[HBR] Radio design advice

[Mike Hanz]

Ian Wilson wrote:

>Pullen Mixer: you will probably need to increase the triode count. A Pullen
>should have a cathode follower driving both sides of the mixer; so that's 3
>triodes (you can use the 'spare' triode of a dual triode as one of the
>oscillators).
>

 From a perfection seeker's standpoint, Ian is absolutely correct.  The 
use of a second dual triode will completely isolate any pulling effects 
from the two inputs, and Keats in fact noted this approach in his book 
as a *disadvantage*.  On the other hand, others with a more pragmatic 
(or cost cutting) view have observed that the mixer itself has a cathode 
follower configuration with relatively high input impedances, and the 
elimination of the second dual triode does not affect its operation to a 
degree that bothered their perception of acceptability, especially 
compared with other mixer designs.  I like Ian's observation because 
toobs are cheap in an HBR design and it is a point of perfection that we 
tend to seek in these sets, but if you have space difficulties in your 
particular chassis, you can still use the Pullen approach in its "dumbed 
down" configuration and still get good results from it.

73,
Mike