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Old 8th May 2008, 02:37 PM   #1
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Default 6AS7 ppp

I've been looking at the tubes in my nice old scope and the 6080 (6AS7) tubes caught my attention. Big twin triodes! I've seen these used in OTL power amps and headphone amps.

I've also been contemplating a PPP 6aq5 design (not S5 please! Yuk.)

Anyone see any PPP schematics out there for 6AS7's? OTL power amps?
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Old 8th May 2008, 03:40 PM   #2
Horexx is offline Horexx  Germany
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A heater :
http://www.jogis-roehrenbude.de/Lese...schreibung.htm

Horexx
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Old 8th May 2008, 03:50 PM   #3
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Now there's a good winter amp!
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Old 9th May 2008, 06:53 PM   #4
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I've seen a couple more designs and there seems to be a couple common themes. These guys aren't real too stable and you'll need 32-64 ohm speakers for the best results.

An octet of 8ohm little Tang-Band speakers for each channels should have interesting results.

It is true that the more 6AS7's the lower the impedance gets?
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Old 9th May 2008, 08:54 PM   #5
Horexx is offline Horexx  Germany
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Quote:
t is true that the more 6AS7's the lower the impedance gets?
Yes, Robert Kunz takes four in parallel, eight per channel. He recommends minimum 8 Ohms, better 16 Ohms. His amp is astonishing stable.
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Old 10th May 2008, 12:58 AM   #6
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I’ve been playing around with the curves and simulations (generic triode model from Duncan Amps for LTspice) with the 6AS7 for the last ~20 hours to develop an OTL amp with 8 of these per channel that has evolved from a SE class A to PP.

Other than in the output stage of an OTL amplifier or as a series pass element is a PSU where low internal resistance is a requirement, I wouldn’t use these tubes. Their linearity is not particularly good and neither is their mu.

Coupling transformers avaliable with a low enough primary impedance for a bunch of these in PPP may not have highest primary inductance and best bass response either.

Cheers,
Glen

EDIT:
Doh! I just read your post again and I see that you actually are talking about OTL amps.
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Old 10th May 2008, 02:13 PM   #7
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After many hours of playing around I've come to the conclusion that the circolotron topology is the best around for building an OTL amp.

I've simulated Robert Kunz's output stage. THD at 60W into 8 ohms is ~6% (1kHz), almost all 3rd harmonic.

Cheers,
Glen
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Old 10th May 2008, 08:18 PM   #8
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My gut feeling has always been that the 6080 tube can give most of its performance with output transformers. With the low ratio the transformer can avoid most of the issues generally encountered with transformer coupling and achieve very large band widths. Because of the low output impedance it will have plenty of bite on low inductance transformers. I'm certain that in your researches you will have come across a number of designs successfully using the 6080 with mains toroidals as outputs. This works surprisingly well and the amp that results is very sweet. PP seems the best way to go to avoid the intrinsic none linearity of this tube. My version beat SY's Red Light district by a long margin. Class A and no feedback gives a very SE sound from these valves.
OTL just seems so damn wasteful of heat and valves.

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Old 10th May 2008, 08:40 PM   #9
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That scematic that was posted is definitely a Circlotron! Look at the Atma-Sphere designs for the best in Circlotrons. The Atma-spheres run most standard speakers just fine.

I have Two of Ralph Karstons early prototypes myself! each with 8 tubes per channel and i am going to build a 14 tube version next winter.


Zc
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Old 14th May 2008, 06:14 PM   #10
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Shoog,

Do you have a schematic handy for your implementation? I've read some of your other posts and I'm curious about the results.

It's just that I've got a TEK power supply which a ton of 6.3v juice. I guess I could also create some monster ppp 6aq5 with it too.
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