Thanks Bob!
That's exactly the kind of help I need.
One question from what you've said so far...
You said you saw a variable capacitor in the plate tank.
I can't find it. I see the neutralizing cap(s) but I don't
see any other capacitors connected to the plate. You lost me
on that one.
I think I understand about the swinging link and the balanced
antenna transmission line. My main antenna is a horizontal loop
fed with 300 ohm TV twin-lead.
The driver to this PA is a circuit with a 4-125 tube in it.
I'm not sure if that would count as a balanced input to the PA.
(This thing was on the air in times past, so I'm operating from the
assumption that "as built" will actually work.)
It sounds like I need to make a good-guess on the tank coils
before firing anything up. Will a grid-dip be useful there?
Thanks for pointing out the meaning of that 'C' and 'B' on
that switch! I wondered what that was about. For AM, I think
I generally want to use 'C', right?
Maximum grid current... that sounds familiar. My experience
with tube-final transceivers has me dipping the plate and
peaking the grid. Plate, Load, Grid, maybe I'll be able to
do this! There are some pots in the exciter deck, so maybe one of
those is a drive adjuster of some sort.
I'm looking forward to hearing more about the neutralizing too.
And one other thing: A guy has offered me some 250TL tubes
instead of the 250TH. I see the grid voltage is different,
will that work?
I have ARRL Handbook and the Bill Orr Handbook from 1960.
But neither have a swinging link in them. I think I'll
keep an eye out for some older books. Any suggestion in
that regard?
Thanks very much, you are making me much more confident.
Chris
kc0atc
Estes Park, CO
On Fri, Jan 07, 2005 at 07:34:10PM -0500, Bob Good wrote:
Dear Chris. Oh Boy! Now this is just what an old timer like myself
really
enjoys. I remember well my old push pull TZ-40's with a swinging link
tank.
You have asked a chapter full of questions, so lets get started. The
coils
that fit with the swinging link are the final tank coils. If they are
numbered, they might indicate 80, 40 etc on them. If not, the one with
the
most turns will be the 80 meter coil and so on decreasing the coil turns.
As they decrease in number, they will likely get larger in wire size.
160
meters back then was limited to low power and other restrictions and
would
have required additional plate capacitance to be switched into the
circuit,
so I feel sure that we can rule out that band. The number of turns on
the
swinging link allows you to match the output to the characteristic
impedance
of the feedline. The feed system on popular antennas would likely have
been
balanced line. These may have consisted of zip cord, twisted cotton
insulated line, twin lead or home made balanced feeders of up to 6 inches
spacing. You can feed coax from a link coupling. More on that another
time. The plates are fed through the center of the tank coil. The
variable
capacitor in the tank circuit is your plate tuning control. You control
the
plate loading by closing the link into the center of the coil axis. You
must use push pull in the input tuned circuit as well or otherwise you
wind
up on DSB ! Don't want that. These coils will likely be smaller in size
and will also have individual units for each band. Most likely here we
will
find a fixed link coupling to the exciting stage. You will resonate the
circuit by tuning for maximum grid current in the finals using the input
variable capacitor. You have not told me enough to determine if you need
to
limit the drive. This can be done in the grid of the driver tube via a
linear taper pot. I also do not know enough to give you information
on
the keying for cw. It should be biased near cutoff to keep the quiescent
current within safe limits. I noticed in your schematic that appearantly
the amp switches between class "C" and "B". He is using the filament
transformer to derive grid bias needed for class "B". The capacitors
between the plate and grid circuits are the neutralizing "condensers".
Neutralization needs to be done before firing up the unit. More to
follow
there also. The 250TH use 75 watt filaments. You should expect to see
color in the plates under load. Tell us more so we can enjoy writing
more
epistles. Best wishes.....Bob , K4BG, Sylvania, GA
--
Chris Howard YipYap.Com
chris@yipyap.com Estes Park, Colorado USA
current Estes Park weather -- http://www.yipyap.com/wscurrent.html