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click any of the thumbnails below for enlargements or explanations (click here for schematics) |
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Finished Amplifier |
Inside View | The Tube | Control PCB | Top View |
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If you have a 2C39 or a 7289 in the parts box, you can build the cavity for this amplifier from the plans in the ARRL Handbook. The original cavity design article was written up in the 80's by Chip Angle, N6CA. Because the documentation in the handbook is adequate, I won't go into it's construction here except to say that I used copper instead of aluminum for the cavity ring; as for the rest of the amplifier, most of the techniques are the same as in the 300w amplifier listed in the left-hand navigation bar.
The tube is being operated at many times it's normal ratings, and can run like this in SSB and CW because of the low duty cycle and water-cooling of the anode. However, if you intend to use this amp in a high duty-cycle mode like FM, note that a full key-down for more than a few seconds will cause overheating of the grid and cathode, and the tube will drift thermally due to internal changes. For this type of service you'll need to lower the drive to keep grid current under 50 ma, and bias the tube close to cutoff at idle. Under those conditions the amplifier can operate continuously at 125-150w output.
The youngest 2C39/7289 tubes are 26 years old now, and even with unused ones, there is quite a variation in performance (see test data below).
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Water cooling is required at these power levels |
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| Tube 1 - 7289 #1 | Tube 2 - 7289 #2 | Tube 3 - 2C39WA #1 | Tube 4 - 2C39WA #2 | |
| Output Power | 200 w | 160 w | 170 w | 150w |
| Drive Power | 10 w | 10 w | 10 w | 10w |
| Anode Voltage | 1350 v | 1350 v | 1350 v | 1350v |
| Anode Current | 300 ma | 260 ma | 270 ma | 250 ma |
| Grid Current | 80 ma | 60 ma | 70 ma | 60ma |
| Idling current | 25 ma | 25 ma | 25 ma | 25 ma |