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Old 30th December 2011, 04:53 PM   #1
wicked1 is offline wicked1  United States
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Default Amp ideas for my spare 3.3k output transformers.

I've got a pair of O-Netics 3.3k output transformers. Obviously designed for 300b.

I want to build a relatively simple/inexpensive amp for a friend of mine, and was wondering if I can use these transformers for something other than 300b's, or if I should stick to what they were designed for.

I'm also looking for schematic/amp suggestions, as I don't want to spend a lot of time thinking about this, so I think I'll go by a preexisting design.

The tubelab amps come to mind, but it's been a few years since I last worked with tubes, and maybe some other simple quick to build amp designs have come out since then.

I'd appreciate any ideas you've got to help me get started.

Thanks!

--edit, I just noticed my forum picture is of my old tubelab amp w/ these transformers! That amp has since morphed and changed and gotten different transformers, and isn't really the same tubelab circuit anymore. Still, it was a great amp, so that is one direction I could go... again...

Last edited by wicked1; 30th December 2011 at 04:56 PM.
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Old 30th December 2011, 06:25 PM   #2
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I think triode wired 6L6GCs will be OK, for about 6 W. O/P each. If you want to get the 8 or so W. a 300B yields, an AB2 setup is OK. G1 in 6L6s is rated for a a positive grid current regime.
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Old 30th December 2011, 06:55 PM   #3
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If your Iron was intended for 300B, then it is probably also designed for 90mA or 100mA idle current. If that's the case, I'd use a KT88 triode strapped. If it is gapped lower at say around 70mA then 6L6GC would work great.
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Old 30th December 2011, 09:04 PM   #4
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Under stressing "iron" is never bad. I recommended the 6L6GC given the fact that the 3.3 KOhm primary impedance mentioned compares well with the 6.6 KOhm end to end impedance frequently found in PP 6L6 amps.
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Old 30th December 2011, 09:57 PM   #5
kevinkr is offline kevinkr  United States
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Strikes me that the EL34 triode strapped would also be a good match.. The triode curves for the EL34 IIRC are pretty decent, significantly better probably than a triode connected 6L6.. Most classic 300B SE implementations run at roughly 60mA, and this would be OK with a triode strapped EL34 up to 350Vp (eff) or if you are brave or have really good EL34s a bit higher..
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Old 30th December 2011, 10:46 PM   #6
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Kevin,

Safe choices in current production EL34 family tubes are the EH fat bottle 6CA7 and the "reissue" Genelex KT77. Both are beam power tetrodes made in New Sensor's Saratov, Russia plant. I don't know if any good true pentodes are in current production. JJ's Octal production is, in general, very shaky and SED (=C=) seems to be "belly up".
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Old 30th December 2011, 11:10 PM   #7
kevinkr is offline kevinkr  United States
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Concur on the JJ thing, not sure exactly what is going on with distribution and production with Svetlana but I did find this: SED Tubes - Vacuum Tubes Manufactured in St Petersburg, Russia

Last time I knew New Sensor offered both pentode and beam tetrode versions of the 6CA7/EL34..
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Old 31st December 2011, 06:44 AM   #8
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Quote:
Originally Posted by Eli Duttman View Post
Kevin,

Safe choices in current production EL34 family tubes are the EH fat bottle 6CA7 and the "reissue" Genelex KT77. Both are beam power tetrodes made in New Sensor's Saratov, Russia plant. I don't know if any good true pentodes are in current production. JJ's Octal production is, in general, very shaky and SED (=C=) seems to be "belly up".
I have JJ KT77's and they are not shaky at all. Very fine valves.
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Old 31st December 2011, 02:20 PM   #9
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Quote:
Originally Posted by Eli Duttman View Post
Under stressing "iron" is never bad. I recommended the 6L6GC given the fact that the 3.3 KOhm primary impedance mentioned compares well with the 6.6 KOhm end to end impedance frequently found in PP 6L6 amps.
Some manufacturers rate the dc current for the air gap at the center of the magnetic flux for the transformer (Magnequest and Lundahl for ex) to stay in the most linear region of the magnetic curve.

So to get the Best sound, you'd want to use a tube that will idle at or close to the design center point. (presuming it is loaded correctly)

So, I'll re-state what I said eariler, find out what the transformes are gapped at (recommended current vs max) and go from there.

Last edited by rock4016; 31st December 2011 at 02:32 PM.
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Old 31st December 2011, 04:42 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by costis_n View Post
I have JJ KT77's and they are not shaky at all. Very fine valves.
You're very fortunate, the last JJ KT-88 tubes I purchased were not very good, and one failed catastrophically well within the ratings.. You could tell looking at them that the QC is pretty marginal, friends using their KT77 have reported similar issues. OTOH I have several pairs of JJ300B that are over 10yrs old and still work well, and have recommended these in recent times to others who report no issues with them. Jim McShane won't even carry them any longer because there were so many quality issues with the octal product range. I'm not sure why there is so much variability in quality, I've found the 6BQ5 and small signal stuff to be generally OK on a consistent basis.
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