----- Original Message -----
From: "Floyd Sense" <sentek@sprintmail.com>
To: <amps@contesting.com>
Sent: Thursday, February 06, 2003 7:53 PM
Subject: [Amps] Question on Ameritron amp wattmeter circuit
> This question relates to the Ameritron AL-800H, but the circuit in
question
> is identical to that in the AL-80B and perhaps other Ameritron amps. I
had
> to replace the toroid used to sense forward/reflected power, as well as
the
> diodes, a couple of resistors and a couple of disc caps in the circuit.
The
> secondary winding of the toroid is loaded with a 51 ohm resistor and the
> forward and reflected voltages are taken off of the ends of that resistor.
> The winding is center-tapped and goes to ground through a 1.8K resistor
> bypassed with a 270 pf mica cap. The center tap is also connected via a
10
> pf fixed cap and a series 3-12 pf ceramic trimmer to the RF output line
> (which passes through the toroid).
>
> The question is: what is the function of the caps connected between the
> center tap and the primary conductor? I assume that the adjustment has
> something to do with the calibration of the forward/reflected power
meters,
> but don't know how it comes into play. There's a pot to ground connection
> in each of the legs to calibrate the meters with. Perhaps this variable
cap
> is used to correct the power reading at higher frequencies?
I'll take a shot in the dark: Could the trimmer cap be used to compensate
for a phase shift caused by XL of the winding? If so, it would seem that
the adjustment would be made on 10 meters where it would have a greater
effect, using a purely resistive 50 ohm load.
Jeff - K1LE - CT ><>
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