[HBR] HBR transmitter

[Walt Hutchens]

> I am working on a small CW mixer/ transmitter using the HBR-14
> oscillator to control the frequency. Is anyone else doing this?

I'm not doing it, but there's no reason this shouldn't work. You can
either key the conventional way -- all signal frequency stages -- or
bias everything appropriately and just key the IF frequency injection
oscillator.

There are some issues:

The HBRs are a well-engineered approach to a high quality receiver (by
1950's-60's standards) that any careful and determined ham could
build. They are NOT the most stable receivers possible, even using the
technology of that time -- some loss of warm up stability was one of
the trade-offs for other good things in the design. Thus your
transmitter will drift some during warm-up.

The use of the second harmonic of the oscillator frequency on some
bands will need to be considered.  The injection signal for the
transmitter should be taken from the receiver mixer grid, via a
suitable buffer.

(That buffer will need to be left on when receiving in order to avoid
changing the load on the oscillator.)

The second harmonic trick works well for the HBRs because the mixer
(signal) grid coil (where the oscillator is also injected) serves to
select that harmonic. If you try to take take the oscillator signal
from somewhere else, you'll get so much fundamental that you'll have a
very tough time.

Without having the circuit in front of me I can't remember how the HBR
does muting but you'll need to be sure that the technique doesn't
change the load on the oscillator. If it does, then the oscillator
will be on a slightly different freq. when you are transmitting and
the amount of this change will vary from one band to another so it
can't be permanently compensated in the IF freq. oscillator.

Even the change in the load on the receiver power supply causing
oscillator voltages to change slightly may be an issue. It may be
necessary to mute by shorting the receiver antenna terminals and audio
output and even more may be needed on the higher bands, since AGC
voltage will change, changing the IF/RF stage currents.

VR tubes aren't perfect ...

I think I would start by studying the problem of keeping the receiver
oscillator frequency from shifting when the receiver is muted.  Once
that's taken care of, the rest should be do-able.

The usual timing issues will apply -- the receiver should be silenced
before the transmitter signal comes up.  Of course if muting is
done manually that's not a problem.

This is a very appealing approach to a low to moderate power vintage
ham station and should be a lot of fun to build up and use!

Walt
KJ4KV