[ARC5] Help a Dummy Out on Modulation.

[Al Waschka]

I'm guessing here, and relying on my tube design experience from many years ago, but as you change the load and move the operating point up and down the load line, the gain of the tube may be changing substantially.  That would account for the variation in antenna current.  Similarly, the impedance of the tube will change as you move up and down the load line, so variations is plate voltage caused by the modulation will have different variations in plate current which is what transforms through the tank circuit to RF output.
 
Depending on the correlation between the gain curve and impedance curve, it might be possible to explain the effects you are observing.
 
Al K5TAN

--- On Mon, 2/8/10, David Stinson <arc5 at ix.netcom.com> wrote:


From: David Stinson <arc5 at ix.netcom.com>
Subject: [ARC5] Help a Dummy Out on Modulation.
To: "Old Tube Radios" <boatanchors at theporch.com>, milsurplus at mailman.qth.net, "ARC5" <arc5 at mailman.qth.net>
Date: Monday, February 8, 2010, 4:09 PM


I don't understand something, so thought I'd ask the
people smarter than me...

Take a look at this diagram of a BC-230 transmitter:

http://home.netcom.com/~arc5/SCR-AH-183/BC230TX.JPG

If operated and tuned according to the manual, using the
called-for 4-ohm/100pFd phantom antenna and using a
normal, well-working carbon microphone, I can't get more than
30% moduation and about 40 mils of antenna current.
Tubes are good.
Now, I understand about the modulators not
delivering enough power to get 100% at full load, and the manual
says to keep the loading down so you can get more mod.  However-
no matter where I load the rig, light or heavy, tuned or mistuned,
I still get about 30% mod, unless I yell.  Even more funny- if I use an 
external
antenna tuner, a wire antenna and just tune for maximum "smoke,"
I get about 1.2 amps antenna current and nice, 100% mod!..... (tilt!).
To add more "huh?" to the mix- when I use one of the xtal-control
coil sets with a 3880 KC crystal installed, I get less power out
(probably due to lower grid drive), but I can over-modulate.
I'm scratching my pointy head on this one, guys.
What is it I don't understand here??

73 Dave S.




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