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Old 15th February 2007, 08:48 PM   #1
Fuling is offline Fuling  Sweden
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Default OTL questions

Ok, hereīs the story:
A couple of days ago I finished my first ever OTL amp, an SE piece with 4 x PL504 per channel in a choke loaded CF configuration with a very simple input stage and a bit of NFB.
It burns 200W to get 2x1W on the outputs, but the sound is actually quite marvellous. My speakers (FE 167E in 24L BR) doesnīt need much power or damping factor, theyīre perfectly happy with a couple of watts and <10 ohms output impedance.

The SE beast has inspired me to build another OTL amp, something a little less controversial with slightly more output power. PP class AB, that is.
Iīm thinking something like 4-6 6C19 per channel in a Totem Pole topology with +-150V rails and virtual ground.

1: What kind of phase splitter would do the job best with lowest parts count?
It stands between an inverted Futterman:

http://www.tubeaholic.com/projects/page.view?RowId=51

or this LTP-variation:

http://www.tubecad.com/january2000/page10.html

6C19 seems to require quite a bit of voltage swing, so Iīm leaning towards the LTP for this reason.

2: Power supply... It takes quite a few different voltages to get an OTL working, but what concerns me is the +-150V high current supply for the output stage. I have a bunch of 1000uF 200V `lytics, four 112mH 2A chokes and a few suitable power transformers.
The problem is that I donīt have two identical PTīs, can I feed both channels from the same transformer?
My gut feeling tells me that this would cause problems since the PSU is floating with a virtual ground.
Buying two new power transformers is of course an option, but they donīt come cheap.

BTW: Before anyone starts arguing about the number of output tubes, remember what I wrote about my power and DF requirements.
Iīll probably build a high power, high DF monster OTL someday but not this time.

Cheers!
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Old 15th February 2007, 10:28 PM   #2
coresta is offline coresta  France
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Hi ! I prefer the Kamijo topology very regular in testing , more than the futterman . (a 12BY7 is CCS of a pair twin triodes as splitter ) . The floating ground is a good solution that protects the speaker . Try the 6AS7g as they are very affordable and reliable from Russia (real mil spec stocks ) . I'll provide in a few weeks a full report on my new OTLs
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Old 16th February 2007, 02:03 AM   #3
ulibub is offline ulibub  France
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Hi!

Already considered building a Circlotron design? I've made only best experience acoustically with my amp. But it turned out to be quite an immense project. I've presented it already during the thread "6C33 design comments" several weeks ago:

Uli's Circlotron

Although I use a low-ratio OPT (4:1, my first goal was output power), I think that with lower plate voltage and 4 (instead 2) power tubes per channel impedance could be lowered down to get "real" OTL service.

Good success!

Uli
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Old 18th February 2007, 07:52 PM   #4
Fuling is offline Fuling  Sweden
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Thanks for the replies.

After many hours of reading and calculating I begin to se the outlines of something:

PC86 input stage with CCS plate load cap coupled to a 2xPC86 LTP phase splitter. The balancing and hum cancellelation signals are fed to IRF820 followers on top of the phase splitters plate resistors.
B+ around 450V, 22k plate resistors, 10mA plate current through each PC86.
The "tail" goes to an SS CCS connected to a -60V rail.
Feedback goes from the amps output directly to the phase splitterīs inverting input, no global NFB to the input stage.

Output stage: 3+3 x 6S19 (russian low Rp triodes), +-160V and virtual ground.

Right now Iīm waiting for PSU caps from the US, tube sockets from Ukraine and power transformers from Belgium.
Canīt do anything until I get at least the tube sockets...
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Old 18th February 2007, 07:54 PM   #5
Fuling is offline Fuling  Sweden
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BTW: The SE OTL is almost finished, and it sounds really good!
Attached Images
File Type: jpg seotlklar.jpg (90.6 KB, 864 views)
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Old 18th February 2007, 09:49 PM   #6
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Very nice cabinet too, if I may add. Do you have anything inside, like a speaker, etc., or just using it as a base?
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Old 18th February 2007, 10:25 PM   #7
Fuling is offline Fuling  Sweden
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Thank you!

I could have made the enclosure 2-3" lower, but I wanted to use the full width of those oak boards.
The cathode chokes and PSU takes up a lot of space.

Ooops, I just noticed something: the SE OTL belongs i another thread
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Old 18th February 2007, 10:35 PM   #8
Jeb-D. is offline Jeb-D.  United States
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Wow! Nice job, looks great.
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Old 9th March 2007, 07:29 PM   #9
Fuling is offline Fuling  Sweden
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Iīve been quite lucky sourcing parts for my new OTL.
The PSU caps costed 5 bucks at Ebay and Iīve found toroid transformers with all the voltages I need except output stage B+ at local surplus stores.

This picture also show the reason I donīt want to use tubes with top caps, that cat costed more than the budget of this amp...
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Old 9th March 2007, 07:32 PM   #10
Fuling is offline Fuling  Sweden
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Left channel chassis, the right channel is mirrored:
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