[R-390] Using the 6BZ6 tube

[Jim Whartenby]

Perry
I found the quote you mention in Chapter 11 of R-390 Y2K Rev 3.0 manual.  In searching this chapter further for "6DC6" there was also the below which seems to contradict his later statement that you have mentioned:
"Potential Low-Noise Tube Substitutions: 
I don't do the following unless I run out of tubes. I tried all of these in my Collins R-390A forseveral months, but could not really tell any difference. AGC action may have been a bit worse, andsensitivity measurements didn't really change. Resting noise level did decrease with no signalinjected, but it was compensated by less gain when signals were applied. 
Net result: no measurablechange in sensitivity. If I had an abundance of the following tubes that would allow me to "tweak"the performance a bit, I might have had better results. Regardless, here are some easy substitutionsfor you to try. 
6BZ6 instead of 6DC6 in V201 and 6AH6 instead of 5749/6BA6W in V508 (National Radio Clubreprint R-57) 
6JH6 instead of 5749/6BA6W in V501, V502, and V503 (Electric Radio issue 26, pages 22+) 
6AB4 instead of 6C4"

The 6BZ6 has much higher transconductance, a much lower noise figure and a more remote cutoff characteristic.   This may result in a yin-yang effect with some cancelling the other.

I get the impression that he has selected 6DC6's and other tubes for ultimate performance.  He also mentions adjusting B+ and resistor values to improve performance so he was really getting into the nitty-gritty.  
Out of tolerance resistors is a real but often ignored issue so perhaps replacing all carbon composition resistors with carbon film will have just as much an effect as tube substitution?  I know, that is a bit much but replacements for out of tolerance resistors should be with carbon film instead of carbon composition.  IMHO a much better resistor technology.
Regards,
JimLogic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Friday, August 16, 2024 at 03:33:13 PM CDT, Perry Sandeen via R-390 <r-390 at mailman.qth.net> wrote:   

 Bryon and list.
Chuck Felton (SK) noted in his R390A repair and mod business that the 6DC6 is not a semi-remote cutoff tube (which it was supposed to be) so he used the 6BZ6 instead.  He also recommended that it be used as the 1st RF amp tube in the SP 600.
Chuck was a prolific experimenter as well. He wrote numerous notes on upgrades for the R390A, SP 600 and 75A4 to name a few.  Also he wrote an article for ER radio for simple and effective R390A receivers. 

Hope that answers your good conservative question
73,
Perry KM6FQV
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