[ARC5] Receivers and mixer noise.

[Kenneth G. Gordon]

Peter Schleuss HB9RUZ reports testing his restored BC-455 with and 
without that reversible triode (12AT7) mixer/HFO mod mentioned here a bit 
ago.

With the mixer-mod, MDS is around -124 dBm, even with the 12SK7s.

Trying a 12SG7 in place of the 12SK7 does not, then, improve the S/N ratio. 
In fact, swapping to the 12SG7 actually makes the situation worse, possibly 
due to the fact that the 12SG7 is a "semi-remote cut-off" type.

Paralleling the RF amplifier's cathode resistor with another resistor (as 
suggested by Bill Orr) of the same value, thereby adjusting the grid bias so 
the tube operates closer to its Class A point also does not help.

We suspect that the tube and bias swap simply increases the RF amp gain 
enough to overcome the mixer's noise output a bit better.

When the mixer noise is reduced, there is no need for the added gain.

Since the mixer noise can be dramatically reduced by using a simple plug-in 
adapter, there is no need for any further operation on the receiver. It works 
just fine as is.

Too bad ARC engineers had so little choice for a suitable mixer/HFO tube at 
the time.

Then again, knowing what we do about ARC engineers, perhaps they chose 
the noisy 12K8 for a very good reason.

Ken W7EKB