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Old 15th January 2008, 04:28 PM   #1
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Default Andrea Ciuffoli's KT88 PP

Hey all.

Anyone tried this one?
http://www.audiodesignguide.com/my/pp3.html
Worth building? The specs are pretty much what I'm looking for. A simple KT88 PP with about 20W output.
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Old 16th January 2008, 07:20 PM   #2
Svein_B is offline Svein_B  Norway
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Sorry, I have not tried this design.
But I love the simplistic beauty of the circuit.

There are a couple of related threads on the K&K (Lundahl) forum:
http://www.audioasylum.com/cgi/t.mpl?f=kandk&m=4663
http://www.audioasylum.com/cgi/t.mpl?f=kandk&m=1133

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Old 16th January 2008, 07:23 PM   #3
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It's a classical 1930'th design. Results depend very much on an interstage transformer's selection.
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Old 16th January 2008, 09:23 PM   #4
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Results depend very much also on the quality, condition and age of the tubes, since there is no NFB to normalize them.
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Old 17th January 2008, 10:11 AM   #5
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Experience tells me that interstage phase splitting sounds very good and it is well worth the investment in suitable quality iron (these can stay with you for multiple builds).
However the choice of KT88 as the output devices seems quite a poor one. Damping factor will be poor because of the relatively high rp of the KT88, even when triode strapped. If you can live with less power some type of high powered triode would be a much better option. Even a 300B would be a good choice.

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Old 17th January 2008, 01:43 PM   #6
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Very similar to Sun Audio SV-KT88PE.

Manufacturer web site: http://www2.big.or.jp/~sunaudio/sv/svkt88pe_e.html

Schematic:
http://www.meta-gizmo.com/Tri/sunschematic.html

(actually, this is for the SV-300B MKII, but the schematic is very similar for the KT88 version).

Attached is a simplified schematic of my current project, using 6CG7 valves. Not shown are Aikido noise cancellation capacitor/resistor divider, "safety" resistors in the Aikido circuit, and the grid bias for the KT88 output valves. Also not shown is ultra-linear taps for the output stage (if desired). I'll probably build this as a Class-AB1, with about 50mA idle (each) on the output valves.

The LL1660S has about 360 Ohms of primary winding resistance, which means the transformer winding also serves as the cathode bias resistor to Aikido valve V2B (upper valve).
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File Type: jpg smoore kt88pp.jpg (39.1 KB, 1284 views)
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Old 17th January 2008, 03:51 PM   #7
Svein_B is offline Svein_B  Norway
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Quote:
Originally posted by Shoog
However the choice of KT88 as the output devices seems quite a poor one. Damping factor will be poor because of the relatively high rp of the KT88, even when triode strapped. Shoog
What would be a ralistic output impedance of a triode strapped PP output stage with KT88 (or EL34) ?

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Old 17th January 2008, 04:36 PM   #8
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Quote:
Originally posted by Kashmire



Attached is a simplified schematic of my current project, using 6CG7 valves. Not shown are Aikido noise cancellation capacitor/resistor divider, "safety" resistors in the Aikido circuit, and the grid bias for the KT88 output valves. Also not shown is ultra-linear taps for the output stage (if desired). I'll probably build this as a Class-AB1, with about 50mA idle (each) on the output valves.

The LL1660S has about 360 Ohms of primary winding resistance, which means the transformer winding also serves as the cathode bias resistor to Aikido valve V2B (upper valve).
No comments.
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Old 17th January 2008, 05:36 PM   #9
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Cryptic response. Elaborate.
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Old 17th January 2008, 06:02 PM   #10
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Quote:
Originally posted by Kashmire
Cryptic response. Elaborate.

You loose gain such a way and increase impact of oddnesses of an interstage transformer.

Try to shunt a CCS in cathode of the cathode follower by a capacitor, you'll be surprised.
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