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#10
December 7th 08, 05:49 PM
posted to rec.radio.amateur.homebrew
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6L6 substitute
philo wrote:
"ken scharf" wrote in message ... philo wrote: "Stev eH" StevehkhhSDJvhvbjjxbvvbhnvbhzjdnxzvzhzdshbvnjzvnb vnvjbvbcjbvvvvnmxvzjhjzsdg fgsfghgjsghgsljhglhdjfghfufgfhzysgfhczgugfvzlvufzv llgfzlyfyvlgbylvdfghvblyry wrote in message ... JIMMIE wrote: After years of avoiding morse code I am finally getting into it. the ideal of operating a very simple transmitter appeals to my junkbox/ trashcan construction mentality. I am looking for a substitute for a 6L6 that has the plate brought out the top. I was trying to build a little Glowbug transmitter but was having all kinds of problems neutralizing the the thing. I think I would have better luck if I can separate the grid and plate circuits from each other. I am considering an 807 as a replacement Suggestions would be appreciated. Jimmie An 807 IS a 6L6 with a top cap anode connection. Any of the TV line output valves will also do the job if you want cheap, the 6DQ6B works well and has an octal base (The sockets for the 807 are rare). 6146 or 2e26 etc work well but will cost you more. Steve H No way! Back in the old days I've used both 6L6 and 807's and they are not the same tube at all. You could probably use a 6L6 to drive an 807 though IIRC, the usual driver for an 807 was often a 6AG7 but I'd sure a 6L6 would be fine Just for grins I got out the RCA tube handbook and compared the plate curves for the 807 and the 6L6. Up to the limits for the 6L6, they are IDENTICAL. The 807 curves go to higher plate voltage and current. The 807 and the 6L6 have the SAME cathode, grid and plate structures. The differences are in the shielding added to the 807 to reduce grid-plate and grid-cathode capacitance, and in the plate support spacers which are ceramic in the 807 to increase the operating voltage rating of the tube (which is also helped by the plate top connection). You can see the added shielding just above and below the plate supports. The 807 is a MUCH better performer at RF than the 6L6, it usually does NOT require neutralization and would have more power gain than the 6L6. At Audio frequencies the two tubes will perform IDENTICALLY within their maximum ratings. Wow...thanks for the info. They are considerably closer in specs than I realized. Of course, you could run an 807 quite a bit past their official ratings and never burn one out. I recall tuning my transmitter for absolute maximum output...and that occurred right around 50watts input. (probably on 40 meters. I'm sure that on ten meters it would have been a lot less) Ran my 807 rig for years like that. Don't really know what my 6L6 transmitter would have done... just ran it on 40 meters at about 25 watts input...we always just considered them to be audio tubes. If I were going to build a tube transmitter today, I'd probably use a 6146B. Well the 6L6 has changed over the years, while the 807 never did (except for the 'W' version). Later 6L6 versions might have changed the outward appearance of the plate in response to cost cutting and automated assembly lines, but the internal spacing of the elements remained the same. If you look on the QST archives (are you an ARRL member?) on line for those old 'QSL 40' series of articles on home brew transmitters using the 6L6G tube you can see in the pictures how the plate structure is the same for the original 6L6G tube as the 807. And if the claims for this tube in those home brew articles is true, the 6L6 was JUST as rugged as the 807 in being pushed past it's ratings. Only the 6L6 would probably arc over at the socket before the plate voltage reached what the 807 would handle (I've heard of 807's taking over 1000 volts on the plate just fine!). The official ICAS ratings for the 807 allow up to 750 volts at 100ma plate current, and there is a typical operation shown at 75 watts input, so at 50w your transmitter wasn't even breaking a sweat yet! A pair of 807's in AB2 (SSB linear) will put out 120w PEP, just about the same as a pair of 6146's. The 6146B officially will do about 135-140w in the same service. Note that ICAS ratings for the 6L6 were never published, but you can assume that the 6L6 is identical to the 807 EXCEPT for the plate voltage rating, which I would guess is good to 500v or so. |
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