[ARC5] ARC-5 Mixer Substitution

[Kenneth G. Gordon]

On 4 Apr 2018 at 9:38, Scott Robinson wrote:

> Folks,
> 
> The 12K8 (triode/hexode) and 12J8 (triode/heptode) mixer tubes have a 
> major advantage over regular pentagrid types like the 12A8 or 12SA7. 
> It's this: the LO pulls *far* less with AVC action on the -K and -J 
> types. Example:
> 
> GE G-106 wood radio, 1939 vintage. Has RF amp and 6A8 mixer. Aligning it 
> at the top end of the SW band at 18 MHz was nearly impossible, as the 
> AVC moved the LO over 20 KHZ from the start point. Substituted 6K8, 
> which is a triode/hexode so the LO is better separated from the mixer 
> and the deviation decreased to 1 KHz.
> 
> The noise advantage of the 12SA7 is trivial. The equivalent noise 
> resistance of the 12SA7 is 240K and of the 12K8 is 290K. The RSF amp is 
> used partly to overcome mixer noise. 12SK7 noise resistnce is 11K, far 
> lower than the mixer. These numbers come from the Radio tron Designer's 
> Handbook, 4th editoin, page 938.
> 
> FWIW,
> 
> Scott Robinson

Your information is worth quite a lot, Scott, and I am grateful for it.

I HAVE subbed a 6SB7Y for a 12SA7 in an old Hallicrafters S-41G, and the result was a 
HUGE reduction in noise, and thus an increase in signal levels, at frequencies above about 
10 MHz.

I would be curious about subbing a 6SB7Y for the 12K8, although of course some 
allowances would have to be made for the different filament voltages. A resistor in series 
with the filament of the 6SB7Y may work as long as the filaments in the receiver had been 
changed to 12V from the normal 24V.

There is no 12V version of the octal 6SB7Y, although, as I remember it, there may be a 7 or 
9 pin equivalent.

I have also been very, very curious over the past several years concerning what a sub of a 
complete Pullen mixer using a pair of 7963 sub-mini dual triodes (that is, 4 triodes) for the 
6/12K8 would accomplish. 

One could build a plug-in adapter using an old octal tube base. 

Each triode in the 7963 has a tranconductance of 13K, which should translate to very low 
noise output.

Of course, although noise output would be greatly reduced, I have no idea what effect this 
sub would have on both stability and tracking.

Besides, the lower noise may not be worth the effort, since the 12SK7 RF amp does a very 
good job of overcoming the horribly noisy 6/12K8 mixer noise anyway.

Even so, it would be a very interesting experiment.

In my previously (1950s) modified BC-454 which now covers 10 meters, the receiver's 
sensistivity is still less than 1 microvolt for MDS, even on that band, still using the original 
tube lineup.

Ken W7EKB 

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