R
RI126
Guest
I recently bought a D&A PDX-400, and I'm looking for a owners manual and any information on how to tune it,how much do I need to drive it etc. RI126
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That amp is a little more complicated to tune because it has a "driver" stage with it's own tune and load controls.
Here's how to tune it (at least one way to do it):
While giving the amp a little drive, adjust the plate tune to max output. Then adjust the main load to max and increase the load just a little to bring the output back down 5-10%. Then repeat the same procedure, but this time by adjusting the driver stage. Finally, check the tune and load one last time on the main stage to make sure that nothing changed dramatically since you just adjusted the drive. I don't believe that amp needs much drive, so be careful how hard you push it.
Here's how to tune it (at least one way to do it):
While giving the amp a little drive, adjust the plate tune to max output. Then adjust the main load to max and increase the load just a little to bring the output back down 5-10%. Then repeat the same procedure, but this time by adjusting the driver stage. Finally, check the tune and load one last time on the main stage to make sure that nothing changed dramatically since you just adjusted the drive. I don't believe that amp needs much drive, so be careful how hard you push it.
Radio drive ONLY ! !
Since it was sold in the form of a "Ham CW transmitter", you had to convert it BACK to a linear after you got the thing new. Most folks simply hooked a wire from the input side of the antenna relay to the cathodes of the 3 driver tubes. This causes the driving radio to "see" a high SWR, usually 3-to-1 or so.
Best advice is to find a gray-cabinet "Phantom" amplifier and copy the input circuit. The "Grid Tune" trimmer cap that was mounted on the rear panel in the "Phantom" model is soldered to a tie strip inside the PDX, with the same coil soldered in parallel with it. It was used in the "crystal oscillator" circuit that made it a "CW" transmitter as delivered by the factory. You still have to add the four-turn input winding to the bare-wire coil found inside. Never have seen any instructions anywhere for doing this. Once the original "Phantom"-style input circuit is in place, the trimmer cap can be adjusted for a good impedance match (low SWR) between the radio and the PDX.
Tends to improve the output power, along with making the final in the radio breathe a bit easier.
73
Since it was sold in the form of a "Ham CW transmitter", you had to convert it BACK to a linear after you got the thing new. Most folks simply hooked a wire from the input side of the antenna relay to the cathodes of the 3 driver tubes. This causes the driving radio to "see" a high SWR, usually 3-to-1 or so.
Best advice is to find a gray-cabinet "Phantom" amplifier and copy the input circuit. The "Grid Tune" trimmer cap that was mounted on the rear panel in the "Phantom" model is soldered to a tie strip inside the PDX, with the same coil soldered in parallel with it. It was used in the "crystal oscillator" circuit that made it a "CW" transmitter as delivered by the factory. You still have to add the four-turn input winding to the bare-wire coil found inside. Never have seen any instructions anywhere for doing this. Once the original "Phantom"-style input circuit is in place, the trimmer cap can be adjusted for a good impedance match (low SWR) between the radio and the PDX.
Tends to improve the output power, along with making the final in the radio breathe a bit easier.
73
I have a D&A PDX-400 and the SWR trimmer (small tuner knob) in the back had went bad and I replaced it with a new trimmer but it still didn't fix the problem. The SWR between the radio and the linear is sky high. Before the stock one went bad it was always good. Any ideas of what's going on with it or how to fix or bypass this completly and get a good SWR between the radio and linear?? It worked Excelent befor the trimmer went bad. Output was 800-850 watts. Any help would be very much appreciated.
