[Collins] R-388/51J mods - BCB included

[Dr. Gerald N. Johnson]

On Sun, 2007-11-11 at 14:47 -0500, Bill Cotter wrote:
> That's a good point about the value chosen, Jerry.
> 
> I think a much lower value would be a better choice for matching 
> and keeping the mixer happy. The value 0.01uF was likely a 
> 'brute-force' option. This mod was conceived a long time ago, and I 
> cannot remember the source (CQ or QST, I believe). Howard Mills, 
> W3HM, brought it to my attention when I was working out the kinks 
> of this receiver.
> 
> 73 Bill
> 
> 
K5JV, Lon Cottingham found it in a vintage Hollow State News. Its a one
paragraph note on page 4 of issue 20 Hollow State News, fall 1988, in
the midst of 5 pages on receivers by Dallas Lankford. It may have been
published other places, but there he says write him for more information
as if that was the first place it was published. Other paragraphs do
reference QST or CQ articles.

Vacuum tube mixers are the roughest spots in a receiver. They can't
stand strong signals, but they make much more noise than a gain stage,
partly from the low gain as a mixer and partly from partition noise of
the electron stream which makes a mixer tube noisier the more grids it
has. Whatever the noise source, the fact that a mixer tube is noisy
requires more RF gain in front of it to have the RF stage set the noise
figure of the receiver. And triple conversion with vacuum tubes as the
51J family does on the BCB makes it all worse. Raising that gain with
the .01 instead of maybe a 30 pf across the 3 pf surely hurts the
blocking range and intermod performance there on a band where really
strong signals are quite common.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer