[HBR] WARC bands?

[Walt Hutchens]

Brian said:

> So, if you try to extend the range by going to a larger tuning capacitor,
> you end up in the "general coverage" two dial receiver category, and need
> the extra complication of a bandspread capacitor and dial?

You can go that route but in addition to the extra complication you
get a performance hit: Stability and dial accuracy are going to be
substantially hurt.

The larger a tuning capacitor is, the more difficult it is to make it
stable.  The W6TC sets use accurate and stable FIXED capacitors for
most of the capacitance; you can then use a fairly inexpensive tuning
cap to do the small amount that must be variable.

I've had good results using caps designed for the FM receivers of that
day. They're just 'consumer grade' but because the FM sets needed
reasonable stability at 88-108 mcs they're not bad at all for an HBR.
Of course the specified caps are nicer but I think any improvement in
stability will be masked by other factors in the design -- wiring and
chassis movement, changes in tube parameters, the fact that zero temp.
coefficient caps aren't, and so on.

The thing about the W6TC sets is that they were designed to allow a
ham of modest technical skills and wallet produce a receiver that was
generally the equal of the best commercial sets for the ham market and
FAR ahead of anything for similar money. You could -- still can --
work about anything on any of the bands they cover. Once you add a
larger variable capacitor (even as a band set cap) and the wiring that
goes with it, equal stability really isn't possible. So you can do
okay on the lower bands on AM but SSB and the higher bands probably
not.

It is true that the Collins receivers -- particularly the military
sets -- did many bands and even general coverage with first-rate
performance. It's also true that they cost ten or more times what a
ham set cost and used techniques that are well beyond hams -- like
precision gears and linear permeability tuning.

As others have said the way to go for WARC bands is to wind coils for
them. Get your receiver built and make at least a couple of coil sets
for the standard bands. Then wind the coils for the additional bands
as desired. You'll end up with a first rate receiver for those bands
and any time you want another, you can wind those coils.

Walt
KJ4KV