[R-390] RE: Nuvistaplug?

[Drew Papanek]

Mark Huss wrote:

>Here is one for all the experts.  A plug-in modification for SP-600's was 
>the Nuvistaplug.  This was a replacement for the first RF amplifier in 
>SP-600 that substituted a pair of 6DS4's in a cascode circuit.  The 
>resulting 6 to 9 dB increase in Signal to Noise level made the SP-600 sound 
>like a new receiver.  My brother recalls that there was a similar 
>modification using a 6CW4 for the R-390A, popular with MARS operators.  I 
>happened across the schematic today, and it looks like with changing one 
>pin in the Nuvistaplug, you could drop in the replacement for the 6DC6.  
>This should increase the signal to noise level of the first RF Amp.  So has 
>anybody even heard of this being attempted?

Perusing the R-390 Final Engineering Report (at r-390a.net) I see where the 
Collins engineers tried a cascode circuit using a dual triode TV front end 
type tube.  They found that the RF amplifier stage so configured had 
insufficient AGC control range.

Perhaps the 6CW4 is different; that tube was not available back in 1948.  A 
cascode circuit using 6CW4's might have a comparable AGC control range to 
the 6DC6 circuit if the 6CW4 has a variable mu characteristic.  I don't know 
what  dual tiode types were tried at Collins nor do I have a set of 
characteristic curves for the 6CW4 to compare.

Maybe in our application we could tolerate less AGC control range.

That said, the 6DC6 circuit in a properly functioning R-390A can hear right 
down to the thermal noise level and no additional gain would be required.  
More gain would reduce reduce the radio's dynamic range ( more tendency 
toward the dreaded intermod).

The R-390 and R-390A have quiet 6C4 triodes as mixers and do not need much 
RF amplifier gain to override the small mixer noise contribution.  The 
SP-600 uses a 6BE6 pentagrid converter as a mixer; that is one of the 
noisiest mixers known to mankind.  The SP-600 needs plenty of RF amplifer 
gain ahead of the mixer, particularly on the higher bands, so that signal 
can drown out the mixer noise.

Drew