[AMRadio] Re: Modulator Plate Voltage

D. Chester k4kyv at charter.net
Wed Nov 14 11:50:09 EST 2007

> Very well said Don and if I might add, for the newcomers to generating
> audio.  That in any XFMR coupled stage (ie push pull plate modulator), the
> max audio voltage that can be produce is when the audio tube draws so much
> current during its half audio cycle that it reduces it's on plate voltage 
> to
> zero for a moment and on the next half cycle its plate voltage doubles the
> supply voltage.  This is saturation for the circuit.  Adding more tubes or
> bigger tubes to the modulator will not get more audio voltage from the
> modulation XFMR. ...

> John Coleman, WA5BXO

The problem is, that even when a tube is fully saturated, it cannot pull the 
plate voltage all the way down to zero.   Usually, the minimum plate voltage 
will be someting like 20% of the power supply voltage.  That is why one 
cannot modulate 100% using a class-A Heising modulator, unless the rf final 
plate voltage is reduced, using a series dropping resistor with an audio 
by-pass capacitor across it.  In any case, it is impossible to pull the 
plate voltage below the potential of the grid.  In class-B service, this 
imposes a limitation because the grid is driven positive on audio peaks. 
With a screen-grid tube the plate voltage can never be pulled below the 
screen voltage. .

Sometimes the modulator tubes don't like working into a low plate-to-plate 
load impedance, and become  non-linear.  That is when adding tubes in 
parallel will help.  The parallel tubes will also pull the plate voltage 
lower,  since they will pull twice the current at the saturation point.

You can achieve high positive peaks two ways.  Increase the voltage on the 
modulator, or reduce the step-down ratio of the modulation transformer.  The 
latter may require parallel tubes to develop the needed "torque" to  work 
into the low impedence load.

Don, k4kyv

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