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Weak Signal VHF by Tim Marek - K7XC

Conversion Notes - AM-6155 to 222 MHz

by Tim Marek - K7XC

Back in December of 2003, I purchased a mint unmodified AM-6155 on Ebay for $202 (+ shipping) from a fellow in the Midwest. I knew of their reputed reliability and relative ease of modification from articles I have seen published over the years so I felt reasonably assured I could make one work on 222 MHz. As I get older it seems that motivation, and having the free time to do something about it, seldom occur at the same moment. Which is why it’s now December 2005 and I am finally closing in on having the unit ready to operate in amateur radio service.

Part of the reason for my delay in finishing is that there is so much conversion data available on this particular unit. They have been on the surplus market over 20 years now and many folks have made what they deemed the correct modifications and published their findings in newsletters, journals and on the Internet. Some of these documents are very complete while others are rather sketchy in their descriptions, and at times the discrepancies between them has been confusing. This is a result from the varying philosophies of those involved which range from "do as little as possible to get it on the air" to "lets take this opportunity to make it better with improvements in design and function".

When the need for a 222 MHz amplifier became a priority, I began to sit down and digest everything written to date in an effort to find my own way through the process of creating a reliable workhorse from a mint AM-6155. I visited several wonderful web sites devoted to the subject (which will be listed at the end of this document) and printed many of the articles. Between the published mods and clean copies of the original Air Force TO’s (schematics), I created my own guidelines based on desired functionality and reliability with a reasonable amount of effort and minimal cash expenditures involved.

My plan is to incorporate the best of all the mods that I have seen to date;

1) Build and install a supply to power the economical surplus 28V coaxial relays.
2) Rewire the HV supply to be more reliable in amateur service.
3) Rewire the front panel and A5 board to support a operate/standby switch, metering screen reg current, and overall improved accuracy.
4) Provide for total cutoff during receive with fully adjustable transmit and idle bias.
5) Install a PTT RCA jack on the rear panel that keys the amp when grounded. (I use blue jacks from Digikey for PTT)
6) Build and install a PTT relay driver circuit that switches the bias and RF coaxial relays.
7) Use the economical surplus 28V Coaxial Relays available today.
8) Incorporate the latest component values based on the work of hundreds of fellow VHFer's.

Lets start with the mods that are common to any of the AM-6155 or AM-6154 units outs there.


1) Replace R1 and R2 (10 ohm 1/2W) with 2W versions. The 1/2W resistors will not handle the increased current.

2) Install a separate ground wire from the grounded ends of the R1 and R2 to chassis ground as the existing circuit ground will not handle the increased current.

3) Short across one of the two 100K 91W Resistors. Only one is needed, making the other a spare.

4) Remove R16. Every mod I have read says to do so but none describe why.


1) Install the red wire from tie point E8 to the positive side of the large filter capacitor in the bottom of the module. This will provide the +15V for the metering circuits (The voltage doubler for +28V to the coaxial relays is fed from this same location).


1) Remove R1 and disconnect the wiring to it (to be used later).

2) Disconnect the wiring to S3, the tune/operate switch. It will become the Operate / Standby switch once your done. Wire it in series with + the 12V feeding the PTT relay driver circuit to provide a standby function when S3 is open.

3) One of the two red wires that were removed from S3 goes to tie point E8 on the main chassis. Route this red wire to the +15V feed on the AC/AC converter module to power the meter circuitry.

4) The second red wire removed from S3 goes to J6 Pin 4 and should be soldered to tie-point E8.

A5 BOARD (On the front panel)

1) Remove C1 (15uf 20v) to speed up meter response.

2) Remove CR4 and CR6 and reinstall them directly across the meter itself, which greatly improves its accuracy.

3) Change R3 from 2.7Kto 15K to keep the meter from pegging at the higher power levels.

4) R4 (5.1k) may need to be changed to 22K - 28K to make the fwd/rev positions read correctly.

5) R18 may be increased in value to make the meter read a nominal value of 24 in the heater voltage position.

METER UNIT (On Front Panel)

1) Clip the jumper wire from switch S1A pins 1 and 3.

2) Move wire from S1A Pin 11 to Pin 3. This wire also goes to pin 16 of plug 1 (Doing this makes the key position display screen reg current)

A3 BIAS SUPPLY MODULE (Loosen 3 screws and 1 cable to remove)

1) Remove the 5 screws holding the circuit board and gently remove it from the unit without removing or breaking any wiring.

2) Locate VR1 (1N3044B - 100V Zener Diode) and remove it. Install in its place an ECG5098A (130V Zener Diode).

3) Cut the trace between E10 and R17. Using a pair of the wires removed earlier (22 gauge or larger), bring out both sides of that junction to the rear of the amplifier chassis. These will go to the new 5K 1W bias pot that is to be installed there.

4) Reinstall the modified circuit board back into the A3 module and mount the A3 Module back into the main chassis. Route the new wiring along the right side to the rear panel of the main chassis.


1) Drill 3 holes in the rear panel, one for the new 1W 5K Bias Control pot, one for the RCA connector used to key the amplifier (Ground to Xmit) and one for the RCA connector feeding +28V on Xmit to the coaxial relays.

2) Install the 5K pot. Connect the wire from the E10 side of the cut trace to the high side of the pot and connect the wire from the R17 side of the trace to the wiper. Install a 56V Zener (ECG5090A) from the cold side of the pot to one-pole of the DPST relay. This will be switched to ground on Xmit. The other pole is fed +28V with the relay output firing the coaxial relays when switched to Xmit.

3) Install the two RCA connectors. One of these connectors will be used to key the 12V DPST PTT Diver relay. The 2nd RCA connector feeds the two 28V coaxial relays switching the RF through / around the amplifier. I found color coded RCA jacks available via Digikey and used “black” for the +28V feed to the relays and “blue” for the PTT line.

PTT RELAY DRIVER CIRCUIT (Built on perfboard and mounted near the blower motor)

The amplifier bias and coaxial relays are controlled by a single DPST 12V relay. The 15V feed used to energize this “PTT driver relay” is routed through S3 on the front panel to provide an Operate/Standby function. The ground side of the relay coil is then fed to the center conductor of the “PTT” RCA connector on the amplifier back panel. When it is shorted to ground the 12V relay energizes and switches the bias voltage from the 130V Zener to the 56V Zener on the back panel. At the same it time it feeds 28V out the 2nd RCA connector to the pair of 28V coaxial relays switching the RF through / around the amplifier.

I removed all the unused wiring that goes to J7 on the back panel of the main chassis then remove J7 itself. I mounted this board in the same area where J7 used to be, using one of the original mounting holes and drilling 3 new ones for the remaining standoffs.


To derive the +28V needed to energize the RF coaxial relays a simple voltage doubling circuit can be used. All that’s needed is two 1N4004 type diodes and two 470UF 50V electrolytic filter capacitors. The original drawing calls for 100uf but the 470’s seem to hold it closer to 28v during transmit. The schematic can be found on the AM-6155 information depository website. It is in the form of a .GIF file from a KF0M schematic available at http://www.n1rwy.com/am6155/kf0m/kf0m5.GIF

That should complete the needed changes to any AM-6154 / AM-6155 main chassis. These modifications now allow the amplifier to cut off (almost) in standby conditions, provide front panel standby/transmit control, and yield improved regulation of the HV supply.


Now we need to modify the RF drawer assembly to readily accept drive power. Making the changes for 220 MHz operation is by far the simplest of any band. It consists of (1) adding a 22pf mica capacitor across the existing 4pf series capacitor and (2) adding a 2pf mica capacitor across the input tuning capacitor. Basically all that we are doing with these changes is increasing the input coupling.

Many of the published mods call for a 33pf mica cap but many folks have seen much better results with a 22pf mica, reporting easier grid tuning with less drive power required. I couldn’t find a 2pf mica and used a 5pf in its place with great success.

Be sure to remove the bandpass filter and directional power detector and replace them with N female to N female adapters. Both are only rated at 60W or so and will not handle the new power levels. This would be a good time to check the RF drawer N connectors and make sure the nuts are all nice and tight on the shields. Now would be a good time to give everything a good once over and verify all your added wiring.


1) Operation of the new bias control circuitry is as follows: The original bias control pot located on the A5 board, (Accessible through the top cover of the amplifier) will now control the cut off bias at very close to zero ma. The new pot located on the main chassis rear panel controls the operating bias which should be set to 60 ma +/-.

2) With these modifications in place on my AM-6155, 10 watts of drive produced 400 watts output at 222.100 Mhz. I run it at that power level in typical CW/SSB service and at 300 watts when operating the WSJT modes. Your mileage may vary depending on the quality of the PA tube in your amplifier.

3) Plate current readings of 250 to 275 ma are typical at these power levels. DO NOT EXCEED 300 ma !!!

4) Operation into a high SWR is not encouraged as the HV insulator and plate load insulator (KAYTON from Dupont) are known to break down under these conditions.

5) It is highly recommended to strap the input for 220V operation but 110V is no problem with a stable AC source.

6) I suggest you feed it a watt or two and adjust all the controls until you find the sweet spot on each and then slowly work your way up till the tube saturates at 300 to 400 watts out. My cavity peaked at 00068 on the turns counter.


Here is a list of websites I have found to date that have AM-6155/AM-6154 conversion data, schematics, recommendations, and typical meter values during normal operations.

The AM6155 VHF+ Conversion Repository (The Mother Lode Of AM-615x Information)

Packrat's Technical Info

WZ1V's VHF Modifications Page

Fair Radio Sales - Amplifier Section

Many thanks to all those who published their modifications over the years. Without the work of people like N3AHI, KF0M, W3RJW, N1RWY, K0TLM, WA3AXV, WA5VJB, WZ1V, WB0VZW, and others, none of this information would be available.

Best of luck and I hope to work you on 222MHz from DM09ol.

Tim Marek - K7XC, 3310 Alcorn Rd, Fallon NV 89406
Updated 12/15/2005