|[AMRadio] Home brew AM Cathode modualtion|
John Coleman, ARS WA5BXO
wa5bxo at pctechref.com
Sun Oct 3 15:37:44 EDT 2004
The "Rice" neutralization circuit works better with a higher Q
grid tank and, if possible, some input swamping or grid load resistor.
The biggest trouble with the circuit is that the loading of the grid
tank is not constant over the full RF cycle as a result the "Q" and
balance is shifted somewhat over the period of the RF cycle. Starting
with a higher "Q" or C value is one way of minimizing this effect.
The circuit does work because I used it as well and also with a 304TLs,
back in the days of experimenting with the ultra modulation or balanced
high level modulation circuits. I eventually went to standard "Push
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Donald Chester
Sent: Sunday, October 03, 2004 12:45 PM
To: amradio at mailman.qth.net
Subject: RE: [AMRadio] Home brew AM Cathode modualtion
>The 450th is a big triode, so I don't think you can run it single ended
>net output, you would need to do 2 250th tubes in push pull or
>but the 450th would do the cathode modulator quite well.
You could use a single-ended pi-network type output circuit, with a
grid tank coil. That is knows as "Rice" neutralisation. You don't get
perfect null as you can with a balanced plate tank cincuit or a pushpull
arrangement, but it works well enough that it became the standard design
tube type broadcast transmitters starting in the late 50's.
I used that circuit with the first high power rf final I ever built,
304-TL in the final, because I didn't have a large split stator tuning
capacitor on hand.
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