The "will not work" reference will come off of the FAQ web site.
I checked back to the source and it had been around for a while - traced to a
1950's R-390A manual reference. As far as the 'the "FAQ" and "Y2K" manuals
are full of statements that are simply wrong' accusation in a previous posting - I
say 'put up or shut up' - if you have knowledge of factual errors in either, please
share them with the group and me, so they can get corrected. They will not get
fixed unless you provide data instead of rhetoric.
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Date: Sat, 22 Apr 2006 17:39:59 -0500
Subject: Re: [R-390] Re: 6C4? 6C4W? 6C4WA?
I'm like Tim, I don't think I've seen any difference between the different flavors of
6C4 in any of my radios. All seem to work just fine.
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Date: Sun, 23 Apr 2006 08:32:27 -0400
Subject: Re: [R-390] Re: 6C4? 6C4W? 6C4WA?
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
4: noise figure
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.
(14 pages)
(10 pages)
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