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Author Topic: 6M Amp Conversion using AL-80A  (Read 1729 times)
W5DQ
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« on: February 17, 2010, 11:03:13 AM »

I know that many folks use older Heathkit SB-220 HF amplifiers to perform the 6M conversion on but has anyone here ever used an Ameritron AL-80A amplifier to convert to 6M. If so, was there any issues with using that particular model. I have been told that they will convert easily but I have also heard that the AL-80A had some issues (don't recall off hand what they were exactly) that the AL-80B version corrected and not sure if those issues migrate over in the conversion process.

I have access to use a AL-80A for 6M conversion but before I head down that path, I'm just doing the required homework.

For now, please refrain from suggesting other amps to use as I know that there are many plausible candidates that can be used but for now I am only interested in hearing about successes or problems concerning the use of an AL-80A amp.

Thanks for the help,

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
K6AER
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« Reply #1 on: February 17, 2010, 09:21:11 PM »

It looks like some of the regulars are off doing other things. So here goes.

You will have to do the following things.

•   The input and output band switch will need to be removed.

•   Input Pi networks will be removed.

•   Tank coil with taps will be removed.

•   The tune cap needs to be about 5-45 pF with about the same spacing as the original tune cap.

•   The load cap will need to be about 5-100 pF same spacing as the old load cap.

•   Depending on the capacitor design you might be able to just remove capacitor plates.

•   The tank coil will need to be about 4 turns, 1.5 inches in diameter with 1/8 inch coil. Number 8 gage solid copper wire will work fine.

•   The RFC choke will need to be reworked, fewer turns. Check that the choke is not resonant from 44-60 MHz. I would use a grid dip meter.

•   The input Pi network will have to be built to provide matching for the 3-500Z. Look in the ARRL Handbook for values.

•   Some coax routing will need to be addressed for stray inductance is more of a issue on 6 meters.

•   The parasitic choke should be fine.

•   Power supply, by-passing caps and metering should not change.

Good luck.
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W1QJ
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Posts: 1521




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« Reply #2 on: February 18, 2010, 04:48:02 AM »

The AL-80A has no issues being converted to 6 meters when done properly.  Makes a nice KW output with no issues. The key point is, it must be done right.  Been doing them for 15 years now. Lou
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WB2WIK
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« Reply #3 on: February 18, 2010, 06:32:41 PM »

I've done this exact conversion and it wasn't difficult but I did *not* use the parasitic suppressor, it had too much inductance for 50 MHz use.

Besides winding a new plate RFC (same form, fewer turns) I also changed the filament chokes (same ferrite rods, fewer wire turns) and changed the plate blocking/coupling cap and B+ bypass cap at the bottom end of the plate RFC to smaller values.

I used the original 10m cathode input tuning form and core, but removed turns and changed the mica capacitor values to dip at 50 MHz.

I re-used the original plate TUNE cap as the new LOAD cap, and changed the TUNE cap to a 3-30pF 5kV Jennings vacuum variable I happened to have on hand.  That worked brilliantly.  My plate tank coil is 3-1/2 turns of 3/16" copper tubing wrapped around a 1-1/2" form, spaced over about 2-1/2" or so.  You can squeeze or compress the turns with the TUNE cap set at about 10 pF to achieve resonance indicated by a grid dip meter.

I had to add a wide copper strap between the front panel and the chassis, as there was a problem prior to doing that (you could feel the heat generated at this interface after transmitting for 60 seconds key-down).

The resulting amp is "single band," of course, but does run as much power on six as it ran on HF.  With a stiff AC line, about 900W PEP.  It's pretty clean if you keep the grid current down around 100mA, which is very possible with proper loading.

It's a good candidate for a 6m conversion, and they're pretty cheap.  I bought my AL-80A "conversion" unit in very good condx for only $550 about six years ago.

Of course, if you find an SB-220 for the same price, even better and it's a fine candidate as well.  The 220 will run slightly more power, but only slightly -- few would even notice the difference.

Good luck!

WB2WIK/6
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K6AER
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« Reply #4 on: February 18, 2010, 07:17:02 PM »

I went back and looked at my notes. The RFC choke was 28 turns of number 22 ga wound on a 1 inch form with about two wire diameter spacing.

Glade to see one of the regulars back.
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