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6L6 Without Screen Drive?

Not sure what data sheet you are reading, but the term "screen" means grid in some countries.

The original RCA data sheet from 1939 reads "When the push-pull connection is used with a 6600 ohm load, the 6L6 will give 34 watts of audio power without the need of grid driving power."

This means a pair of 1939 vintage metal 6L6's can put out 34 watts in AB1 (no grid current, therefore no "driving power"). The conditions specified for this power are 400 volts plate, 300 volts screen grid, 2% distortion.

Todays 6L6GC's can easily hit 50 watts or more with a bit more voltage.
 

jazbo8

diyAudio Moderator Emeritus
2011-01-05 8:34 am
In Transient
Last night I read in a 6L6 datasheet that it can output"up to 34 watts without screen drive." Is this true?

Please at least provide a link, and more importantly quote it correctly... This is from the RCA 6L6 datasheet:
"When the push-pull connection is used with a 6600-ohm load, the 6L6 will give 34 watts of audio power without the need of grid-driving power."

No where does it say screen drive.:rolleyes:
 
This is a 1947 application note directed at using 807's in an amateur radio transmitter. Note the modulator schematic. It makes about 125 watts in class B.

An 807 IS a 6L6GA in a package with the plate brought out through a cap on the top. This removes the voltage limitations due to the base. A 6L6 or any of the common audio tubes are limited by the fact that pin 2 is the heater and often grounded. Pin 3 is the plate which will normally see twice the B+ voltage, and often 3 X B+ or more in a guitar amp clipping into a speaker operated near its resonance. The tube will arc over inside the base, or at the socket.

Yes, you can extract a large amount of power from a 6L6 type tube in this manner, but it sounds like %$^%@#.......even in a guitar amp.

http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips0702.pdf

There is a thread where unusual drive methods used to extract big power, higher efficiency, or lower distortion from tubes in a similar fashion.

http://www.diyaudio.com/forums/tubes-valves/285100-those-magnificent-television-tubes.html
 
Last night I read in a 6L6 datasheet that it can output"up to 34 watts without screen drive." Is this true?

The spec sheet: 6L6, Frank's gives just ESSSSS-LOADS of possible operating points for both SE and PP audio amplification. The most I see there is 26.5W of output in Class AB1.

There are also Class AB2 operating conditions specced for 31W and 47W. Maybe you're thinking of the electronically identical 807? The 807 Application Report does spec an output of 38W in PP Class A1. However that requires VPK= 500V and V2K= 300V -- the 6L6,, since it's not a top cap tube, and the pinout places the highest voltage pin (the plate) right next to the lowest voltage pin (heater) it is rather voltage limited, and 500V is likely to lead to a flash-over.

You could also substitute the 6BG6 -- either a 6L6 with a plate cap, or an 807 with an Octal base.

Maybe that's what you had in mind?

Either way, all the 6L6-oids are probably gonna need some extra help from local NFB if sonic performance is what you're after.

As for screen drive, that's gonna be problematic, as the screen voltage runs fairly high at 300V. The more volts you need to swing, the harder it becomes to do so while maintaining good linearity. There are better candidates for screen drive than the 6L6-oids.

This is a 1947 application note directed at using 807's in an amateur radio transmitter. Note the modulator schematic. It makes about 125 watts in class B.

Yes, you can extract a large amount of power from a 6L6 type tube in this manner, but it sounds like %$^%@#.......even in a guitar amp.

That design is about as hideous as they come. No NFB, and 750V is a bit too much for oxide cathodes to last for very long. Probably didn't matter, as that same magazine advertises 807s for $2.00 a pop, so you could blow 'em up. Of course that thingie was intended for AM plate modulation on the ham bands where fidelity is neither necessary nor legal.So it didn't matter how horrific it sounded, and open loop, 807s sound nasty Those high order harmonics are like fingernails on a black board.

I can get 100+ watts from a pair of 36LW6's with much lower (and safer!) voltages, and probably better harmonic spectrums as well, even before connecting the gNFB to take the "edge" off.
 
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The original 6L6 (metal) data sheet from 1939 shows 34 watts. See the bottom of page 88.

http://www.mif.pg.gda.pl/homepages/frank/sheets/021/6/6L6.pdf

That design is about as hideous as they come.....Those high order harmonics are like fingernails on a black board.

As I said it sounded like %$^%@# No details about how long it will last in the article.

I can get 100+ watts from a pair of 36LW6's

I have seen 250 watts from a pair.....650 volts plate supply, 175 volts screen, drive on G1 only, 2500 ohm load. No glow, no explosions, just good clean power, and it doesn't sound like %$^%@#. Complete amp details will be outlined during the build up, after I finish building my lab. You do need the fat bottled 36LW6's to run this kind of power. The skinny ones without heat radiating fins that AES sold several years ago don't cut it at 250 watts. They are OK at 150.
 
The original 6L6 (metal) data sheet from 1939 shows 34 watts. See the bottom of page 88.

http://www.mif.pg.gda.pl/homepages/frank/sheets/021/6/6L6.pdf

OK, so that's what the OP was referring to. Those must've gotten pretty hot in operation. Surprised there's no mention of cooling. Even though the metal 6J5s used in the Vixen design run with a whole 56mW of PD, they get surprisingly warm.

"As I said it sounded like %$^%@# No details about how long it will last in the article".

After doing the Vixen design, I happened to come across this Portuguese (Brazilian?) article (attached) describing this plate modulator. That's where I got the idea for the Le Renard project, and then it became clear why the Portuguese (Brazilian?) hams went with 6BQ6s where you'd normally expect to see 6L6s. Open loop, they sound better. None of the irritating harshness of the 6L6-oids, just an overly "aggressive" or "edgy" sound. All they need is just enough gNFB to take the "edge" off. Included variable gNFB so's you could dial in more edge for Techno and Power Metal, more softness for Andre Rieu or Karen Carpenter.

I have seen 250 watts from a pair.....650 volts plate supply, 175 volts screen, drive on G1 only, 2500 ohm load. No glow, no explosions, just good clean power, and it doesn't sound like %$^%@#. Complete amp details will be outlined during the build up, after I finish building my lab. You do need the fat bottled 36LW6's to run this kind of power. The skinny ones without heat radiating fins that AES sold several years ago don't cut it at 250 watts. They are OK at 150.

I have a bunch of these 36LW6s, but prefer to run 'em a bit more conservatively. They'll need custom OPTs ($$$$). I can do my own PTXs, so the 36V heater power won't be a problem. 115W is more than enough, considering that the Vixen's 30WPC gets the neighbors complaining if I crank it.
 

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mr2racer

Member
2009-07-09 12:53 am
Here's the link; http://www.mif.pg.gda.pl/homepages/frank/sheets/021/6/6L6.pdf

Yes, I misquoted it. It mentions "...without the need of grid-driving power." Am I right in assuming that is G2 they are talking about? Also, this is in the 6L6 datasheet not in 6L6GC. I was curious and assumed it was because 6L6 was such an early pentode. What I wanted to know is if this would be some sort of triode connection and would it sound any different?
 
The datasheet you referenced is the same one I found. It is for the early metal envelope 6L6 tube. The 34 watt characteristics are for pentode mode class AB1.

Am I right in assuming that is G2 they are talking about
?

No, G1 will start to draw current, and consume power if driven positive. According to the same data sheet, page 89, you need 0.35 watt to drive the grids. This is no big deal today....we have mosfets and tubes good enough to make a decent cathode follower. In 1939 you needed a good driver transformer, and a small power amplifier to drive it. Including transformer losses, about 1 watt was needed.

G2 will always consume some power in any NORMAL pentode operation. Yes, I know it is possible for some tubes, especially RF tetrodes to actually put power OUT the screen grid due to it's negative resistance region, but this should not happen in a normal audio amp.

would it sound any different?

It will sound pretty much the same as a modern 6L6GC running the same power in AB1. The basic construction of the 6L6 hasn't changed. It got glass with the 6L6G, straight sided glass with the 6L6GA, better materials with the 6L6GB, and got a bigger plate with the 6L6GC, but inside the plate, it's the same tube.

The original 6L6 had lower ratings than the modern 6L6GC and 34 watts was about all you were supposed to squeeze out of them in AB1. You could get up to 60 watts in AB2 which does require some driving power to push the grid positive.

The 6L6 types have a reputation in some circles for being harsh sounding. Other people like them. I find them OK in a guitar amp, but they have never been my favorite tube. They can be tamed by local negative feedback, and the RCA engineer named Schade wrote a dissertation on this when the tube was introduced. A modern 6L6GC can make about 30 watts in push pull triode connection, and does sound a bit different than the same tube in pentode mode. I have always found that a 6550 or KT88 betters the 6L6GC in most every way, so that's what I tend to use.

that's what the OP was referring to. Those must've gotten pretty hot in operation.

The old metal tubes did a fairly good job of transferring the internal heat out of the tube. That's why they get so hot on the outside.

I went to a technical high school in 1967 - 1970. We learned electronics on vacuum tubes because that is what we had. Much of our lab equipment and parts were donated by the local Air Force base, including several hundred brand new RCA metal 6L6 tubes. I personally blew up maybe 25 of these, and extracted somewhere around 200 watts out of 4 of them. Did they get HOT, well the paint peeled off! I don't know how long they lasted. They were still kicking when I graduated leaving the mother of all guitar amps.....and a rather large Tesla coil behind.

We cut several metal 6L6's open after I fried them, and they had less internal damage inside them that some similar glass tubes I blew up.
 

mr2racer

Member
2009-07-09 12:53 am
Thanks tubelab! I'm building an HK A500 but I didn't want to use the 7355's. And I didn't have any. I planned to use EL34 but the HK filament winding can't cope with the EL34 1.5 amp rating. The 6L6 at .9 amp is much closer to the original 7355's .8 amp. Jim McShane said 6L6 would work better than EL34. I have a quad of winged C 6l6 so I'll use those.

The output transformers, according to Jim, are good for about 30 to 32 watts. So triode operation might be perfect. Tubelab, could I just connect grid 2 (pin4) to the plate (pin 3) directly? What would be the PP to the output transformer? Would it sound better than 6L6 in pentode?
 
The output transformers, according to Jim, are good for about 30 to 32 watts. So triode operation might be perfect. Tubelab, could I just connect grid 2 (pin4) to the plate (pin 3) directly? What would be the PP to the output transformer? Would it sound better than 6L6 in pentode?

Frank's has a spec sheet that gives the pseudotriode plate characteristics: 6L6 It doesn't give any operating points, though, so you'll have to do your own loadlines to determine what's a good plate load. As for how that'll sound, that's something I can't say since my design was pentode mode. Once you get the NFB connected, they sound great. I'd guess pesudotriode operation would sound better than open loop since pseudotriode basically means max lNFB. You still might need some gNFB to take the 'edge" off. That comes down to try and lhear.

Definitely include screen stoppers since the 6L6-oids like to make snivets.