[R-390] R-390A s/n improvement 6HA5 mixer

[Larry H]

I've been playing around with this for a few months and in my original post on 4-19-16 I let you know that replacing the 1st and 2nd mixers with 6HA5's was a very noticeable improvement in weak signal and SSB reception.  I knew that Ray Osterwald had written an article on this subject, but could not find a copy of it. 

Renée, K6FSB, graciously offer to send me a copy of it (Part Three - 'The Competition Grade R-390A') from ER #26.  Thank you very much Renée.  I also thank Perry Sandeen for the schematics he sent to me on this subject.  All have been very helpful.

After reading it, I realized that trying the 3rd mixer also would probably be worth while.  And, I am here to tell you,  at my surprise, that it was.  The level required to produce 10 db of quieting is much more difficult than the usual 10 db s/n + n (this rx before using 6HA5's, the average uv level for 10 db s/n + n was .7 uv).   The same rx with 6C4's had an average uv level required to produce 10 db of quieting was 2.4 uv.  That's a huge difference in measurement methodology.  Here's my results for 10 db of quieting while using a 6HA5 for the 1st & 2nd mixers and then for all 3 mixers:.


1st and 2nd mixers:
   .9mc: .64uv, 3.9mc: .95uv, 7.9mc: 1.0uv, 9.9mc: .78uv, 15.9mc: .90uv, 21.9mc: 1.0uv.
All 3 mixers:
   .9mc: .47uv, 3.9mc: .70uv, 7.9mc: .63uv, 9.9mc: .57uv, 15.9mc: .35uv, 21.9mc: .70uv.

As you can see, there is substantial improvement in the measurements using 6HA5's in all three mixers.  And yes, I can tell the difference when listening to both AM and SSB.  My antennas are not the quietest, but I can tell the difference.

Regards, Larry Haney