[R-390] R-390A 6C4 Mixers

[Les Locklear]

Well, back in the mid to late 60's the company I was working for was making the transition to computers to process our parts orders, etc. We had multiple problems with the original 6C4's. Of course as everyone else in those times being it for hf receivers or mainframe computers, we tried the "Mil Spec" 6100's, and the 6C4WA's with the ruggedized filaments. We eventually settled on the computer rated (new at the time) 6135 tubes. 
 
They solved all the longevity and all other problems. Fast (or maybe slow forward) 40 years and I'm having problems with flaky 6C4 tubes, especially on Hammarlund receivers. The last several years I have gone through multiple SP-600's and in recent years, modified HQ-180C receivers. I can tell you without reservation or tube data sheets that 6135's are the way to go, 540 KHz to 54MHz they work without any complaints, losing oscillation or getting weaker on higher frequency signals than and of the other 6C4 variants. The 6135 is a permanent fixture for my choice in place of the common 6C4. Especially on the Hammarlunds where they seem to be finicky. Not so much on the R-390A's. Try one, you'll like it!
 
As usual, YMMV.
 
Les Locklear


----- Original Message -----
From: Byron Tatum <bjtatum1 at att.net>
To: <r-390 at mailman.qth.net>
Sent: 6/29/2024 6:44:17 PM
Subject: [R-390] R-390A 6C4 Mixers
________________________________________________________________________________

> Hello-    If any members have the time I wanted to ask some questions regarding 
> the 6C4 tubes used as mixers in the R-390A.    I have been going through my 6C4 
> type tube stock here, testing them on a TV-7D/U, preparing for some work on a 
> few receivers.    Is there any advantage in using the variants of the 6C4, such 
> as the 6C4WA, 6100 or 6135 instead of using the plain 6C4? Can someone explain 
> the differences between these variants and the common 6C4. As I understand it 
> the -WA, 6100 and 6135 variants mostly differ in ruggedness and possibly 
> quality of construction. Just curious about other's opinions here. I have researched these variants and studied the data sheets. Was wanting to know if any electrical properties of variants would benefit mixer service.    I know that sometimes the particular brand/variant of 6C4 can be critical in oscillator service. I experienced this some in restoring a few HQ-180 receivers and picking out the most stable 6C4 oscillator tubes.    Thanks for comments,Byron W5FH  
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