[R-390] R-390A 6BE6 Product Detector changed to 6BY6

[Larry H]

I made the Capt. P. H. Lee 6BE6 product detector change to one of my 390As about 15 years ago.  After making some minor adjustments to it and installing the Dr. Langford 2 diode AGC change, I've been very happy with it.  I recently improved the S/N of the RF and IF decks, but it seems like there is still too much tube noise.  When I converted it to a product detector, the noise level went down 2 db from using the BFO and diode detector, but when I turn the product detector on, the noise is still 2 db more than without.  When I did the conversion, I paid close attention that the audio level would be the same when switching between SSB and AM.

When I did the change, I thought that the signal levels would be high enough, that a low noise tube would not make any difference.  Well, that was a bad assumption.  I started looking for a low noise alternative, and found a few: 6BY6, 6CS6, quiter 6BE6, and others.  I had more of these tubes, so I started with them.  I did not make any circuit changes and tried them.  I found a quieter 6BE6, 1 db.  The only 6CS6 I had was also 1 db quieter, but the oscillator output was down 3 %.  Not a big issue.  I tried the 6BY6, and it was 2 db quieter, had the same oscillator output, and the same audio output, and the noise level is now the same in SSB or AM.  I don't have any spare time to look for a better solution, so 6BY6 it is.

Regards, Larry