AM-6155 Amplifier, 432MHz RF Deck Conversion by K4HV

 

There are several ways to convert the AM-6154 and AM-6155 RF decks on 432MHz. Most Iíve seen describe using a hollow Teflon cocoon of sorts and feeding the HV through an inductor inside or a makeshift PC board capacitor on the outside, etc. Some have been successful using this method. I found it to be too much work and aggravation. I also use the following method for my 144MHz and 222MHz amplifier. The plate choke dimensions only change. The rest of the power supply modifications exist elsewhere on the internet. I strongly suggest that you do your own research and not blindly follow all those instructions. Figure out why they were done then do those that make sense.

 

The following describes a re-routing of the HV feed to be somewhat more conventional. Unsolder and remove the original HV wire from the center of the cavity. Unscrew the RFI bead assembly and pull it out, cut the wires off, then put the plug back in the hole to keep the bugs out. This is not used. It eliminates most of the arcing problems in the plate/cavity coupling capacitor that punch through the Kapton insulator. Clean the surfaces carefully with alcohol and check for burrs, etc. before reassembly. Put the plunger in the UHF position and lock it down.

 

Hereís the overall cavity set up and is ready to go. I added a 10ohm HV glitch resistor in case of internal tube arcing.

The resistor can be seen toward the lower right mounted on small HV standoffs. The white HV wire runs back to the capacitor. It was obtained from RF Parts.

 

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AM-6155 cavity converted for 432MHz

 

Below are pictures of the inside of the plate compartment. I just happen to be using a Russian 4CX400. I run the same tube in all three of my converted amplifiers. It makes it easier for me to standardize everything in the shack. The feed-through capacitor is soldered to a 7-turn inductor. The inductor dimensions are in nearly any ARRL handbook that has a 432 amplifier construction article. If the inductor isnít big enough (uH wise), excessive RF current will run back through the capacitor, blow it apart and blow the HV fuse. I had to add a turn on my 222MHz amp because of that small problem.A small hole is drilled in the plate ring just large enough for the inductor wire to pass through. The small tab of the wire is then bent back and soldered. Trying to get solder to stick to the ring is pretty hard. Use needle nose pliers to tighten the inductor connection. Keep the inductor away from the sidewall to prevent HV arcing. Itís pretty tight quarters in there and the leads are short. I used a threaded HV feed-through capacitor 1300pF @ 3kV. They have a 5kV test voltage. I was able to get these from Surplus Sales of Nebraska. Part number (FRI) GRN-1300. They have several to select from, and prices range $7 to $10 each. Get a couple in case you screw up or for another amplifier.

 

Important You will not be able to use the supplied nut on the capacitor due to the thickness of the side wall. Drill and tap the sidewall of the plate compartment for the capacitor and thread the part in the wall. (Not a through hole)

 

Loading Capacitor Modifications

There is a black rubber/plastic washer inside the loading capacitor gearbox to limit travel of the loading capacitor. Using a sharp knife, whittle the washer away and cut it out. This will allow extra loading adjustment for the amplifier. When tuning the amplifier up on 432MHz the loading is very light and hard on the tube. Efficiency is low on a 432MHz conversion.

 

 

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AM-6155 Plate Compartment

 

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Close up of the plate connection with the Teflon chimney removed.

 

The capacitor is sticking out of the side of the plate compartment. How can I get the cavity back in the amplifier? Easy, just make a notch in the chassis like the one shown below. Get a pencil and use the RF drawer to mark where the notch needs to be. Use a file and just keep working until there is enough clearance to tip the cavity to slide back in the chassis. Thereís no need to go too far. The front cover will hide the notch when it is re-installed. Be careful not to hit the capacitor when tipping the RF drawer back into the chassis. They are somewhat fragile and can crack if abused. At least now itís easy to change out!

 

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AM-6155 Capacitor Clearance Notch

 

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Putting the Cavity back in the Amplifier Chassis

 

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Input circuit compartment

 

Summary

I get a reliable 375-400W out of my 432MHz amplifier. It has seen heavy contest use for several years. Those who say they get more are driving it too hard and create splatter up and down the band. My grid voltage regulator circuit can be found here. You must use a grid voltage regulator on a 432MHz conversion. It also helps on the 144MHz and 222MHz conversions.

 

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