[AMRadio] Mod xmfr?

Patrick Jankowiak recycler at swbell.net
Sun Oct 10 13:27:48 EDT 2004

For a close idea of the 813s' needs as far as using them for AM 
goes, have a look at the transmitting tube manual such as the RCA 
TT-4. Pick a set of data that gets you close to the RF output you 


For the PA, 1600VDC at 150mA per tube (300mA) provides 360 watts 
of carrier from 480 watts input.

For the modulator operating AB1 at 1500VDC, a 9300 ohm P-P load 
is called for in order to get 260 watts output.

The rule of thumb is that the modulator should be able to provide 
an amount of power equal to or greater than half the DC input to 
the RF PA stage. This rule if accurate for sinewave modulation 
and a condition of proper impedance matching.

So, the modulator, operating at either 1500VDC, or 1600VDC just 
like the PA, would have more than enough power for the rule. (RF 
PA 480 watts DC input /2 = 240 watts, and the modulator makes 260 
watts) Just barely!! and you might lose a bit in the modulation 
transformer, but it will still be OK for the sinewave operation.

Voice is not a sinewave, so it is useful to either make use of 
speech processing, or make the modulator able to put out twice or 
more of what the rule of thumb gives, if you can get away with 
it. That is another topic, and does not really reflect upon the 
impedance choices.


Next, what is the secondary impedance of the modulation 
transformer? It is the E/I of the RF PA stage, so that it is in 
the example 1600/0.3 = 5333 ohms.

The modulation transformer could have an impedance ration of:

9300 to 5333 = 1.74 to 1

The turns ratio (voltage ratio) is the square root of that:

SQRT 1.74 = 1.32 to 1


813's can put out more power operated class AB2. 813's as a 
modulator might be better somehow triode connected (G1 through 
20K to G2 and drive G2) and operated closer to class B. I don't 
have any data for that but I would like to see it discused if 
anyone has done it.

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