Hello Stephen,
The use of 813's for both the class AB modulator and class C RF amplifier
is a quite wise move, as it simplifies the power supply considerations a lot.
A worked example, assuming the use of maximum ICAS ratings for the class C
813's:
RF amplifier
(carrier values for 2 parallel or P-P RF tubes in grounded-cathode)
Plate input: 2 x 400 W carrier
Plate voltage 2000 V DC
Plate current 2 x 200 mA DC
Screen voltage 350 V DC
Screen current 2 x 40 mA
Grid bias -175 V DC
Grid current 2 x 16 mA DC
Grid drive power 2x 4.3 W RF (at the grids, much more must be available)
Common screen dropping resistor approx. 20000 ohms, or 40000 ohms per tube.
Modulating impedance is approx. plate voltage/ DC (plate + screen current) =
4200 ohms approx
Available modulator required power for 100 % modulation is approx.
(plate + screen input)/2 + 20 % safety factor due to losses and mismatch = 500 W
Modulator
(values for 2 P-P AF tubes)
DC plate voltage 2000 V DC
Zero signal plate current 2x25 mA DC
Max signal plate current 2x180 mA DC
Screen voltage 750 V DC
Max signal screen current 2x55 mA DC
Grid bias -95 V DC
Grid drive power 2 x 0.5 W AF (at the grids, more must be
available)
AF output power approx 500 W
Plate to plate AF impedance approx. 14000 ohms
The modulation transformer should then have an impedance ratio of 4200 ohms
single-ended secondary to 14000 ohms push-pull primary,
and be capable of an AF power transfer of at least 500 W to provide some safety
factor.
Actually, the impedance ratio of the modulation transformer is not so critical,
especially if there is a power margin in the modulator,
and the operating conditions of both modulator and the class C stage can also
be adjusted to some extent to fit an available transformer.
The screen supply for the class C 813's deserves some additional mention.
Ideally, the screens of the class C 813's should be modulated to the same
extent as the plate voltage.
This can either be done by a special screen winding on the modulation
transformer, or by using
screen dropping resistors from the modulated plate supply.
If the dropping resistor route is used, it is advisable to provide a clamp tube
or a potential divider
between the screens and ground which reduces the screen voltage when the RF
excitation is removed.
Otherwise, should the class C 813's be operated with fixed grid bias and RF
excitation be removed, the screen voltage
will then approach the plate voltage and a flash-over is almost certain.
The use of dropping resistors in the screen circuit is somewhat wasteful of DC
and AF power,
so another way is to "self-modulate" the screen, using a 20 Henry or so audio
choke in series with
the screen voltage supply, which then should be reasonably well regulated.
The usual precautions to insure the application of grid bias and plate voltages
before screen voltage apply here.
Also, it should be kept in mind that the plate voltage on the RF stage is
doubled on the modulation peaks,
corresponding to a 4 times increase of plate input, so capacitor voltage
ratings will have to be doubled compared to
the CW or SSB dimensioning, and also the stability of the amplifier may be
found to be critical, as self-oscillations are prone to occur
on the modulation peaks. It is strongly advisable to include neutralization
circuits for the RF stage.
Detailed guidance about AM modulation of class C amplifiers can be found in
pre-1970's ARRL Handbooks
and in the W6SAI "Radio Handbook".
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: <StephenTetorka@cs.com>
To: <amps@contesting.com>
Sent: Sunday, October 10, 2004 2:11 PM
Subject: [Amps] Plate modulator xmfr ?
> 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.
>
> 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.
>
> Tnx,
> Steve
> WA2TAK
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> Amps@contesting.com
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