And if you don't want to go to the trouble of building
or buying a return loss bridge, you can also use one
of those hand held impedance analyzer's like the
Autek VA-1, MFJ-269, or Tempo Research HF-CIA.
A couple of watts from an HF exciter and a QRP
wattmeter would even work if you make the dummy
plate resistor big enough (caution: if you use this method
be careful - 2 watts of RF across the 1000 to 2000 ohm
dummy plate resistor resistor will generate 40 to 60
Volts RMS RF voltage).
73 de Mike, W4EF.......................
----- Original Message -----
From: "Radio WC6W" <wc6w@juno.com>
To: <w4lcn@frontiernet.net>; <amps@contesting.com>
Sent: Tuesday, November 25, 2003 10:35 AM
Subject: Re: [Amps] amp. Pi-L tuning question
>
> On Mon, 24 Nov 2003 19:34:33 -0600 "hm" <w4lcn@frontiernet.net> writes:
> > Sir:
> > I wonder if you or some members here could help me with set-up
> > problem or point me to some place to find the information I need?
> > Some time ago I built a 4-1000 linear amplifier with tuned input and
> > Pi-L output. The tuned inputs is good, and after many hours of
> > trial and error tuning the output network, I have the 40 meter band
> > tuned very well, and the 20 meter band somewhat close. Now to my
> > problem. I have acquired an H/P 8601A sweep generator(50 ohm
> > output) and also have an H/P 606 signal generator for
> > centerfrequency marker, as well as a Millen grid dipper and a
> > tektornix 547 scope. I would really like to be able to set the
> > remaining bands using the sweep generator/scope as I think it could
> > make tuning the Pi-L much easier, plus learning something new.
> > Unfortunatley I don't know how to set-up the sweeper to the
> > amplifier output network to do this. I think where my efforts have
> > failed is with the matching the sweeper to the 4-1000 plate, or to
> > the network itself. If you or one of your readers could help me to
> > do this or perhaps tell me where I could find this information I
> > would really appreciate it...
> > thank you for you time...H. B. Moore...w4lcn...
> > _______________________________________________
>
> Hi,
> Well, if you insist on doing it this way... it sounds like you have
> most of the required stuff.
>
> First tune your amp up into a 50 ohm dummy load on 40 meters where you
> state that you have it working well.
>
> Now shut it down and disconnect the amp. from the power outlet for
> safety.
>
> Obtain a Return Loss Bridge. This will allow plenty of time for the
> HV to bleed down. :-)
> HP 8721A or equivalent (ridiculously pricey on the used market!)
> or
> Build one:
> QST September 1997, pp. 34-36
> or
> http://www.highfrequencyelectronics.com/Archives/May03/HFE0503_Kurzrok.pd
> f
> or
> http://www.qsl.net/it9xxs/reflectometer/reflectometer.htm
>
> Connect the RF from your generator to the bridge input and connect
> your amp to the test port. Connect the vertical input of your scope,
> along with a 50 ohm termination, to the sample port.
>
> Connect the generator's sweep output to your O'scopes horizontal
> input. Set your sweeper to scan 5-10 MHz or so (to cover 40 meters where
> you left the amp set). Adjust the 'scope as necessary to fit the
> display.
>
> With the amplifier still disconnected from power, HV discharged, etc.
> Connect a 10K carbon potentiometer between the plate of the 4-1000A and
> ground next to the tube base. An old Allen Bradley Type J will be fine.
>
> Adjust the pot. for minimum return loss as indicated on your scope.
> The pot should now be set to your effective plate impedance.
>
> Leaving the pot at this setting you should be able to sweep the other
> bands and adjust your tank components for minimum return loss.
>
> Happy Sweeping,
> Marv WC6W
>
>
>
>
>
>
>
> *
>
>
>
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