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Old 9th January 2010, 11:53 PM   #1
Loren42 is offline Loren42  United States
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Default My KT88 Williamson Amp Build

It was suggested that I open a thread on this beast I just built. It is a Williamson design with KT88s driving Edcor CXPP100-MS-4.2K.

The amp is actually two mono-blocks on a single chassis. The two power supplies only share the AC line. A delay circuit is used to delay start the B+ about 60 seconds after ignition of the power switch.

Tubes are Sovtek 6SN7s as drivers and Ruby KT88. I selected these tubes as "disposable" in case of catastrophic meltdown, but they have behaved flawlessly.

I have yet to make quantified measurements of distortion, power, frequency response, and phase relation, but our ears give it two thumbs up.

Stereo imaging is much better than the H/K AVR-525 that I had used for my system. I use the 525 as a preamp for now.

The outputs are set at 505 VDC at the plates and the fixed bias is set so the tube idles at 28 Watts of plate dissipation.

Click the image to open in full size.

Click the image to open in full size.

Love to hear your thoughts and suggestions.
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Old 9th January 2010, 11:56 PM   #2
SY is offline SY  United States
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Great job and a terrific implementation of a classic design!
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Old 10th January 2010, 12:34 AM   #3
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That's a very nice build. Two thumbs up Looks like the transformers are mounted on a thicker piece of metal because of the weight.

You made one goof on the schematic though. The control grids and screens are reversed. (Obviously not in the actual wiring)
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Old 10th January 2010, 12:43 AM   #4
TheGimp is online now TheGimp  United States
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Very Nice!
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Old 10th January 2010, 12:47 AM   #5
Brit01 is offline Brit01  United Kingdom
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congrats! One day I will get there.. still on my pre-amps

very nice piece. good job.

chassis is home built?
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Old 10th January 2010, 01:05 AM   #6
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by HollowState View Post
That's a very nice build. Two thumbs up Looks like the transformers are mounted on a thicker piece of metal because of the weight.

You made one goof on the schematic though. The control grids and screens are reversed. (Obviously not in the actual wiring)
Good catch! The amp is wired as per the pin numbers, but the symbol is obviously wrong.
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Old 10th January 2010, 01:33 AM   #7
Loren42 is offline Loren42  United States
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Originally Posted by Brit01 View Post
congrats! One day I will get there.. still on my pre-amps

very nice piece. good job.

chassis is home built?
Partly. It is a Hammond chassis, but a 1/4" aluminum plate is added on top where the transformers are mounted and another 1/4" plate is mounted as an internal divider at the forward lip of the top plate for additional strength and to act as a electrical barrier between the power supply section and the audio section. The whole amp weighs in at 42 lbs.

The bias pots are mounted on another 1/4" aluminum block off of that center divider so that the pot shafts are just below the chassis top. There are three jacks that are for DVM probes to assist in setting the fixed bias. A star ground resides in the center of the power supply section under the delay board PCB.

Speaking of PCBs. There are four PCBs in the amp as you can see here:

Click the image to open in full size.

The center board is the delay circuit. A single PCB is used for the driver circuit, but the layout actually is separated down the middle so that the circuits are really independent, if that makes sense.

Under the stereo driver PCB is another aluminum plate that shields the PCB from the twisted pair filament wires. There are six short wires that connect the 6SN&GT tube sockets to the stereo driver PCB. The filament pins do not run to the PCB. There is also a blank PCB that acts as a shield from the AC switch.

The two LEDs on the front panel are as follows; one is yellow and indicates filament voltage is on and the other one is blue when the delay circuit has kicked in.

The delay circuit is a 555 timer that engages two Omron 5VDC relays (one for each channel) mounted on the delay PCB. Much to my surprise when I had those two relays top mounted they would not engage. The coil voltage was there, but they would not engage! Turns out that the coils are polarized and I was forced to mount the relays on the solder side of the board!

The front bezel is another 1/4" plate aluminum piece. All aluminum is type II black anodized.

A short story about this amp I call the Push/Pull T-Rex:

When I was in high school I was befriended by this really weird guy named Rex Reichert. He built custom tube amps and sound systems for local bands and lived in a hut on his parents property in southeast Pennsylvania. He even built equipment for Todd Rundgren. His last act was developing and marketing a board game named Nuclear Armageddon. Then he disappeared years ago and was never heard or seen from again.

Anyway, Rex taught me how to build tube amps and speaker cabinets. I built a number of tube amps while in high school, then stopped for a long, long time. Nostalgia got the better of me a few years ago and I decided to build another amp that I call the T-Rex.

Rex was a great guy, but his 200 Watt guitar heads were legendary for extraordinary bursts of flames in the middle of a performance. Unlike his amps, this one does not share the same pyrotechnic outbursts.

All of us that knew him miss Rex very, very much, but I am glad to provide this tribute for all to see today.
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Old 10th January 2010, 03:17 AM   #8
a.wayne is offline a.wayne  United States
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Seems like Rex is very wanted :

Federal Bureau of Investigation Featured Fugitive - Rex C. Reichert


Yeah , you might want to modifiy that a bit
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Old 10th January 2010, 03:19 AM   #9
Bob_345 is offline Bob_345  United States
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Default nice

i wish i had skilz like that!
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Old 10th January 2010, 02:56 PM   #10
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by a.wayne View Post
Seems like Rex is very wanted :

Federal Bureau of Investigation Featured Fugitive - Rex C. Reichert


Yeah , you might want to modifiy that a bit
Very shocking. That was not the person I knew him to be at all.
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