> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Jim Brown
> Sent: Tuesday, February 02, 2010 4:54 PM
> To: Tower and HF antenna construction topics.
> Subject: Re: [TowerTalk] Using Stubs to Reject Harmonics
>
> On Tue, 2 Feb 2010 15:55:10 -0500, Gary Schafer wrote:
>
> >You should be able to get an idea of the output impedance of the amp at
> the
> >second harmonic frequency by checking with an impedance meter at the
> output,
> >like with an MFJ. You may need to place a resistor from the plate to
> ground
> >of the tube to simulate the tube load. Do all this of course after the
> amp
> >has been tuned up on the operate frequency and then shut down.
>
> Yes, I thought about doing that, but didn't get as far as simulating the
> tube. You
> can't just shut down the amp though -- you've got to leave it up with
> the antenna
> relay pulled in. All in all, a lot more work than I have time for right
> now. But I
> think you're right on target about measuring it. Any suggesting as to
> the
> resistance to use to simulate the tube?
>
> 73,
>
> Jim K9YC
>
You don't want to have the amp on when you connect a measuring instrument to
the output. Just the static conduction (noise) developed in the tubes will
probably zap the front end of your meter.
Use a small power supply to connect to the relay to pull it in while doing
the measurement.
Tube plate impedance can be found by plate voltage /plate current /1.8 that
will get you close. Example: 3000 volts at .8 amp will be around 1875 ohms.
Don't worry about disconnecting anything in the plate circuit. Just haywire
the resistor between the plate and ground. A quarter watt resistor will be
fine.
I do this to set up the tank circuit when building an amp. Gets you right in
the ball park for tune settings.
73
Gary K4FMX
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