[ARC5] Receiver Tube Substitution

[Arden Allen]

> ......The BC-779 has two RF amp stages ahead of the 6SA7, which as you 
> state
is a very noisy mixer.  ..........

As you all know multi-grid tubes used as mixers are noisier than triodes. 
One of the tradeoffs was for gain.  Which sounds naïve because you need even 
more gain ahead to get the same signal to noise ratio as what a triode would 
provide.  I guess it was all about automatic gain control and RF-OSC 
isolation which pentagrid mixers (6SA7, 6SB7Y, 6BE6, etc.) did well for AA5 
radios which had only two gain stages ahead of the detector.  With the 
additional grids of those converter tubes even more noise was generated due 
to the partition effect, i.e., electrons playing bumper cars around the 
grids.  The added grids were needed for isolation between the RF tank at the 
control grid and the local oscillator tank.  That avoided unnecessary 
interaction between the two tanks which included radiation of the oscillator 
signal from the receiver antenna.  However above 10 MHz the isolation 
becomes progressively less with frequency.  The RF tank pulls the oscillator 
when an antenna trimmer is adjusted and the AVC bus is biased negatively 
from rectification of the oscillator signal at the RF grid.  No doubt for 
years the conundrum caused Trouble in River City for designers trying to 
come up with cost efficient designs.  A 6SG7 ahead of a 6SA7 proved to be a 
successful approach for moderately priced multiband receivers like my Halli 
SX-110 which I converted to product detection and does a fair job on the 
bands.

Arden Allen
KB6NAX

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