This installment will largely concern itself with what we now call the 6L6 power oscillator, or "PO". These days, the 6L6 and it's descendants finds a ready audience with the electronic music crowd, but from time to time, the single valve (or, tube, but I prefer the more accurately label "valve") 6L6 or 6V6 transmitter turns up either in discussion, or as a project transmitter.
Very recently, we had an event on the air called "Novice Rig Round-up", in which about 300 hams trotted out their beginner rigs, whether stored in the cellar or purchased and restored for the event - and exposed the world to some very hard core nostalgia. Tones and signal dynamics that I haven't heard in years. And among the Home Brew offerings was the single valve 6L6'er, and I believe more than one. All of these were, of course, crystal controlled, as per the Novice protocol.
It is remarkable that a valve on the brink of turning eighty years old is still stirring up the Ether! Shades of Grammar and Goodman!
The 6L6 is a ubiquitous as the day is long and icicles form on a frosty winter's day. As the photo above hints, a lot of this use was portable, as per the Depression Era concept of "portable". The valve began life as an intended Audio Beam pentode, but was from the very first announcement of it's introduction in May of 1936, this tube found itself the frequent visitor of the Ham Shack Work Bench. I thought it might be fun to catch a few early episodes of the Saga of the 6L6 Power Oscillator, beginning with the announcement itself, as it appeared on the pages of QST, fetching the attention of the 10,000 hams in North America:
The original 6L6 was, of course, a metal valve. The idea was shielding, and in truth, it was an effective shield. Eventually the concept of the metal valve went by the wayside simply because just as effective shielding can be obtained using metal slip cans, which became the norm with the advent of the 9 and 7 pin "mini-tubes". The 6L6 gave a respectable amount of output at Class A, to the tune of about 8 watts, realistically. But as we shall see, a lot more was realised as a Class C cw transmitter.
The very next month's issue of QST saw the first mention of the 6L6 as a transmit tube. We see the first, and probably the most enduring morph of the 6L6 in cw service in Frank Edmonds' June '36 article "The 6L6 Beam Power Tube as a High-Output Crystal Oscillator":
And here was shown the most enduring schematic. We will find this oscillator circuit very nearly duplicated in a later and more famous article from February 1938. The opening line of this article pretty well sums up what the legions of Ham Radio Homebrewers held as principle for the past Century: "The advent of a new tube always kindles the fire of conjecture as to it's adaptability to transmitter oscillator design, even though it may have been intended for other uses." (italics mine)
And adapt we did. Now figure, this tube sold for about two or three 1936 dollars. Depression dollars! This tube could have purchased dinner, a cigar, and an overnight stay at a not-to-disrespectful boarding house. It wasn't cheap . . . about 40 dollars today. Thus, the 6L6 was not an economy move. It was, rather, an efficiency move. You got more bang for the E x I buck, input to output! The concentrated electron beam flow was a thing of genius!
Check out that Crystal Oscillator Performance Data Table. This was the article that most inspired the later creator of the famous "QSL-40", Fred Sutter. Check out that last line: 425vdc plate, 285vdc screen, 70.1 watts input!! Yikes!! At 51.5% efficiency at Class C, it realised 36.2 watts output. Oh, but hold on to your electrodes, this puppy could be driven higher than that!
What about the xtal current? Maybe I should have shown a close-up of the xtal, but it is the size of a tumbler combination pad-lock! Bliley made them, we call them "Door Knob" crystals today. They handled enough current to light a #47 pilot lamp. No kidding! There were a few for sale at the recent Orlando Hamcation, I should have purchased one just for display purposes. They really are a marvel to behold, the ancient Bliley.
Later, in November 1936, there appeared what to this author was the real impetus that put the 6L6 on the radar for the more commonly resource endowed ham of the middle Depression Era. Byron Goodman published an article in that month's QST called "A Simple Two-Band 6L6 Tri-Tet Transmitter". The TTT. Ok, so that monicker never caught on, but boy, the portable 6L6 PO transmitter sure did!
"In these days of inexpensive crystals and tubes, there is really no reason for the beginner or amateur of limited means to deprive himself of the advantages of a crystal controlled transmitter." Thank you, Mr. Goodman! This article, in my opinion, is what put the 6L6 in the Shack. Here it is, in all it's Masonite and Plywood Glory. And yes, fans, she is still built very much like this today. Only, uh, with a little less grid current. Oh, but Byron was not one to hyper-charge this valve, much more sensible voltages are utilised. While he holds out that this rig can be powered up to 400v, he speaks largely about using the 250v "power pack".
I, personally, am a big fan of Masonite, if it is double tempered. This would be the type used for clip-boards. Masonite has an oil impregnation that permits longevity that is quite impressive. As a Letterpress Printer that deals with very old wooden type cases, I find my Masonite cases have outlived their wooden counterparts in un-air conditioned Florida climes 2 and 3 to 1. Wood rot sets in down here fairly quickly, Masonite is very nearly immune to this. Polished, it is a dead ringer for Bakelite. In the 1930s, often they were backed with aluminium below deck, for shielding and to create a ground planing effect. It was also easier to ground, but cheaper than an all metal chassis. This construction mode lasted all the way up into the 1960s. Not a bad run, boys!
The back-page finish of the Goodman article. A foundational piece of literature from a radio pioneer in his own right, Goodman published great construction articles, along with the likes of Millen, Lamb, and Grammar. And a lot of their work is informational to this day. Why? Because the Laws of Physics have not yet changed, ya young Squirt!
The last entry closes out the 1930s to a large degree, and is probably the most famous of the 6L6 articles. Entitled "The QSL-Forty", by Fred Sutter, this article uses the second morph of the 6L6, the glass "S" shaped envelope 6L6G.
The QSL-Forty was so named because the chassis dimensions were QSL-card sized, 3-1/2 by 5-1/2 inches, and stood 3 inches tall. Oh, and beware that schematic. It was corrected the next month: the cathode needs to have a key inserted between it and ground, and not shunted across cathode and ground. Yeah, guess what? As a Novice I built this rig right out of this magazine, and I didn't catch that. So I can hardly blame the QST layout guys!
Fred Sutter cites as his source the Edmonds article, but I think he also read Goodman's article, too. Just a hunch. To me, of equal value in this article is Sutter's description of how he made that tank/antenna coil! I followed his instructions, and you need to know, this was a very nice coil !
This is my own version of the QSL-Forty, using what I call a "Modified Slat Board" chassis, consisting of angle aluminium with hardwood end-blocks. Very, very solid chassis construction. The transmitter was a bucking bronco, though. To keep a good note, you cannot simply tune to a dip in current like a MOPA. I found I had to detune slightly to the high capacity side, or "Hi-C" of rez tuning. This brings the efficiency of the PO down considerably, but it keeps the FCC off your back. This rig here could do 20 watts, but if I wanted to keep the "youp" down to a dull roar, it was detuned to about 5 watts output. And it had to be touched up constantly, regardless of the xtal. But it made contacts, and was very much a 1930s romp on 40 meters, no doubt! But, back to the Sutter article. . .
The power supply, with the tapped bleeder to supply the screen. I might imagine these days you'd want to stack a couple OD3s or V150s for some regulated 300vdc on that screen. Notes on cw signals in 1936 were more tightly controlled than they were "Pre Cairo", in '27, but a far cry from what is required today. This was why these ops simply tuned to 'dip', and let 'er rip!
Heh, I have to point out that pilot lamp in the grid circuit. It served both as regulator and as a warning indicator to unkey! Ahh, the Thirties . . .
And this about wraps things up. The final page and finishing comments by Fred Sutter. He was a very candid writer, and some of the comments are as interesting as the tone they take! But then, all these old radio mags have an aire of authenticity, a very "face to face" nature to them. Very plain language was used: the publisher understood that the average Ham was the average "Joe", educated maybe through 6th grade, many completed high school, some collegians in the mix. Mostly tradesmen. Very easy to understand, and the humour is priceless.
I hope you liked our little fifty-cent tour of the Legacy of the 6L6 PO. Perhaps you might like to give one a try for NRR? Just remember: Plate and screen voltages bite, use common sense, and don't tune for power, tune for a good note. Keep yer dits and dahs short, and yer QSOs long. And above all, have fun!
Paper Wren Press.