[Amps] Advice on how to work this thing...

[Bob Good]

Glad you are learning Chris.  there has to be a big variable capacitor 
across the tank coil assembly.  It's an LC circuit.  the single 4-125 is 
more than adaquate to drive the PP 250T's. The input LC intwork is the same 
design as the output and is correct.  I suspect you will find a pot 
connected to the 4-125 control grid that will allow you to set the final amp 
grid current.  Yes, a 250TL is low mu, TH is high mu.  A matter of no 
concern to you.  Your antenna sounds great for this transmitter application. 
Yes you will use Class "C" for AM and CW operation. ( Oh by the way,  you do 
HAVE to run cw if you are going to operate a fine nostalgia rig like this. 
It is absolutely prequisite!  Lest the wolf hong rattysnitch befall you. 
Don't ask!)  Now this is not a linear amplifier as such.  You haven't 
mentioned what happens prior to the 4-125 driver stage.  We need to go there 
also.  Don't get spooked by the coils.  Just take a digital picture of them 
and e-mail it to me.  Shouldn't be any mystery there.  I'd like to see pix 
of everything about the rig Chris.  You should be able to look up link 
coupling in your 1960 hanbook and find out what you need there.  While in 
the Handbook, also look up neutralization.  Oh one final thing, Do you know 
anything about these new fangled things called "solid state"? 
Gee............(((73))) Bob, K4BG
----- Original Message ----- 
From: "Chris Howard" <chris at yipyap.com>
To: "Bob Good" <k4bg at planters.net>
Sent: Friday, January 07, 2005 10:08 PM
Subject: Re: [Amps] Advice on how to work this thing...


>
> 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 at yipyap.com                                 Estes Park, Colorado  USA
> current Estes Park weather --  http://www.yipyap.com/wscurrent.html