[ARC5] Receiver Tube Substitution

[Kenneth G. Gordon]

On 17 Dec 2016 at 14:08, Richard Knoppow wrote:

>     While the 6SG7 will plug in place of a 6SK7 and work OK I am not so 
> sure its noise will be any lower.  The bias for maximum gain is about 1 
> volt while the 6SK7 needs 3 volts so in a receiver designed for the 6SK7 
> the bias will reduce the gain and Gm of the 6SG7. I have not tested this 
> but think the resulting noise would be the same.

All the above is pretty much the case, especially the biasing issue. 

Furthemore, internal capacitances are enough different that simply plugging a 6/12SG7 in 
place of a 6/12SK7 in the RF amp results in the "Align Input" capacitor of an ARC-5 not 
working correctly.

Replacing the 12SK7 with a 12SG7 in several refurbished ARC-5 receivers here resulted in 
no noticeable improvement and the "Align Input" issue I mention above.

However, I also have a BC-454 here which was converted to cover the 10 meter band 
sometime around 1950 or so. I finally got that one to work, and the 12SG7 does make a 
noticeable improvement in the noise level on 10 meters, even without adjusting the biasing 
so that it is correct.

Even so, the 12SK7 RF amp still shows a sensitivity on 10 meters of 1 microvolt or better 
MDS on CW.

I am reasonably certain that any advantage to using the 12SG7 over the 12SK7 would not 
become apparent at any frequency below about 20 MHz.

Ken W7EKB

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