[HBR] Radio design advice

[N2EY at aol.com]

In a message dated 6/3/06 4:23:54 PM Eastern Daylight Time, 
brianehill at charter.net writes:


> Yep 6K8. I got a couple and was just wondering if anybody had used one and 
> if it's true that weak signal performance suffers or is there a magic 
> circuit someone has come up with? hi hi :)
> 

The problem with multigrid mixers like hexodes and heptodes* is that they're 
noisy. They have lots of conversion gain and are more immune to pulling than 
most other mixers, but the noise can't really be cured because it's a function 
of all those grids (partition noise).

The classic fix was to precede the mixer by a stage or two of low-noise RF. 
This works but it puts more gain before the selectivity.

Still, good receivers have been built with those tubes. The Drake 2-B and 
Collins 75A-4 are two examples.

One big difference between the 6K8 and other multigrid mixers like the 6L7, 
6SA7, 6BE6, and related types is that the 6K8 has a separate triode unit for 
the oscillator, so that one tube can be used for both oscillator and mixer 
without the problems of the 'self-excited' pentagrid circuits. 

The best use of the 6K8/12K8 I know of is in the ARC-5 receivers.

73 de Jim, N2EY

* A 'heptode' is a tube with seven elements, not a beatnik amphibian.