Re: [Amps] Plate modulator xmfr ?

[R.Measures <r@somis.org>]

On Oct 10, 2004, at 5:11 AM, StephenTetorka@cs.com wrote:

> Hi all:
>
> I need to better understand the workings of a plate modulator for AM.
>
> Considering the RF amp with 2 x 813's and the modulator with 2 x 813's 
> - what
> are the equations and math to work out the required wattage, 
> impedance, step
> ratio etc.

-  My first clue that some strange things were going on during "plate 
modulation" was when I attended a lecture by Eimac's Ham-rep, John 
Reinartz    Plate modulation is a strange animal because the Class-C RF 
stage load R on the modulator is assumed to be constant in the standard 
formulas, however, the instantaneous load R at 100% modulation varies 
from c. half of what the standard formula predicts at the 
positive-peak. when 2x the indicated current flows, to over 1M-ohms at 
the negative peak when zero current flows.  Another complication is 
that much less power is required from the modulator during the negative 
half of the modulation cycle than the positive half  -- which the 
standard formula does not take into account.


An old timer (W6SDM) told me:
1.  In order for a modulator to produce the peak audio power needed to 
100% positively modulate a Class C RF stage, it needs to be able to 
produce more power than the standard formula says is needed.   For 
example, a 1000V, 100mA, 100W Class C RF stage:  The formula says that 
50w-rms of audio is needed to modulate it.  However, at the positive 
peak, the modulator must produce 1000V-pk at 200mA-peak -- which =s a 
peak power of 200w, or 100w-rms.
2.  The most important design consideration for determining the 
turns-ratio in the modulation transformer is to make sure that when the 
positive peak secondary V is produced, the instantaneous anode-cathode 
V in the modulator tube that is conducting during the positive-output 
half of the cycle remains in the linear region, and is not in the 
non-linear/saturation region.

- - The short version:
-  One needs a modulator that can produce the same RMS output power as 
the input power to the Class C RF stage being modulated.
-  The mod-xfmr secondary Z that will produce the best modulation is c. 
half of what the standard formula says it should be.

Mr. Murphy said that things are usually not as simple as they first 
look.

Ref:  
http://lists.contesting.com/archives/html/Amps/2001-06/msg00326.html
> And, just how much can one depart from the ideal arrangement and with 
> what
> effects? ( should I not be able to find the right xmfr for this amp 
> I'm planning
> to build ).
>
> PS: I am seeking that modulator xmfr at this time.

Free, unsolicited advice:     Using c. 2500vdc, 4, 813s in g-g will 
linearly produce 1500w-pep.  When driven by a modern transceiver that 
uses low-level modulation followed by linear amplification, the 
distortion produced will be virtually undetectable to the human ear.  
However, "plate modulation" produces noticeable distortion.

cheers, Steve
> Tnx,
> Steve
> WA2TAK
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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