[ARC5] ARC-5 Mixer Substitution

[Scott Robinson]

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



On 4/4/18 8:36 AM, Phillip Carpenter wrote:
> Has anyone substituted a 12SA7 for the 12K8 in an ARC-5 receiver?
> 
> I’m interested in hearing about any details/performance.
> 
> Thanks in advance to all who reply!
> 
> Phillip W4RTX
> 
> Sent from my iPhone
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