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Old 7th February 2006, 05:33 PM   #1
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Question Metal 6L6s?

Hi,

I was recently given a quartet of matched RCA 6L6 metal tubes. They've hardly been used at all, only powered up a couple of times, so I thought something fun could be done with them. One of the tubes unfortunately has a dead filament, so no stereo PP, but a stereo SE (perhaps UL?) should be possible.

I was thinking of building something from scratch, using Boozhound's article as a guide, but if anyone has a complete schematic it would be nice to use as a guideline. The cheap Edcor SE OTs should be a good match, I think.

Now, before I get started and making a complete fool out of myself, are these tubes worth going with? Any suggested schematics?

Thanks,
Rolf
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Old 7th February 2006, 08:11 PM   #2
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There's one here: http://tinpan.fortunecity.com/saints...mer/6l6-se.gif

Voltage should be lower for 6L6G, GA, GB, or metal - 300V should be fine. I would use 1/2 6SL7 or 6SJ7 as driver, as I have a bunch...

Oh yeah - heat pins 2 and 7 on that "open" one - it might be open OUTSIDE the tube...
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Old 7th February 2006, 08:31 PM   #3
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Hi there

I have six NOS of those black in metal 6L6...
Brand new (of course I have put them to service for a couple of hours to check if everything was OK...)
Very nice sound and if used in a good project, within their limits, I guess we could solder them directly into the circuit....
Very very rugged (someone told me they did just that: solder them to circuits in military vehicles, to resist "the small bumps" in battle fields...)
BTW: how much money would them cost actually?

[ ]
Ricardo
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Old 7th February 2006, 09:37 PM   #4
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Beware the metal case of these tubes is connected to pin 1. Some old amps use the extra pins on the tube socket for tie points. The old Stromberg Carlson amp that I used for a stereo in the late 60's used pin 1 for the common connection for all of the screen resistors. I learned this the hard way at exactly the same moment that I discovered that that cool looking Steppenwolf the Second album (vinyl LP) cover was conductive! Make sure that nothing (except ground) is connected to pin 1.

Metal 6L6's can often be found cheap, and can be used in many 6L6 circuits. Just keep the power dissipation and voltages within the ratings. That being said I have abused these tubes (the old Stromberg was one example) to the point of the paint peeling off without failure. Don't plug them into a 50 watt guitar amp though, you can't see the red glow of death, and you won't know that failure is emminent until it is too late. Within these limits they sound a lot like modern 6L6GC's.
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Old 8th February 2006, 12:59 AM   #5
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These tubes were installed in a 100W bassman at a point (not very long, mind you), but we quickly discovered that it wasn't the optimal operating point. Could some of the tubes have been ruined by this? I'm not able to check the tubes as I haven't got a tester, but I hope they work OK.

BTW, Tom Bavis: Your link doesn't work

- Rolf
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Old 8th February 2006, 01:33 AM   #6
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If the bassman survived, and the owner didn't complain of loud popping or frying sounds from the speaker (tube arc), the tubes are probably OK. If the amps bias was set really hot the tubes would have been subjected to excess dissipation. Examine the paint for signs of peeling or blistering. Check the lettering. Continued overheating can cause outgassing from the internal metal elements which can raise the tubes distortion and cause a runaway condition due to grid current.

I used one of the previously mentioned Stromberg Carlson PA amps for a guitar amp for about a year with 4 metal 6L6's. The tubes were still alive (with peeling paint) after a year of abuse when the power transformer finally gave up. The amp was designed for 6L6GB's and put out about 100 watts with the metal ones. They were GE's from the early 60's. Homestead AFB donated about 100 of them, and about 10 of the Stromberg amps to the high school that I attended. I got to take 3 home in exchange for teaching the (vocational electronics) class how to rebuild and test them. That was one of my last tube projects before a 25 year detour down the sandy path.
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Old 8th February 2006, 02:04 AM   #7
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Try this - http://tinpan.fortunecity.com/saints/668/primer/#

"6L6GC ultralinear amp"
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Old 8th February 2006, 02:13 AM   #8
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Wow, 100W from 4 6l6s? That's a *little* bit more than the specs....

Visually the tubes look perfect, and there were no scary sounds from the Bassman, so I guess they have survived. I was thinking I could check them in my friend's 6V6 guitar amp (just to check if they made any sound at all, not for long periods of time). That shouldn't be too hard on them.

- Rolf
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Old 8th February 2006, 02:20 AM   #9
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Tom Bavis:
Hey, that looks perfect as I've already got a couple of Mullard EF86s around. The OT primary seems quite a bit lower than the datasheets indicate, though. Would a 5K primary be OK for pentode mode? How about UL?
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Old 8th February 2006, 02:38 AM   #10
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100 watts from 4 6L6's is not terribly hard. You can test the tubes in a 6V6 guitar amp. Do NOT leave the amp on for long. The 6L6 draws a lot more filament current than a 6V6. Way back (again) in the 60's I begged my parents for ever to buy me a Fender Champ. I finally got a used one. I liked the sound it made when I put a 6L6GC in it and fed it with a home made fuzz box. Unfortunately the power transformer didn't like it and eventually fried. I think it lasted about a year of daily use. (sound familiar?)
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