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Old 18th December 2003, 12:48 AM   #1
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Question OTL design recommendation? (6C33C maybe?)

Greetings, all. I was hoping I could solicit recommendations for OTL designs that folks here have built and enjoyed. I'm looking for something capable of at least 25-50 W/channel, and I appreciate simplicity, so I've been leaning toward something using the 6C33C. Candidates so far include Andrea Ciuffoli's version 8.2 (regular, OCL, or cheap & easy), COVI Mark II Circlotron, and Hans Beijner's 6C33C OTL amplifier (I'd have to contact him and ask for a schematic with component values).

Would anyone like to recommend an OTL design that they've enjoyed building and listening to? Thanks in advance,

Aaron T.
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Old 18th December 2003, 05:55 AM   #2
Colt45 is offline Colt45  Serbia
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6S33S, 6AS7/6080/6N13S/6N5S, 6S41S, 6S19P, 6336

all of them should work ok.

6S33S is the big one
6336 is like a double dissipation 6080/6AS7
6S41S is pretty much a both sections of a 6AS7 paralleled. (same septar base as 6S33S)
6S19P is basically 1 6AS7 section, in a 9pin mini bottle.
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Old 19th December 2003, 12:17 AM   #3
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Hi,

Andrea's circuit should be O.K. if you add more individual bias possibillity...As it's shown, biasing one (set of) tube(s) is just taking from the other.

Hans seems to be happy with his OTL. If he wants to help you with the details then I'd say go for it.

The 6336A's arent exactly cheap and are often used ones...

Beware all of them require good current drive to overcome Miller capacitance.

Cheers,
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Old 19th December 2003, 05:55 AM   #4
arg_ is offline arg_  Greece
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Quote:
Would anyone like to recommend an OTL design that they've enjoyed building and listening to?
I have build the Ciuffoli OTL v8.2 OCL (output capacitorless) and I am very happy with it. The schematic is ok and it will work right away as it is; however I can suggest a few things for improvement, one of them is Frank's hint.
It outputs 24W/8Ohm but you can use two 6c33's per rail in case you really need more power.

Hans' OTL should be a very good alternative too

Cheers
Argiris
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Old 21st December 2003, 06:46 PM   #5
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Here's another link to an OTL from Alan Kimmel, although it's a little late

Sweep Tube Monoblock
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Old 23rd December 2003, 12:55 AM   #6
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If I'd built my OTLs a year or two later, I would probably have used the 6C33C's which were not readily available when I was constructing mine, but at the time, 6AS7G's were available new for $1-2 so I basically bought a 'lifetime' supply of them for my OTLs.

I actually bought 35 6LF6's to build a couple monster OTL monoblocks (4-500) watts for which the triode OTL I built (picture in photo section) was supposed to be a warmup, but I liked what the triodes did so much sonically, even though I was maybe only getting a quarter to a third of the power per output tube that is achievable with big horizontal sweep tubes that I eventually sold the 6LF6's instead to Richardson Electronics at a good price except for one that had lost vacuum.

The Kimmel amp looks like it really gets the most out of its output tubes without beating them. My topology uses a totem pole output and if I had gone the tetrode route, I was planning to either allocate a pair to drive the screens of the the rest to get the 'enhanced' mode similar to what Kimmel did with sand, or maybe used two halves of a 6AS7G or 6336 possibly driven off a 'common grid' gain stage. My topology only passes the signal through three current or voltage amplification stages (counting the input differential cross coupled cascode circuit as one), is fully balanced, and is dc coupled, unlike Kimmel's.

As far as damping factor, I added positive current feedback, which is cool because it actually reduces negative feedback when it kicks in, & with a relatively simple adjustment, I am able to get a damping factor of several hundred from 20hz to above 10 khz. However, I haven't done this for years thanks to my inherent laziness, so I think I recently measured 8 ohm damping factor as about 120 in one channel and approximately infinity or a slight negative source impedance at the test frequency in the other
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Old 23rd December 2003, 01:11 AM   #7
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Hi,

Very interesting, Thoriated.

There is at least one more OTL thread I know you may want to go through here.
Just do a local search on OTL and it should surface.
I think I was the threadstarter of one of them just you know I'm the guilty one.

Good to have you here,
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Old 23rd December 2003, 02:47 AM   #8
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Thanks.

DIYAudio is a very interesting place. I just went through the thread you mentioned. Btw, since I read your post I fired up my upstairs stereo with the OTL again. The OTL is fine; my preamp which I brough back from the brink a few weeks ago apparently still has some intermittent internal continuity problems & I nearly lost a LS3/5A woofer when one channel of the phono preamp went into sustained subsonic relaxation oscillation that may have had a mechanical coupling aspect. Time to open up the preamp again and find the continuity gremlins which weren't obvious when I had it on the bench the last time. Aargh!

Btw, with my OTL, supply rail fuses for each channel have been adequate for protection when the fault has been within the OTL itself. In the few instances when a 6AS7 developed a short (which usually seems to clear itself, interestingly enough- maybe a loose flake of cathode coating?) or once when an inebriated friend fell into the OTL (ouch!), one or both supply rail fuses would open before any appreciable DC hit the speaker. This of course is not the case if the DC or VLF high amplitude AC is applied to the input of the OTL and I've fried up a couple of woofers this way.
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Old 23rd December 2003, 03:09 AM   #9
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Hi,

Are you using 6AS7Gs exclusively and if so are these US or USSR made?
The reason I ask is that they're as different as the US 6080 to the US 6AS7Gs.

The problem with the fuses popping at switch-on is not unknown to me but I feel this has more to do with bad quality fuses not taking the inrush current at start-up.
These lousy fuses can take out output tubes in the process just the same.

More later, I really need some sleep.

Care,
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Old 23rd December 2003, 03:47 AM   #10
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Hi, Frank -

It looks like I have both types of 6AS7G in the amp - maybe a few more Russians than American tubes. Come to think of it, the momentary breakdowns occured with only the American tubes - I can't recall it happening with the Russian tubes. But even if a tube did do this once, it would work properly after the supply rail fuse(s) were replaced for that channel and I wouldn't have trouble with that tube again.

I agree that they're different - they sound different to me -& seem to have different transconductances and mus. To me the American tubes have a more punchy sound & produce a little more power in the amp. The Russian 6AS7G's sound a little cooler and ultimately provide a little less current drive before clipping.

I've had little luck with 6080's in this amp for some reason - maybe because I'm running the output stage supplies between plus and minus 160 to 170 which may be a little bit on the high side for some varieties of the 6AS7G/6080 family. I even tried a spare 6336 or two but the main toroid transformer is not robust enough to handle much more filament and plate current than just the 6AS7G's draw.

Btw, which speakers do you like to use?
I'm using my backup Chartwell LS3/5A's for the moment, but I plan to build a 2-way high efficiency with a highish impedance that this OTL should love to drive since it'll range between 24 ohms to 100 ohms through most of the audio range.

Talk soon -
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