[HBR] RE: HBR 2006 and a question

[Walter A. Hutchens]

Don wonders:
> I have a question. Most of the HBR designs show a capacitor across the
> primary of the audio output transformer. Would someone explain the concept
> and use of this capacitor? I assume it is to either flatten the response
> of the audio output or peak it.

Bingo!

In an HF communications receiver you don't want the high frequency audio 
response to go too far.   Above 2500 or 3000 cps the amount of intelligence 
conveyed by a voice signal in proportion to atmospheric noise falls rapidly.   
One easy way to limit the highs is a capacitor across the primary of the 
output transformer.  

If the transformer were completely unloaded (no speaker or tube) then 
you'd get a pretty sharp peak at the resonant frequency of the 
transformer's inductance and the capacitor.   However with the tube and 
speaker hooked up you get little or no peak -- the Q of the tuned circuit is 
too low.   Below the resonant frequency you have the tube plate resistance 
in series with the inductance of the transformer (high inductance) which is 
shunted by the load resistance reflected back to the primary.   More or less 
flat, once you get above the frequency at which the inductance of the 
transformer has such a low reactance that it shorts out the load.   Above the 
resonant frequency the load is shunted by a capacitive reactance and 
shorted out -- the power is dissipated in the tube plate resistance.  

The main reason to get rid of those highs is to improve the signal to noise 
ratio.   You can start with a .001 or .002 mf capacitor (or if copying an 
existing design, use the specified value).   Then once you have the receiver 
working, try other values from maybe 500 mmf (which will be too low) up to 
.004 mf or so.   Pick the value that sounds right to you when listening to the 
kinds of signals you will be copying.     

A second reason for this capacitor is that while the very high audio 
frequencies contain little voice power, noise spikes can develop high enough 
voltages to break down the insulation of a transformer leading to a carbon 
track and eventually a short.    Killing the unneeded highs with a shunting 
capacitor can prevent this from happening.    Usual voltage ratings for this 
cap are 500-up, often 1000 volts.   

Older military communications receivers often added a neon bulb across the 
transformer and cap pair.  That not only completely swallows high voltage 
spikes but provides some amount of noise limiting when listening under 
thunderstorm conditions, as for example a comm watch or aircrew are 
required to do.  

Walt 
KJ4KV