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Old 4th April 2008, 09:18 PM   #1
JesseG is offline JesseG  Canada
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Default My KT88 SE project

Hi all:

It is up and runing, so I thought I would share this project with you all.


Click the image to open in full size.

It is a single-ended KT88 in Triode mode, with a 6DJ8 SRPP driver stage, James OPTs and a HUGE power supply made from trafo and chokes rescued from some very classy radio equipment.

The whole thing is built on an open-frame chassis made from 1/2-inch (yes, thats 13mm) aluminium plate. All up it weighs about 55lbs.

Construction is very straightforward. However the power supply is a bit wierd. The Trafo outputs 375-0-375. Since I did not have room on the chassis for a rectifier tube, and SS rect. brings the voltage up so high that I would not have been able to use reasonable value caps, (plus, I had 2 chokes) I decided to split the output of the trafo into 2 seperate half-wave supplies.

The result is a seperate supply for each channel, which share a trafo secondary. So an almost doal-mono design

Some more photos:
Click the image to open in full size.
Just starting construction

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Things well along

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Nearly done

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first power-up = measuring voltages and waiting for smoke

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In the room and making tunes - mighty good-sounding, if not overly powerful.

and the schematic
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Old 4th April 2008, 10:26 PM   #2
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Looks great.

- glad I wasn't punching the chassis holes though
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Old 5th April 2008, 12:09 AM   #3
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I've got the lovely task of makign my chassis holes soon....but thankfully I'm only dealing with ~1.5mm!

Are those Fostex TE166Es?
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Old 5th April 2008, 12:19 AM   #4
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Nice work Jesse.

Quote:
Originally posted by Psychobiker
Are those Fostex TE166Es?
Those look like Dave's tweaked FE127E.
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Old 5th April 2008, 01:19 AM   #5
JesseG is offline JesseG  Canada
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The holes were all drilled. The square holes that mount the trafo and chokes were drilled at the corners, then cut with a jigsaw, then filed smooth. Very time consuming and rather mad, really.

However, the chassis is nothing if not solid



Quote:
Are those Fostex TE166Es?
Quote:
Those look like Dave's tweaked FE127E.
They most certainly are Dave's enABleD FE127s.

BTW: Thanks very much, gmilitano, for the KT-88 SE you published on your site. It is the source for the KT-88 section of this amp.
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Old 5th April 2008, 02:06 AM   #6
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Quote:
Originally posted by JesseG
BTW: Thanks very much, gmilitano, for the KT-88 SE you published on your site. It is the source for the KT-88 section of this amp.
Actually, that is Mikael Abdellah's Single Ended KT88 Tube Amplifier.

Here is Alex's Single Ended (SE) KT88 Tube Amplifier with a beefed up supply.

Cheers,
Gio
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Old 5th April 2008, 11:58 AM   #7
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Hi JesseG,
well done, what about cathode decoupling?
Kind regards,
Darius
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Old 7th April 2008, 12:35 AM   #8
JesseG is offline JesseG  Canada
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Quote:
what about cathode decoupling?
Good eyes

I left off the cathode bypass for two reasons - 1) because I wanted to get a good idea of the effect on the sound with and without a cathode cap. and 2) because the schematic calls for a 220uf cap - I don't have any 220uf of any kind of quality right noiw and the idea of putting a cheap electryolytic in there just makes me cringe.

Right now, the amp sounds fantastic. When I can get some 220uF Black Gates (or similar) the I will try them.
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Old 20th April 2008, 04:26 AM   #9
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Hi JesseG,

That looks super and that solid construction is wicked!

Is there a grid leak resistor on the KT88 not shown in the schematic?

Those speakers look familiar too

Cheers!
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