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Old 15th November 2007, 09:39 PM   #1
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Default 6L6G Amp recommendations

My first post and first desire to build a Tube Amp. I am trying to figure out what to do with 4 6L6G tubes. This would be my first tube amp build but not my first foray into building things or wiring electrics. I own an Old Land Rover and rewired it...no major burns or ticks and I came out ok ;-)

With the basics first. My room is 10 x 12 feet with 8' ceilings. The room has book shelves and a couch. I will be building full range speakers, probably frugal horns. Music I like to listen ranges from classical (classically trained clarinetist), jazz, blues, swing, rock (all types) and folk. I spent 12 years in service flying/fixing helicopters so some of my hearing is gone. Everything I actually know about soldering/electrical components came from my Grandmother...she was a Rosie the Riveter type gal in WWII who did electrical work at Bendix and never left. Not afraid to dive into the deep end.

I do not care if it is PP or SE, mono or stereo (may like mono because it is more stuff ;-) )just want a good solid amp that works with these tubes.

Does anyone out there have a suggestion?

PS I am reading the sticky for Noobs but ALOT of information in there that will take time to chunk through.

Thanks for the help!
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Old 20th November 2007, 07:31 PM   #2
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Default Would a Tubelab PCB fit the bill?

Bump?

Anyone?

This is for an audio amp.
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Old 20th November 2007, 08:10 PM   #3
7N7 is offline 7N7  United Kingdom
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Yes, If you haven't done this before, I think a Tubelab Simple SE might
be a good idea

If you have horns then power should not be an issue in a room 10' x 12'


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Old 21st November 2007, 01:02 AM   #4
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6L6G's can be used in a SimpleSE amplifier, and I have tried 6L6G's in one of my amps. There are however some considerations.

The 6L6G has a published screen grid rating of 270 volts. Using this tube in a triode or ultralinear amplifier would require a B+ voltage of about 300 volts to stay strictly within the ratings. This would limit the power output to about 1.5 watts in triode and 2 to 3 watts in ultralinear mode.

These ratings were published under the old "design center" rating system which was far more conservative than the current "design maximum" rating system. This means that the 270 volt rating may be "bent" somewhat without serious consequences. How far can you go? Most vintage literature says that the "design center" ratings were 10 to 20% more conservative than the "design maximum" ratings. This would allow 300 to 325 volts on the screen grid which would mean a B+ voltage of 325 to 360 volts in a cathode biased amplifier. How hard do you want to push your vintage tubes, and are you sure that they are good? I have tested 6L6G's at B+ voltages up to 390 volts without issue, but I have no idea what the long term effects are.

I built a SimpleSE to use with 6V6's. It runs a B+ voltage of 325 volts which puts about 300 volts across the tube. I have used 6L6G's and some old metal 6L6's in this amp with no issues, but I prefer the sound of the 6V6. In fact I currently have some vintage 6V6G's in there now. Power output with the 6L6G tubes was just over 2 watts in triode mode.

You could build a SimpleSE for use with 6L6G's but these constraints will limit the power output to about 2 watts, maybe 3 if you push the tubes harder. If this is enough for your situation, the 6L6G's could be used.

My small speakers are 86db inefficient, so I only use the 6V6 amp infrequently since I also have a 45 amp. If you feel that you may need more power the SimpleSE can be operated at much higher voltage, but the 6L6G tubes can not. Newer tubes can handle more voltage, thus offering higher power output, and the cost is still a smal fraction of the total amplifier cost.

On the other hand a KT88 operating near the maximum will make about 5 watts in triode. This is only 3 db more power, and will be olny slightly louder than 2.5 watts.
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Old 21st November 2007, 01:35 AM   #5
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To me, everything looks like a 6BQ5. Mostly because that's the only kind of (working) tube amplifier I've ever owned. From the tube specs, it seems like the 6L6G is similar to the 6BQ5 but with slightly lower plate and screen voltage limits, slightly higher dissipation ratings, and a lower Zout. If it were me, I'd definitely go push-pull. You get a lot more power from a PP set of tubes, and the numbers seem to suggest the linearity is a lot better. Seems like a decent match for an output transformer like a Hammond 1620, which offers a plate load of 6.6k.

You'll spend a lot more (10x) on the power and output transformers than the value of the tubes you are currently holding in your hand. Don't try to build an amp around the 6L6 just because that's what you happen to have lying around. What am I saying? Isn't that what we all do? Besides, the 6L6 is a pretty good tube, from what I understand. Ok, disregard this thought.

I'd scour the forums and the internet for a proven good schematic using 6L6G tubes. It's guaranteed someone else has already done it. Find it, post it, and ask if someone else agrees it should work. Once you've got some buy in, follow the schematic to the letter. You should end up with a working amp.

Good luck, and keep us up to date with your project!


edit...
Oh, I remember where I was going with the 6BQ5 thought. Read here for details on a amp designed for the 6BQ5, refitted with 6L6GC. Oh, and I guess the 1620 is way too small for the 6L6GC.
http://www.diytube.com/phpBB2/viewtopic.php?t=246
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Old 21st November 2007, 04:52 AM   #6
Yvesm is offline Yvesm  France
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One SE design with CFB :
http://www.dissident-audio.com/SE_6L6/Page.html

and a PP one:
http://www.dissident-audio.com/PP_6L6/Page.html

both for 6L6G.

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Old 21st November 2007, 07:21 AM   #7
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Been in there as well as I had broken a tube (50CA10) for another project and started reconsider switching to 6L6.

Here are some schematic I have been looking into:
ECC83+12BH7A+6L6 Fixed bias - wahoo!
PSU to the above
ECC81+???+6L6
12AX7+12AU7+6L6
12AU7+12AU7+6L6
12AX7+12AU7+6L6Nice and tidy.
12AU7+12AT7+KT66/6L6
12AT7+LM317+6L6
ECF80+ECF80+6L6Looks fun but why so lo P-out??

I came acrass some bad schematics for a Mudlark (2X6L6 PF) - do anyone have a component list?
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Old 21st November 2007, 07:31 AM   #8
Yvesm is offline Yvesm  France
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Quote:
Originally posted by soundbrigade
. . .
ECF80+ECF80+6L6Looks fun but why so lo P-out??

What do you mean by lo P-out ?
Near 30W when driving 6 Ohms load

Click the image to open in full size.

Yves.
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Old 21st November 2007, 07:38 AM   #9
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I was reading the short data on the schematic - Pout 16W....

I really like the schematic - looks like nothing else and I have a bunch of Russian ECF80 to spend, though I hate novals.

Forgot to say - THANKS for your in-depth details.
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Old 21st November 2007, 08:36 AM   #10
7N7 is offline 7N7  United Kingdom
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Quote:
Originally posted by Ty_Bower
To me, everything looks like a 6BQ5. Mostly because that's the only kind of (working) tube amplifier I've ever owned. From the tube specs, it seems like the 6L6G is similar to the 6BQ5 but with slightly lower plate and screen voltage limits, slightly higher dissipation ratings, and a lower Zout. If it were me, I'd definitely go push-pull. You get a lot more power from a PP set of tubes, and the numbers seem to suggest the linearity is a lot better. Seems like a decent match for an output transformer like a Hammond 1620, which offers a plate load of 6.6k.

Traditionally 8k is the standard for a pair of 6BQ5 (EL84) in pentode mode. For triode this would be lower of course - 6k perhaps.

One point about primary impedances, citing Valve Amplifiers (Jones) referring to the Quad II, it appears that Peter Walker selected a low primary impedance for the pair of KT66s (similar to 6L6) he used as beam tetrodes, just 3k a-a. This reduces third harmonic whilst increasing 2nd harmonic which is cancelled by the push-pull operation (given matching valves of course). Of course the Quads run their KT66s at just 340V (330 on g2) and are specified to make 15W.

7N7
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