Amp: KT88 push-pull shunt feedback output via p-channel fet - diyAudio
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Old 31st December 2013, 03:54 AM   #1
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Default Amp: KT88 push-pull shunt feedback output via p-channel fet

Attached is a schematic and distortion measurements at 1W and 10W of a shunt feedback output stage KT88 amp that I built for my brother. Just thought I'd share.
Attached Images
File Type: jpg 996064_10201856174958461_1242312272_n.jpg (71.9 KB, 300 views)
File Type: jpg 1426296_10201856175518475_291678444_n.jpg (117.0 KB, 299 views)
File Type: png ClarkDist1WST.png (126.7 KB, 295 views)
File Type: png ClarkDist10WST.png (222.9 KB, 283 views)
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File Type: pdf Audio_Clark.pdf (23.4 KB, 207 views)
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Old 31st December 2013, 06:22 AM   #2
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Lucky Brother !

I like the use of the heavy perf for the top plate. Great ventilation and freedom to mount boards pretty well anywhere . . . .

Looking at the graphs I have a question. Are the peaks at the bottom end 60, 120 etc. from the test gear power supply ? I see that they don't move much with power output but otherwise I don't know how to interpret them.

I'm planning and getting parts together to build a 6BX7 or BL7 > 6336 differential amp and lately, looking for ideas and details, I've been reading threads you've posted on but this is the first time I've seen one of your creations. Thanks for sharing it!
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Old 31st December 2013, 09:16 AM   #3
SY is offline SY  United States
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Interesting circuit and very nice distortion performance. Why is the power supply rail for the MOSFETs so high? And what are you using for the LED?
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Old 31st December 2013, 09:33 AM   #4
AJT is offline AJT  Philippines
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would be interesting to know results for triode mode KT88 without gnfb...
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Old 31st December 2013, 02:45 PM   #5
seppoa is offline seppoa  Sweden
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There is 327 volts across the LED, will it survive? Wouldn“t it be possible to make the most of the circuit DC-coupled, with N-channel source followers instead?

And a Happy New Year to you all fellow tube lovers :-D
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Old 31st December 2013, 02:58 PM   #6
seppoa is offline seppoa  Sweden
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I have some more doubts about this circuit; if the bias circuit, or any of the MOSFETS fail, the output tubes will turn on hard, and selfdestroy in seconds. Also the input tube needs a gridleak, unless the connected source has a dc path to ground.
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Old 31st December 2013, 03:03 PM   #7
wicked1 is offline wicked1  United States
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What are all the PCB's? I recognize your maida regulator boards. I'm guessing the ones w/ the single heat sinks are the CCS's.. What are the PCB's w/ the red caps and no heat sinks? And the ones w/ two heat sinks each? And what is the DIP ic in the back?
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Old 31st December 2013, 03:16 PM   #8
seppoa is offline seppoa  Sweden
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This circuit need a lot of different supply voltages. And some PCB“s are for input selector and start delays, I think
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Old 1st January 2014, 12:12 AM   #9
AJT is offline AJT  Philippines
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Quote:
Originally Posted by seppoa View Post
There is 327 volts across the LED, will it survive? Wouldn“t it be possible to make the most of the circuit DC-coupled, with N-channel source followers instead?

And a Happy New Year to you all fellow tube lovers :-D
scheme says 27 volts...
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Old 1st January 2014, 03:58 AM   #10
seppoa is offline seppoa  Sweden
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Quote:
Originally Posted by AJT View Post
scheme says 27 volts...
+ 27 volts on anode side, and -300 cathode side, thats 327 volts. Even +27 volts would be impossible, as the voltage drop over a red led is about 2 volts, or am I incorrect??
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