Conversion of the Harris AM-7224/URC from the 8877 to the GS-35B
© 2005 Dave Calhoun, W2APE

There is a closeup of the wiring harness for the tube unit which shows where to cut the cathode return circuit to add additional bias (picture 4). The AM-7224 has a complicated bias curcuit including a cathode overcurrent sensor. Original bias was about 8.2 Volts. Instead of attempting any changes to the original bias board, it was decided to simply add some Zener bias in series. The reason the bias needs to be changed is that the GS-35B has a lower Mu than an 8877. It will require a little more drive and a lot more cathode bias to get the zero signal anode current within reason.

The bias modification will apply to any 8877 conversion so it is included in detail here. Modification is as follows: Counting from the rear with the connector while plugged in top view, pins 4 and 5 are both factory connected together as originally designed. Both wires are cut and (2) 8.2 Volt 50 Watt 1N2806B Zener diodes are inserted in series. The diodes are first hooked in series as you would a string of batteries. The connector (actual cathode of the tube) side goes to the pin of the top Zener, it's case is hooked to the pin of the bottom zener the bottom Zener's case goes to the wires in the harness which used to go directly to the connector. A small heat sink is used for the Zeners whose cases must be insulated using the usual parts. The heatsink used was actually a carcass from an old mobile switching power supply (picture 5). It had the correct holes and transistor sockets. Actual Zener dissipation allows for minimal cooling here. 50 Watt Zeners are overkill but why ask for trouble? (4), then (3) Zeners were originally tried and that proved to be too much. ZSAC is now 240 milliAmperes with 3,200 Volts on the plate. This is probably a little more than necessary but it will stay that way until I get my hands on some better test equipment.

The tube socket and chimney is one which was purchased at Dayton 2004 (This appears to be a socket by Tom Adams, WB8WJU - tk). The existing AM-7224 tube fixture is easy to modify. Only simple hand tools were used. The before and after pictures should be enough to tell this story. Since the filament Voltage was different, a new filament xfmr was obtained through R.F. Parts. A current limiter (R.F. Parts CL-60) was placed in the primary. This might be totally unnecessary with in indirectly heated cathode but I sleep better knowing the inrush current is now limited. The replacement filament transformer fits the footprint of the original.

The AM-7224 originally had the filament transformer output directly connected to the filament pins. This is not a good idea when using an 8877. There are some filament transformers which are designed for isolation of R.F but I don't know if the original was. The replacement is certainly not. A bifiliar choke was wound on a length of ferrite rod which as approx 4" long (pictures 6, 7, 8 & 9). This was obtained from R.F. Parts also but was originally longer. Ther can easily be broken to length. The windings were made from #12 THHN and the ends which connect to the tube are tinned. Stainless hose clamps from the auto parts house complete the hookup. There is an additional R.F. choke which handles the cathode return circuit. Note that no attempt is made to return cathode current through a filament transformer center tap. That would be asking for a nice 60 Hertz intermodulation due to 1/2 filament Voltage appearing in series with the R.F. drive. The GS-35B has one side of the filament directly connected to 1 heater terminal. The 8877 did not. Here is a schematic which I found. No permission to use this has been sought http://www.nd2x.net/ea3axv-skem.html.

The AM-7224 needs a little modification to run at legal limit power as it is designed to be completely idiot proof and run well within it's ratings or kick itself offline. The cathode current limit can be adjusted as well as the power out sensor. It will now make 1500 Watts P.E.P. before showing a fault indication. One picture shows 1580 Watts out indicated on the LCD (picture 14). The Bird 43 says this is 1500 Watts so I'm going with that. A 100 Watt output tranceiver will deliver enough power if the plate Voltage stays up in the very high 2000's. The AM-7224 uses a tuned choke input supply so Voltage regulation is superb.

Cathode drive impedance has changed some from the 8877. It is not drastic and the tranceiver's built-in antenna tuner handles matching just fine. The AM-7224 didn't have a real good match even with the original tube. I'd expect to have to retune any tuned input circuit with a GS-35B retrofit in other amplifiers.

This amplifier is used in linear service mostly on AM. It works fine under high duty cycle modes and delivers 1500 Watts P.E.P. with plenty of headroom. I did have to use an auxillary blower on AM though. Effective heat removal through the GS-35B might not be as good as the 8877 with my original high speed fan. The amp just loafs on SSB. I have not used this on CW and probably never will as will not handle even semi break-in (There is a time delay built in to prevent hot switching the R.F. relays).

The GS-35B anode cooler seems to stay clean and not gather dust with an unfiltered airflow. I expected to see more dust when taking the amp apart for inspection as the inlet filter had been purposely left off for the last 3 months.

73,
Dave Calhoun W2APE