[R-390] Alternative tubes for R-390A 5749/6BA6 in 1st IF V501

[Larry H]

Because I'm receiving a lot of stations/signals that are weak and have that annoying background hiss, I've been looking at ways to improve their reception.  My previous post on 3-10-17 was about my pursuit of an improved RF amp, which was successful to some degree.  Previously to that, I went through the tubes that I had on hand at that time and rated the 'merit' using Roger Ruszkowski's method described in the Y2KR3 doc.  That provided some improvement, also.  And, previously to that, I improved my antenna for a better s/n ratio.  These actions have improved my reception quite a bit, but there is one more thing I'm going to try to do, improve the audio/IF s/n to the 30 db level.  As of a few weeks ago it was at 28 db, according to Roger's procedure (which I believe is an excellent test).
I obtained some more 5749's, so I'll be rating them 1st.  However, on my way to this point, I read something interesting about using alternative tubes for them.  They have the 7BK basing diagram, but since pins 2 and 7 are connected together in the R-390A for V501 - V503, the tubes with the 7CM basing diagram will also plug in directly and work correctly.  This opens the direct alternatives up to a few more tubes, and 2 have very good characteristics for improved operation, a 6BZ6 and a 6JH6 (the 1st, 2nd and 3rd IF must be a remote cutoff in order for the agc to work correctly).  I did not try the 6GM6 because the cutoff for it is -15, and that is not close enough to the -20 of the 5749.  These 2 tubes will also work in the 2nd and 3rd IF positions.  So, here's the  list of direct plug in substitutes that I came up with for V501, 2, and 3: 5749, 6660. 6BA6, 6BZ6, and 6JH6.  
Did I miss anything?
So I went through my new 5749's and rated them for merit.  I found 2 that were better than the one I was using in V501.  I also rated the 2 6BZ6's and the 1 6JH6, and found one 6BZ6 to have an even better merit.  I've been using it for a couple weeks now, and it is an improvement, and I don't see any issues, yet. This did improve the audio/IF s/n to the 31 db level.  My best 5749 improved it to 30 db.  Success!
Roger - Thank you for documenting your procedures!
I've been working on improving my reception for more than a year now, inching my s/n down little by little.  It started out at 1.2 uv as an average on most bands.  The biggest improvement made was getting good 5749's in the IF strip, then the 6C4 mixers numerous months ago.  These improved my average s/n to .5 uv.  That brings you up to where I'm at now with the RF amp and the 1st IF improvements of the last couple weeks.  My s/n is now .2 uv +/- 10% on most bands.  
Now I know that a lot of folks are thinking - what's the point of a s/n that low, or really, I don't believe a number that low for an R-390A.  Well, I've been measuring s/n for 55 years since electronics school, and I know the proper methods involved with getting it right.  One thing I do is calibrate my URM-25D and 'line level meter' periodically with my HP 400FL  AC RMS meter.  As to the benefit of the low s/n, a lot of the usefulness deals with the s/n of the antenna and how quiet the bands are.  I doubt that a very low s/n will make a difference on the bands below 23 mc where I'm located and with this antenna, but I can tell the difference on frequencies above 23 mc, as that is the point at which my antenna's noise is considerably reduced.  And it is very low above 27 mc.  I have no way to measure my actual improvement in reception for the last 2 weeks, but I usually listen off and on for most of the day every day, and this is the best it's been.
I know that we all know the importance of a low noise RF amp, but I did not think that the 1st IF was very important, but I guess it is.  I've read through Rogers and others info many times, and did not get that impression.  Now I know.  Good listening to all.
Regards, Larry