Jim - Great input here. Always good to learn about tubes. I liked your point
about carbon film resistors. I purchased a bunch of carbon composition
resistors in 1991 as spare units that in 2024 had drifted outside of 15-20%.
The group of 120 new 2Watt resistors I have just purchased have good copper
leads, and the values are very good. The only observation is that the body has
two small transition caps on either end of the body of the resistor to
transition to the leads, and these caps enable the entire resister to be picked
up by a magnet and I got to thinking about potential magnetic reactive
inductance issues in an RF circuit. After reading up on this I decided it was
a non-issue for the 390A use case. These new resistors will outlast me.VR
John N3JKE
On Saturday, August 17, 2024, 2:15:58 PM EDT, Jim Whartenby via R-390
<[email protected]> wrote:
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
<[email protected]> 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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