What power level do you want? How many tubes of each type do you have? What iron is available?
A 4-125 modulated by a pair of 4-65 tubes or a pair of 4-125 tubes will give 300 watts out at 2500 volts, 150 ma. A pair of 4-125 tubes modulated by a pair of 4-125 tubes will do 600 watts, or say 500 watts at 2000 volts. When running 2000 volts and up, the 100TH tubes make good modulators if you run class B. You could do the standard pi net, or push pull link coupled, depends on what is in the junk box. If you use a big enough filament transformer, you can just plug in 4-250 or 4-400 tubes in place of the 4-125 tubes. If you are limited to a 300 watt mod transformer, the single 4-125 works well. I like the fixed bias plus grid leak bias on the rf stage, plus a separate screen supply with a choke and resistor in it. Its keyed along with the high voltage. The bias supply runs off the filament voltage so anytime the filaments are on, you have bias. Grid input coil with link for each band you want, switched in circuit by a band switch, pi net coil can be a roller inductor or switched taps on one coil. Or, it can be a one band transmitter, with just coils for that band. You can make the bias and screen voltages adjustable, along with switching out the screen choke and resistor, and make it work as an amplifier as well. I would say more depends on what iron you have, tubes can be got on ebay and at fests, or in trade, for low prices, but a good 600 watt mod transformer might be hard to find, as are high power chokes and power transformers, those parts are not as frequent on ebay and fests because of the weight. A pair of 813 tubes gives 600 watts out easy, a pair of 812a will give 250 watts, a single 4-125 250 to 300 watts, etc. For the 4-xxx tubes, you need to get up to 2500 volts before they become efficient, and that means big plate tuning caps, big insulators, high voltage chokes, etc. The 813 runs at 1500 to 2000 volts, 300 watts out each The 812a runs well at 1000 to 1500 volts, 130 watts each, The 4-125 runs well at 2000 to 3000 volts, 300 watts each, The 4-250 and 4-400 like 2500 volts and up, 510 watts each, The 4-65 tube likes 2500 volts, gives 230 watts out. For modulators you have: 811A, a pair at 1500 volts gives 340 watts class B, 813 tubes as modulators give 650 watts at 2500 volts AB2, 4cx250b tubes give 600 watts at 2000 volts in AB1, 805 tubes do 370 watts at 1500 volts class B, 810 tubes do 500 watts at 2500 volts, 450 watts at 2000 volts class B, 572B tubes will do 530 watts at 2000 volts class B, 4-125 tubes do 330 watts at 2500 volts AB1, 400 watts in AB2, 4-400 tubes do 625 watts in AB1 at 2500 volts, 4-65 tubes do 270 watts at 1800 volts in AB2 100th/100tl tubes will do 450 watts at 3000 volts class B The easiest tubes in the world to get are the 811a, 4-125, 4-400, 813, 572b. The Bill Orr handbook 14th edition (red, 1956) is a good book for designs. I would not build a transmitter because I had some tubes, but did build one because I had a nice mod and power transformer...the push pull 812a rig... Brett N2DTS > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Behalf Of Jim Isbell > Sent: Thursday, December 11, 2003 2:28 PM > To: Mail List for AM radio > Subject: [AMRadio] AM from what I have in the junk box > > > Anyone have any nice schematics to construct a AM transmitter > from some > 4-125As or 4-65As that I have available?? Also have a new 833a that > could be used for a modulator. > > _______________________________________________ > AMRadio mailing list > [email protected] > http://mailman.qth.net/mailman/listinfo/amradio
