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Author Topic: Tuning Tube Finals (Ignorant Extra operator)  (Read 3622 times)
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W6FO
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« on: July 10, 2005, 04:30:55 PM »

Ok, pardon my ignorance but this is my first tube rig.  She's a Yaesu FT-102 rice box and thus is a hybrid with 3 x 6146B tubes.  I finally got everything together in my new shack and want to get on the air tonight.  

I know how to tune the rig into a dummy load but what is the proper tuning procedure when you also have a transmatch to worry about?  My antenna is a 125' Doublet fed with 600-ohm ladder line connected with a Dentron Super Tuner transmatch.

It is my understanding that if I ever "retune" the antenna after tuning the finals, that I have to tune the finals again.  Do I tune the finals into the dummy load first, switch to the antenna and tune for lowest SWR, and then tune the finals again?  Or am I just being too paranoid about blowing by finals?

Thanks

JT - W6FO
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K1JJ
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Posts: 8886


"Let's go kayaking, Tommy!" - Yaz


« Reply #1 on: July 10, 2005, 05:22:25 PM »

JT,

Tune up the 102 into a dummy load for max power out.
Peak C1 and C2.

Then reduce the drive so that with your swr meter set at max
gain, you get a full scale reading on forward. The reverse
reading should be close to zero, of course, into the dummy.

Then switch the rig to the tuner input using the same swr meter
in line.  Adjust the tuner for a 1:1 swr. ie, the rig should now
see 1:1 both into the dummy OR tuner/ant.  Another easy way is
to connect your MFJ-259 ant analyzer to the tuner's input and adjust
it without the transceiver. Then hook the rig up and you're all
set. Premark everything so that changing bands later is a breeze.

Since the rig was previously peaked for max power, you need
to load it slightly heavier for best linearity. UNMESH the
loading cap slightly so that under full power you drop off maybe
7% or so. Then repeak C1, plate tuning. The amplifier will now be
running less efficient, generating more plate current and heat for
a given power output. But the peaks will look better and it will
sound cleaner.

Adjust the 102's drive so that you get maybe 20W of carrier or
whatever you need to drive a linear, if so.  Use a scope and look
to see that your modulated AM waveform looks good. Get some
bandwidth/splatter reports on the air.

73,
T
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W1GFH
Guest
« Reply #2 on: July 10, 2005, 05:34:54 PM »

After dummy load tuneup of the FT102 gets you in the ballpark, one shortcut is using your no-tune 100w ricebox (assuming you have one) to pre-tune the antenna tuner for favorable SWR on 3.885 (or wherever you're going to use the FT102). Then switch out the ricebox, switch in the 102, and voila, minor tweaks and you're there.
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W6FO
Guest
« Reply #3 on: July 10, 2005, 06:05:39 PM »

Tom:

Thanks for the reply.

I wish I had a 259, it'd make things real easy.  I'll have to do it the hard way.  Also, no AM or scope here yet.. I'm limited to SSB and CW for now.  I need to start collecting parts for the NE602 modulator to get myself on AM.

W1GFH: I do have another HF rig, 100% solid state (IC-706), but it's in the truck and I don't have a DC power supply to run it in the house.

I'll play with it tonight and see what I can do.

73

JT - W6FO
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