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A 6 METER AMPLIFIER CONVERSION PROJECT USING AN
AMERITRON AL 80
1: Remove the cover and then remove the 3-500Z and store in a safe place with the screws from the cover.
2: Remove the Load and Tune (plate) capacitors and the plate choke.
3:Remove the input circuit board and the band switch with all the wires and doorknob capacitors.
4: Remove the blocking capacitor, leaving the capacitor that attached to the base of the plate choke that is grounded and is located in the back corner next to the filament choke.
4: Remove the piece of coax cable that was attached to the load capacitor and goes through the inter sidewall to the relay. Remove the RFC choke that came off the load capacitor to ground and save it for later use.
5: OK, now let us drag out the old junk box and or get on Ebay for the parts we are going to need for this project.
6: Load capacitor: 10 to 150 pf capacitor;
Tune capacitor: 5 to 50 pf spaced for 3 kv. or more.
A ceramic coil form for the plate choke, ½” X 3” (2” may be ok) with about 41 turns of no. 22 wire, a 500-pf doorknob and a 8.2 uH choke (Ohmite Z50 or equiv.)
A 1000pf / 5 KV doorknob blocking capacitor.
A 10-ohm 10-watt wire wound glitch resistor, the glass type not the sand type.
A female BNC chassis mount, the nut mounting is best here.
Ceramic insulators, use your known judgment here.
A coil wound with 5 turns of 3/16 wire with an ID of 1 & 1/8” and a length of 3” (the dimensions need to be close, not exact, you may need to compress or expand the length of the coil later).
A coil wound with 4 turns of 1/8” wire with an ID of ¾” and a length of 2 & ½” long. ( the same applies here).
Now we need to construct a plate parasitic suppressor network of 4 - 200 ohm resistors of 2-watt carbon or metal-film type (not wire wound) paralleled so as to equal 50 ohms and these are soldered in the open end of a U, shaped from a 5” length of 3/8’’ wide copper strap. On this amp. I used 2- watt carbon resistors and with the resistors mounted in the U, I had 1” from the inter 2 resistors to the inside of the apex. When my U was finished, it looked like a dipper (for the city dweller, I guess it would be a ladle).
Are we having fun yet?
Now we need to construct our tuned input board. This is what worked for me with a 1:1 swr input. I wound a coil of no. 22 enamel covered wire with 7 turns with an ID of 10 mm. and a length of 17 mm. You will need 3 silver mica capacitors at 500 volts, a 1500 pf, a 50 pf and a 59 pf (I’m sure you could vary these a little and still get a low swr. Mount these parts on a piece of prototyping board (any insulated board). From the input coax attach one end of the 1500 pf capacitor (that’s the small dia. coax coming from the relay that earlier attached to the tuned input board) and now solder one end of the coil and the 50 pf capacitor to the free end of the 1500 pf cap. The free end of the 50-pf cap. will be soldered to the braid of that same coax. On the free end of the coil, solder one end of the 59-pf cap. and the coax that goes to the input network of the 3-500Z, the free end of the 59-pf cap. is soldered to the combined ground of the two coax braids and the other cap.
WOW, it’s taking longer to write this procedure than it did to construct the amp.
Mount this new input board on the lugs that held the old input board.
Mount the glitch resistor on two ceramic insulators on the chassis where the old plate choke was located. You will need to drill two holes.
Attach the HV lead and the 1000 pf capacitor (the one I told you not to remove) to one end of the glitch resistor. On the other end of the glitch resistor solder a 4” length of hi-voltage wire and pull the wire to the left of the tube socket.
Look at the pictures that I’ve included with this project to get an idea of how the plate choke, the pi-l network and other parts are placed on the chassis. If I had it to do over again, I would not use a vacuum variable for the tune cap. and I would have used the recommended value for the load cap.
The red wire that went to the tuned input board is a 12 volt supply line that was used to activate the relays is now rerouted to the back panel and connected to a RCA jack.
I hope I haven’t forgotten any thing. Have fun with this project and just maybe I’ll hear you on 6 meters.
73, Paul AB5F