Quote:
Originally posted by Chad Hauris
You don't want to use an 807 for a part 15 unlicensed transmitter...that is a 25 watt tube at least. The FCC will be calling.
Check out Robert Casey wa2ise's web site for info about tube AM micro-transmitters. You can use parts from an old AM tube type radio.
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As Chad pointed out, if you want to build a tube-type transmitter, the 807 is way too much for a Part-15 unlicensed unit. There was a design for a Part-15 compliant AM transmitter published in a Ham Radio book some time ago which I seem to remember used a 6A8, 6J7 and powered by two 6.3V 2A transformers connected secondary-to-secondary with a 6X5 or some similar rectifier tube. The first transformer stepped-down the 120V line to povide a 6.3V heater supply for the tubes and to also be stepped back up to provide an isolated and energy-limited 120V source to be half-wave rectified and filtered to meet the unit's B+ requirements. You could redesign the unit using 6.3V single-ended octal equivalents (6SA7, 6SJ7 and keep the 6X5) or use 12.6V tubes with two 12.6V transformers (12SA7, 12SJ7. 12Z3). Miniature types 12BE6, 12BA6 and 12X4 would also work well with a 12.6V heater supply.
AES sells a transformer with 115V primary, 125V 15mA and 12.6V 300 mA secondaries (AES #P-T442 for $16.95) which would power a transmitter using either a 12SA7 and 12SJ7 or 12BE6 and 12BA6 tube complement. Since the 12SA7 and 12SJ7 or the 12BE6 and 12BA6 would draw a total heater current of 300mA, there would be nothing left for the heater of a 12.6V tube rectifier (12Z3, 12X4 etc.) so a silicon diode or 117Z3 (miniature) or 117Z4 (octal) rectifier would be needed. The 117V 40mA heater of either rectifier tube would connect directly across the transformer primary and is insulated from the plate and cathode UNLESS the tube developed a short. The elimination of the 117V rectifier tube and use of a silicon diode and series resistor instead would reduce the risk of loss of transformer isolation due to a rectifier tube short. There are also reasonably-effective means of protecting the rectifier tube against transients (line-to-chassis bypass capacitors of about 0.01uF 600V shunted by 2.2M 1/2W resistors) which are the most frequent cause of breakdown of the insulation between heater and cathode. The particulars will be the builder's choice. The bottom line is that quite a nice Part-15 compliant transmitter could be built around an octal or miniature tube complement using readily-available parts.