[Boatanchors] Which one tube TX ?

[David Stinson]

----- Original Message ----- 
From: "Bill Cromwell" <wrcromwell at gmail.com>
To: "Doug Forbes" <dougforbes at dslextreme.com>
Subject: Re: [Boatanchors] Which one tube TX ?


> On Sat, 2012-04-28 at 07:44 -0700, Doug Forbes wrote:
>> I am building a one-tube transmitter and am concerned over the 
>> crystal
>> current. ...
>> ______________________________________________________________
>
> Hi Doug,
>
> Do you want to build a one stage power oscillator or do you want to
> build a one tube transmitter....
> For the power oscillator there are some tuning tricks in the plate
> circuit that help the crystal survive. If I remember correctly some 
> of
> the hams tune the plate circuit to the low frequency side of 
> resonance
> to take it easy on the xtal. It may be the other way round. Some 
> others
> put a light bulb in series with the xtal. Some do both....

I've had some fun recently with a 1935-design one-tube pentode
RK20 (814) power oscillator I call "Old Fireball."
Diagrams of the circuits I've used here:
http://home.netcom.com/~arc5/HRO/1935tx.jpg
http://home.netcom.com/~arc5/HRO/1935tx2.jpg
http://home.netcom.com/~arc5/HRO/1935txPi.jpg

Not shown is a 300-ohm resistor that connects to the
fil tranny secondary center-tap and thence to the
key jack.  This provides cathode bias to keep the
tube safe with no oscillation.  At only 600 volts on
the plate of a tube designed for 1200, this probably
isn't strictly required, but I don't want to push this
old tube.  They ain't making more.

Here's the "innards" of the Pi-out configuration
before I added the meters:
http://home.netcom.com/~arc5/HRO/814rig1.jpg
Although this is a more "modern" implementation
of a Pi-net output, I did want to try it out.
Yeah I know... look at them looong leads!
One 160, 80, 75 and 40 meters, I haven't had
a problem.  If I was going to try and build it for
10 meters, I'd lay the tube on its side
to shorten everything.
Check out the world's gawky-est parasitic choke ;-).
All the circuits worked well for me.
One thing:  You need to be able to "tweek" the output tank coil.
Less than a turn can make a big difference
in power out vs. in, especially if you use the "Pi" match.

Here's a picture of the transmitter:
http://home.netcom.com/~arc5/HRO/OldFireball.jpg
It's on the air with an HRO Sr. for receiver.
That's a cathode modulator on the left,
which works great on AM- just plug it into the key jack.
The red window is designed to allow me to change the
crystal without getting zapped.
You tune it to the high side (less C) of resonance for CW.
Tuning has to be "right" to start oscillation.
This one uses 600 volts on the plate and 300 on the screen
from an old Drake AC-4 supply and gets about 30 watts out.
Even so, it's very tough on crystal current and
will fracture FT-243 or smaller.  It will eat HC-49
like potato chips.You have to adjust the feedback
just so to minimize xtal current without losing too much drive.
You'll notice I haven't included a series lightbulb in
the crystal lead. I don't see how this is any different from
using a series resistor. All that did for me was lower the
available drive. I decided to stick with the "big blanks" so
it wouldn't be an issue, but then I dropped the most important
crystal (of course- Murphy lives!), which lead me to try
to "tweek" the feedback to a safe level for FT-243 or HC-6U,
which lead to broken crystals :(

Wanting to be true to the 1935 spirit, I've stuck to large-blank
crystals that can "take it," but they are hard to find on the "right"
freqs and difficult to grind up in freq without "killing" them.
They do, however, "pen" down in freq readily, which helps.

All that being said, I'm thinking of re-working it into
an MOPA rig because I've already broken two smaller-blank
crystals.  They aren't making any more of these
so it's irresponsible, IMHO, to risk them over a "lark."
Either that, or I'm going to innovate a way to mount a
sand-state oscillator in one of these big-blank holders
and use that to drive the 814, or maybe mount an external oscillator
that is safe for HC-49s and FT-243s and "pipe it in."

Anyways- good luck on your project and let us know
about any "bumps in the road."  We have a lot of smart
people here.

GL ES 73 OM DE Dave AB5S