Wednesday, March 23, 2011

THE 6L6 JOB

Not a spy movie but a family of homebrew ham transmitters. The 6L6 is a beam power pentode vacuum tube introduced by RCA in 1936 that kind of caught on -- and caught on big: they're still making them. You'll find them in guitar amplifiers and some esoteric hi-fi amps, too (though purists there are now into triodes and are keeping the WeCo 300B in production for the rest of us).

Hams quickly latched onto the new tube -- it was relatively inexpensive for the power level -- and even though it was marketed as an audio amplifier, the lost no time in seeing if the 6L6 was happy with RF.

It was. Eventually, even RCA had to admit it and someone in the transmitting tube managed to sneak the 807 into production -- essentially, a souped-up, RF-ready 6L6.* But the cheaper receiving tube had won a place in the amateur pantheon and went on the become the heart of thousands of little transmitter designs. (There were plenty of 807 ham rigs built, too).

Fred Sutter's "QSL" series of transmitters -- so named because the little rigs had a footprint no larger than a 4" x 5" postcard (hams exchange QSL cards to confirm two-way contact: talk to enough people, far enough away, and you can get a fancy award!) -- were justly famous; mine is safely (I hope) stashed away Somewhere Around here. Frank Jones (of Radio magazine ham handbook fame) offered his own versions (schematic and a nice example at this link) and I have built a couple of those: The first one is a sort-of breadboard: the DC and heater wiring is all underneath. The empty socket at the left is for the frequency-determining crystal, the first variable condenser is plate tank tuning and the second one is output loading. But its a 1930s design, with relatively low-C (50 pF) and the output circuit is not a pi-network: the "loading" condenser is fed from a center-tap on the tank coil. This means harmonics do get out; at my former location, the first time I fired it up on 80 meters, the ham down the alley was knocking on my hamshack window in minutes, having copied my callsign via his TV receiver. (It did okay on 40 meters). This little rig worked all over the continental U.S. The frames of both tuning condensers are "hot" with plate voltage, by the way, but stubby insulating couplings and nice big knobs reduce the odds of getting bit. (Key jack, lower left, is in the cathode-ground circuit -- touch the cathode side when the key is open and you'll get a nasty tingle).

Another 6L6 rig, I built in a candy tin. Shown here sitting atop my RME-45 receiver to show how small it is (and to show off the receiver; I was copying W1AW and trying to keep up with some QRS guys earlier tonight, with a 6' hank of wire for an antenna since there was a storm outside: RME built good receivers). There should be a coil plugged in the socket at the right -- my box of transmitting coils is also Around Here Somewhere. This one does have a newer-type pi-net output, though a little more C on the output side wouldn't hurt. The crystal socket is out of sight behind the tube. And here's the mysterious inner workings -- a couple of RF chokes (plus the tinned-busbar VHF choke just left of center -- the breadboard rig has one, too), a voltage divider for the screen, grid resistor, feedback condenser, cathode resistor and a whole lot of bypass condensers. The BNC connector was for a VFO but I never wired it up. there are a couple of little braces soldered in place, too -- the sheet metal candy tins are made of isn't as sturdy as a real chassis. Oh, don't be fooled by the meter scale (0 to 5 Amps); that's one of a batch of meters surplussed from a 1950s RCA commercial transmitter, all 1 Ma/1 V/1kOhm. Very easy to shunt for the range desired, in this case 0-100 mA).
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*All-glass, with a low-loss base; 6L6s were made in both metal and glass-envelope variants. Much later, the 807 begat the TV receiver sweep tube 6BG6, little more than an 807 with an octal base. The 6L6 was scaled down (6V6), scaled up (6550) and made in industrial versions, too.

15 comments:

  1. Oh, yeah! My first Novice transmitter had a 6L6 final driven by a 6AG7 crystal oscillator. I think I got about 10 Watts out of it, but I was pushing it pretty hard!

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  2. Great post!

    My RFI story: when I was sixteen I got a letter from the FCC. They had monitored my CW signal on the Marine band (!) on Long Island, and they demanded an explanation. I poked around in the transmitter innards and when I tapped the harmonic choke on the final plate cap, the resistor crumbled into dust. I replaced it and sent the FCC a letter of explanation and the matter was closed.

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  3. You can yell at me: About 15 years ago I tossed my big box of tubes in a move.

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  4. This is great stuff, even though I understand barely half of it. Hopefully I'll come back to this post in a year and say "Oh yeah, of course."

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  5. And I think every ham age 18 or older, has enjoyed a few "Cold 807's"!

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  6. Roberta X remotelyMarch 24, 2011 at 3:01 PM

    Dr. Jim: You'd think, in this day of small-scale brewing, that someone wouldbe botting "807" in bottles with a nice etched label....

    Wayne: why, there's hardly a thing to it! If you're an ARRL member, I think the have online QST archives, where you can find every article Fred Sutter ever wrote. A decade ago, I had the League's permission to do a very short print run collection of all of them; back then, it ground to a halt due to lack of time.

    Old ARRL and "Radio"/Editors and Engineers Handbooks are extremely valuable resources. Pre WW II through about a decade after are IMO the best ones. George Grammer, Ross Hull, Frank C. Jones and a host of other FB OTers are waiting to share what they know!

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  7. I generally buy a new Handbook every 10 years. I have my first one from 1964, and all the intervening ones.
    I agree that some of the League's early publications were absolutely world-class, and I still recommend that new Engineers and Technicians get a Radio Amateur's handbook. It covers ALL the basics, from simple things like hand tools and AC/DC fundamentals, on up through the basics of digital circuits.
    I think the ones from the 60's were the best, but then that's when I was growing up with this stuff!

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  8. That candy-tin rig is *very* clean. Nice work.

    And, yeah, "der finger-poken" is ill-advised. (Don't ask how *I* know.)

    BTW, if you're in the [San Francisco] Bay area, the Computer History Museum in Mountain View has a real (but not "live") Eniac computer that reeks of solder flux. From the smell even without power, you'll know it is nearby.
    (1401 N Shoreline Blvd, Mountain View, CA 94043, (650) 810-1010 -- call for days and hours as their schedule varies.)

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  9. Dr. Jim: Excellent advice, though once they've mastered the basics, for modern stuff, it's hard to beat Horowitz and Hill, "The Art Of Electronics," which assumes the reader has a little math and science and moves (rapidly) from there. Started out as a homegrown text for grad students who needed to "do electronics" to accomplish their PhD work but it's down-to-earth stuff all the way through.

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  10. Ed: Thank you, though the lead length on some of the bypass caps seems excessive in hindsight. --I travel very rarely but SF is a place I visited for a weekend once and didn't see many sights (doing tech work). Always hoped to get back, despite the anti-self-defense bias out there.

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  11. I like your can! I never see 1/2 and 1W resistors anymore. Just 0604 SMT...

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  12. The Art of Electronics, Horowitz and Hill, is currently a tad over $100 at Amazon, and just under that at Ebay. That's more than I want to spend so I'll keep shopping for now but it's moved up real high in my "buy next" list.
    Regardless, I always appreciate recommendations from an expert.

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  13. I think I remember paying $40-60 for mine.

    Had a spare for a while, but gave that away...

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  14. All sorts of 6L6, 6V6, 6550, EL-34, and EL-84 glowing hot things get used here at Casa de' G-98. Thanks for the write-up, Roberta! It makes a wonderful reminder for me to fire up both the Hallicrafter and Hammarlund sets one of these days again...

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  15. The Art of Electronics by Horowitz and Hill can be had second-hand for about $15 (including shipping) from here...

    www.abebooks.com

    If you're like me, you scrounge a lot.

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