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#24
April 6th 04, 05:29 PM
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Uwe wrote in message ...
in article , N2EY at PAMNO wrote on 4/5/04 18:59: In article , Uwe writes: I will just have to fiddle a bit more with the pi network (since at the B+ voltages suggested here my plate current would be way high) and I will have to live with the chirp. Are you getting a "dip" in plate current? If not, the coil is probably too large or too small. Unless you get a real dip, the output network isn;t right. I have used a very similar transmitter with 350 volts on the plate, and the dip is clean and pronounced. Jim, the original docs I got for this tx call, at 40 m, for a 15 turn coil on the coil form provided with the kit, which I hear was 1.25" diameter. If I use the formula for air coils this turns out to be roughly a 22 microhenry coil. 22 microhenries? I get more like 8 microhenries using the formula L = (a * a * n * n)/([9 * a] + [10 * b]) where a = radius of coil in inches b = length of winding in inches n = number of turns The coil which works best with my tx is 8 turns on a 1,125" ceramic core. But do you get a dip? Be aware that the AC-1 went through some changes in its lifetime. Some models used a filter choke, others did not. Some used a 730 uuf loading capacitor, others just a single-section 365 uuf one. Coils changed too. To get guess work out of it I just bought and built a L/C meter and measured my coil to have 2.7 microhenry. So I am way off, but it works, sort of. The air caps are 36 to 420pf at the plate and 15 to 728pf at the antenna, so that seem right. All this happens with B+200V and 35 mA plate current. LC = 25,330/(f * f) so for 7 MHz, the LC constant is 516. Your 2.7 uH coil should resonate with 191 uuf. Older ARRL handbooks give typical values for pi network for 50 Ohm antenna loads and my values are in range for the caps but my coil is too small. The ouput voltage on my antenna measured with a scope is up to 75 volts peak to peak, with a 50 Ohm load that would mean I get more out of the tx than I put into it and I am not of the sort who says this might happen. So my conclusion is, and tell me if this sounds right, that I have an antenna which is far from 50 ohm resistive at 40m and that that makes everything weird. That's defintitely part of the problem. What antenna are you using? Have you tried a resistor or lamp load? The dips in plate current are nearly imperceptible and they are not aided by my 250mA full scale meter. They may be 2 or 3 mA. I tune with the help of my scope. The meter tells more. You can use a pilot light (#47, 150 mA) instead of a meter. Sudden thought: Where is the meter connected? Are you reading plate current, or plate-and-screen current combined? Here's something else to try: Often trouble of this sort is due to the RF choke used. What RFCs are you suing, particularly in the plate circuit? Although the LC meter may say they are a certain L, in real life they may have all sorts of unwanted resonances. To test this idea out, do the following: - Remove the plate RFC - Connect the antenna end of the plate coil to the B+ where the RFC used to be connected. This point should already be bypassed to ground through a disk capacitor of about .01 uF - Disconnect the "loading" capacitor - Remove the plate coupling capacitor. What you will then have is the 200 volts being fed to the plate through the coil, with one end of the coil going to the plate supply and the other end connected directly to the plate of the 6V6. The plate tuning capacitor is connected between the plate of the 6V6 and ground. End result is no plate RFC and a parallel resonant circuit. There's no connection for an antenna yet, but that's not important right now. Test out the rig and look for the plate current dip. It should be very obvious because there is no load connected. This is just a temporary setup to see if the RFC is OK. 73 es GL de Jim, N2EY |
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