[Amps] Plate modulator xmfr ?
R.Measures
r at somis.org
Sun Oct 10 13:23:43 EDT 2004
On Oct 10, 2004, at 5:11 AM, StephenTetorka at 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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