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Mrblack
| | Posted on Sunday, December 22, 2002 - 12:00 am: | 
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I put together a 2 tuber and I'm am having trouble with keying cir. When the cage is on the top of box it will key up but when the cage is off of the box it will not key up. What can I do to fix this. This is a simple box no bells,just as cheap as I could make it.The keying cir. works in other box's.I use npn trans. |
2600
| | Posted on Monday, December 23, 2002 - 2:02 am: |
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Hmmm. An NPN transistor and..... what? Doesn't narrow it down much. A keying circuit that keys reliably down to 1/10 of a watt and one that takes 2 watts carrier or more to kick it in both "work". I'm gonna guess that there is no input matching network between your relay and the tube cathodes, just a blocking capacitor. If you put a SWR meter IN BETWEEN the radio and the amplifier you'll find a 4-to-1 SWR or so when it is keyed.
Putting the cage onto it is adding a "parasitic capacitor" to the output circuit, as if you had wired a 10 or 20 pf capacitor in parallel with the Plate Tune control. The tubes and the output circuit parts are separated from the grounded metal cage by an inch or so (or less?) of empty space. Mother Nature says that when you put conductors close to each other you get a capacitor, whether you wanted one or not. I suggest looking at the plates of the plate tune control. If it appears to peak with the plates nearly all the way apart, spread the turns of your Pi-Network (output) coil for a little less inductance. This will move the "peak" position of the Tune control's plates closer to the middle of its travel. I'm guessing that when you put the cage over it, the capacitor goes to its minimum limit, and can no longer turn far enough to bring the thing back to a true peak. If you peak it with the cage off, and then key it with the cage in place, the position of the peak on the plate tune knob changes, toward the plates being farther apart than they were when you peaked it with the cover off. This is the one thing that putting the cover on can explain easily. If this is pushing the Plate Tune control to its end limit, the input SWR will skyrocket to 6 or 8. Any keying circuit will misbehave under those conditions.
The phrase "a simple box" brings to mind an amplifier with almost enough bypass capacitors. I have seen many of these, built without any parts that didn't add to the meter reading. Making one of these stable usually includes moving input and output parts farther apart, improving marginal ground connections on the tune and load controls, moving both of those controls above the deck, adding bypass capacitors to the heater pins on the tube sockets, to the power supply and fixed bias to the grids to keep the tube current down to a safe level for AM.
Did you build it from printed plans, or is it a copy of an amp built by an old-time TV repairman? If you left out filtering and bypassing capacitors, it will do strange things. Oh, and you DID use parasitic chokes on the tube caps didn't you?
For a 2-tuber, there's a simple way to improve the input match if all you used was a disc capacitor from the relay to the tube cathode pins. Make a 1/2-inch diameter coil from insulated solid wire. Size can be #16 to #24, just something stiff enough to keep its shape, about 8 turns. Insert this coil between the input blocking capacitor and the cathode pins on the tubes. Spread or squeeze the turns for minimum SWR on a SWR meter placed BETWEEN the radio and the input socket on the amp. If the INPUT SWR keeps falling with the turns spread more than 1/8 inch apart, remove one turn, squeeze the remaining turns closer, and try again. This will often get the input-side SWR reading below 2 to 1 all by itself. Tends to make the radio AND the keying circuit happier. Your choice of a RF choke for the cathodes has an influence on input SWR. If you used one that is meant for noise filtering, it may not be suitable for use at 27 MHz.
"Just as cheap as I can make it" might be too cheap to be stable.
73 |
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