[R-390] R-390A s/n improvement 6HA5 mixer, 6GU5 RF amp

Wed Apr 20 10:20:47 EDT 2016

Give a 6BZ6 a try in the RF amp....I see them used a lot.

Cecil
K5DL

Sent using recycled electrons.

> On Apr 19, 2016, at 7:16 PM, Larry H <dinlarh at att.net> wrote:
> 
>      I've been wanting to get some time for a couple years now, to work on this and finally did a few months ago.  I've had very good reception with R-390A's for 37 years since I got my 1st one.  About 10 years ago I added SSB capability to 1 and it works great except for 1 thing, the background noise on weak stations.  I picked up an SX-111A about 6 years ago and it had a better s/n than my 2 R-390A's.  I thought there must be some diffenence in the RF amps, but that was not it.  They are the same.  The 111A has a 6BY6 mixer with a 6C4 oscillator.  
>      For years I've read the 390 forum posts and other online doc about lower noise tube alternatives for the R-390A, but saw no easy way to try them until recently.  I wasn't about to pull the RF deck and do surgery on it.  There would be no way to switch back and forth to measure the real results.  I thought about tube adapters, but was not able to find 7-pin tube bases to make adapters out of until recently.  So, I read through a lot of R-390A doc that is online (thank you all that have contributed to this wonderful info) and decided to try a 6HA5 mixer.  I made an adapter about an inch high that plugged right in.  I used a tube socket for the 6HA5 that has the shield base on it just like those used in the R-390A.
>      My first try was the 2nd mixer and using the 9 mc band.  With the rf gain at the same position as with the 6C4 installed, the no-antenna noise level was 9 db higher.  I wasn't sure if that was a problem or not.  I measured the s/n with the traditional method, but could not obtain the 10 db increase in reading.  I thought: OK,what did I break.  Well, it turns out that when using a 6C4, the rx noise level increases with the signal level more than enough to create the 10 db increase at a low input signal level (about .4 uv +/- .1 uv on all the bands).  But, when using the 6HA5, the 'noise' level gives way to 'guieting' before the 10 db increase is achieved.  This required a different way to measure any benefit that might be obtained.
>      What I wanted know is how much signal level would create clear audio (overcome the 'noise' level).  I'm using the 10 db 'quieting' method.  With signal generator and impedance matching device connected to the balanced input and no output from my URM-25D, I set the VU meter to 0 db.  I then increase the generator output (with no modulation) until the VU meter goes down 10 db.  At this point the 'noise' is barely audible.  When I turn on the 30% modulation, the tone is loud and clear.
> I measured numerous bands using this method with 6C4's installed and this is the results:.9 mc: 3.3 uv, 3.7 mc: 5.6 uv,  7.9 mc: 3.5 uv, 9.9 mc: 2.4 uv, 15.9 mc: 2.3 uv, 21.9 mc: 2.1 uv.
> So, here's the same bands with a 6HA5 in the 2nd mixer position:.9 mc: 1.5 uv, 3.7 mc: 3.5 uv,  7.9 mc: 2.2 uv, 9.9 mc: .44 uv, 15.9 mc: .75 uv, 21.9 mc: .67 uv.
> 
> And, here's the bands under 8 mc with a 6HA5 in the 1st and 2nd mixer position:.9 mc: .47 uv, 3.7 mc: 1.2 uv,  7.9 mc: 1.4 uv.
> 
>      As you can see, this is a marked improvement in the measurements, but the real results are in the listening.  I've been listening to 40 and 80 meter SSB for a few days now, and reception is clearly improved. It's bringing in the weak stations much better and the noise between transmissions is noticeably reduced.  Of course, AM reception is also much improved in bringing in weak stations.  During testing, I did switch back and forth a few times to verify that the readings were real and repeatable.  And of course, having to do the necessary alignment each time.  I tried 6 different circuit scenarios for the 6HA5, settling on the simplest.
>      I also tried a 6GU5 as an RF amp alternative, but could never get it to work better than the 6DC6.  I tried numerous different bias and hookup configurations.
> Sorry for the long post.
> Regards, Larry
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