[HBR] HB rcvr

[Walter A. Hutchens]

Keith asks:
> I would like a monoband cw rcvr for 80 meters.  It was suggested that I
> post here, for comments, etc.  I want to use all metal tubes, mainly
> because thats what i have the most of along with octal sockets.   Heres my
> line up,
> 
> 1) 6SH7 RF amp   2) 6K8 or 6SA7/6C5 mixer, 3) 6SK7 IF amp.  4) 6AC7
> detector 5) 6J5 bfo  6)  6SC7 Audio amp.

Just a few comments:

1.  I'd be inclined to use a 6SG7 rather than a 6SH7 for the RF stage.   
Semi-remote cutoff instead of sharp cutoff, with similar gain.  

You'll probably provide RF and/or IF gain controls.   If you use the same 
type tube in both places (two 6SK7's or two 6SG7's) you have the option of 
putting a single pot in the common cathode return line while if the tubes 
have different cutoff voltages (those three tubes do) you shouldn't do that 
as one will cut off ahead of the other, possibly causing you a distortion 
problem.   

Frankly, I think I'd try a 6SK7s in both sockets as they will handle larger 
signals.   If you really need more gain, 6SG7s are a direct substitution.

2.  HI Q ANTENNA AND RF COILS!   Don't skimp here -- unless you're 
going to cheat and go with toroids, use the biggest forms and the heaviest 
wire you can.   That will give you higher gain, so you can loosen the coupling 
to the antenna meaning better RF selectivity and less trouble with mixer 
overloading.   You have to be careful about coupling between the two coils 
causing instability of the RF amp.

3.  As I remember, the 6K8 is a memorably noisy mixer.   (Mid 1930's tube, I 
think ...)   I'd go with a 6SA7/6C5 or even a 6SB7 -- about twice the 
conversion transconductance as I recall, and even better compared to the 
6K8.

Mixer noise isn't generally an issue on 80 meters, but low gain can be -- at 
the least, by forcing you to use a better antenna or tighter coupling.    With 
just one IF stage I don't think you have enough gain for AGC -- but that's 
probably the way you planned things. 

What kind of detector circuit are you using?  Will you be providing 
selectivity beyond the 12 kcs BW one would expect with a couple of 455 
kcs IFTs?   This would be an ideal design to use some audio selectivity.

Definitely a classic 1946 design!   

Walt Hutchens
KJ4KV