[R-390] A vs non A

[Charles P. Steinmetz]

Cecil wrote:

>As I remember all IFs were not setup to be stagger tuned.  There 
>were caps across the IF transformers to increase the Q in either the 
>ones that the manufacturer designed to be or not to be....I can't 
>remember.  The thought was that those that were intended to be, 
>we're electrically different to maintain spec.

In the 390A, the mechanical filters provide the actual IF bandwidth 
response (the interstage LC filters do clean up the stop band, where 
the mechanical filters have zeroes and ripple, so they give the IF a 
lower ultimate stop band attenuation than it would have had with the 
mechanical filters alone).  Think of the interstage LC filters in the 
390A as a roofing filter, although in this case they follow rather 
than precede the mechanical filters that actually set the IF bandwidth.

In some cases, the LC "roofing" filter in a 390A may have a narrower 
or more peaked response than the 16 kHz mechanical filter.  If that 
is a worry, the interstage LC filters can be stagger tuned to broaden 
the overall LC response so that even the 16 kHz mechanical filter 
effectively sets the IF response.

I'm not aware of there being a version of the 390A IF that was 
designed to be stagger tuned, but that's not to say there wasn't 
one.  In practice, the Q of LC tuned circuits is invariably 
determined by the Q of the inductor.  It is possible that some IF 
strips had higher-Q inductors than others, and that these were 
identifiable by some other characteristic (for example, presence of 
additional capacitors), so radio techs learned to stagger tune those 
in particular.  The only real benefit of stagger tuning a 390 IF is 
to broaden the 16 kHz response, and I suspect most 390As were never 
switched to the 16 kHz position by operators during their entire 
military service lives.  Heck, I virtually never use the 16 kHz 
filter, and I tend to favor SW and MW broadcast targets.

>Stagger tuning one that was not intended to be would probably have 
>low IF gain...and not stagger tuning one that was designed to be 
>would probably have excessive IF gain.

The 390A has so much extra IF gain available, and such a wide range 
of IF gain adjustment, that this would not likely have been an issue.

Of course, the 390 is entirely different -- it has no mechanical 
filters, so the interstage LC networks do set the effective IF 
bandwidth (and are, therefore, designed very differently than the 
390A interstage networks).

Best regards,

Charles