[AMRadio] New Transmitter


John E. Coleman (ARS WA5BXO) wa5bxo2006 at pctechref.com
Mon Nov 27 10:40:48 EST 2006


Jack
	You first need to decide 

1. If you want push pull finals with Link coupled tank circuits and plug in
coils or 

2.  If you want go with the pi net on single or parallel tubes. 

Push Pull can be tetrodes or triodes 

Push pull is very stable and easy to neutralize with either tetrodes or
triodes.

Triode circuitry is simpler but requires neutralizing capacitors.  

Triodes are easy to modulate and get linear modulation characteristics. 

Tetrodes in push pull can be neutralized using just wire stubs.

Tetrodes require that modulation be applied to the screen circuit as well as
the plate circuit.

Tetrodes require either a screen dropping resistor from the plate supply
point or a separate supply.

Tetrodes can be tricky in getting the percentage of modulation applied to
the screens without a phase shift.  If you are an audio perfectionist then
the modulation applied to the screen should be a lesser percentage than what
is applied to the plate.  That is to say that when the plate modulated 100%
then the screen should be maybe 60% and this varies with loading and screen
voltage from 40% to 80% but I have never seen one work as well as triodes
with out audio reduction to the screens.

If you want to go with the pi-net output then you should definitely not use
triodes.  Not that it can't be done, but it will require a lot of special
stuff some of which you may have to make yourself.  Band changing is still
not easy with triodes.

Pi-network with single or parallel tetrodes makes a nice circuit for band
switching with out plug in coils.

Tetrodes circuits can be built to require very little drive.


Pi-network manufactured assemblies may be more available than the plug in
coils required for push pull.

All of that said, my favorite is the triode push pull circuit with plug in
coils.  The coils can be hard to find but a person could home brew them as a
lot of folks do.

My favorite tubes for this service would be a pair of 812s in the final and
a pair of 811As for modulators.

GL, 73
John Coleman, WA5BXO 




-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jack Schmidling
Sent: Sunday, November 26, 2006 10:01 PM
To: Discussion of AM Radio in the Amateur Service
Subject: [AMRadio] New Transmitter


I am in the thinking stages of my next rig and would like some ideas.

I am thinking of a pair of 813's or maybe even 4-250's but I can't seem 
to find any specific plans or designs on line.

There must be lots of them out there.

I found a thing called the 813 Maul which is 813's modulated by 813's 
but that is the only one.

Any help?  Any other ideas?

js


-- 
PHOTO OF THE WEEK: http://schmidling.com/pow.htm
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