But for now, a quick look at the project article that launched Fred Sutter's "QSL" transmitters, all using some version of the 6L6 tube:
Readers with an eye to history -- or who notice the cut-and-paste -- should be aware the original schematic left out the cathode bypass condenser, replacing it with a direct connection. That meant the telegraph key wouldn't have worked, the transmitter would have been on all the time. QST published a corrected schematic the following month and I pasted that over the erroneous one.(I have built my own copy of this little transmitter.)
For the record, this transmitter has a lot of exposed high voltage. It is inherently dangerous. Before building and operating such equipment, be certain you understand the risks.
I remember seeing this in an old ARRL handbook. I started collecting the parts and hit an obstacle. As I remember, the coils were only available through Allied Radio and Allied had just been bought out by Radio Shack. All the old Allied inventory had been sold off (Radio Shack was into solid state and didn't see the need to retain all the old tube based stuff.) Lafayette was going out of business as well.
ReplyDeleteI think I had all the parts except for the coils. I could have wound my own but I went off to college and never completed the project. It was a nice little xmitter for a Novice.
Love the power supply, too -- the screen voltage comes via a 20K 50W (that's 50W!) slider resistor. (Lemme check my spare parts bin for that room heater.)
ReplyDeleteQSL (is that the right code?)
ReplyDeleteARRL joined.
"QSL" means to "I Acknowledge Receipt" of a message. With a question mark, it means "Do You Acknowledge Receipt?".
ReplyDeleteAnd while it's common these days to hear people say "QSL, QSL, I know what you mean", or similar, I'm one of those old farts that doesn't particularly care for using Q-Codes with voice communications, especially when the meaning doesn't really match the way it's used.
It's almost as bad as using 10-codes on Amateur Radio.....
Ed, try Mouser electronics, for an Ohmite D50K20KE, $16.21 (on order). Or go cheap and use their 20- or 40- series 10W resistors, rated for more than the applied voltage, a 5K and a 15K, around $2.00 per each. Tap is 5K down from the B+, 15K to ground from there. (Esg won't be exactly right).
ReplyDeleteFred only used a 50W bleeder because he wanted to make sure the screen dropper section had enough dissipation -- needs 6W or more, assuming 32 mA (!!!) worst-case screen current; the bleeder itself runs 22.5 mA. Oversized tapped bleeder resistors are normal design practice in 1938.
I'm redesigning his power supply a little. 15H, 250 mA chokes are not as common as they once were, and neither is switching in the high voltage AC side of B+ supplies. You end up with three Hammond transformers and probably a condenser-input supply filter; C is way cheaper these days.
Wayne: Bravo! --I need to rejoin; I got vexed at them (over a work-related issue) about a decade ago and dropped out. Silly of me.
ReplyDeleteNow I've got to go get my vertical back up.
ReplyDeleteI've probably got all the parts for this except for the 83 - got a few 866's laying around - and that bleeder resistor.
I dropped ARRL maybe 15 years ago, when business kept me away from the shack. Maybe this will get me back into the hobby.
RobertaX de K7EDS
ReplyDeleteR R
Tnx for the math on the resistor, and the link.
Ur PS is FB. Send ATV when done?
73s
RobertaX de K7EDS
SK
ATV?
ReplyDeleteAmateur TeleVision
ReplyDeleteI know why the HV is switched - the 83 is a mercury vapor rectifier, and you don't want to apply HV to the plates until the tube is warmed up.
ReplyDeleteI've got a homebrew rack of push-pull amplifiers I got from a silent key in Elgin, IL years ago - he worked for WGN as an engineer. The rack is nice and tight, with RF gasketing and copper screen over ventilation openings - but the power supply was transformers, two 866 tubes, caps, and filter chokes sitting on a metal rack in the room behind his shack, all wired together with 12 gauge solid copper wire. Talk about exposed HV! I've got grandkids running around here - so can't do it that way. One of these days, I'll try and get that back up.
I've got some Amperite delay relays in glass packages somewhere - put one of those in series with the HV transformer primary and power the heater from the filament transformer - now you don't have to worry about sequencing the power.
Unexposing the HV is not a big problem. And building on wood adds a measure of safety as well. I prefer making coils from a section of #12 insulated solid wire easily obtained by buying a length of 12/2 house wiring at the local hardware store and opening it up. Gives you good wire, even spacing and insulation all in one shot. I usually use 1 1/2" forms, ABS plastic or hardwood dowel. I have a transmitter circuit almost identical from the 1940s and this week popped a 6550 tube in and upped the HV just for fun (and it is fun!) Great design!
ReplyDeleteVE3LYX