Improvements for the Mikael Abdellah KT88??? - diyAudio
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Old 12th September 2008, 09:16 AM   #1
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Default Improvements for the Mikael Abdellah KT88???

I have collected most parts for this amp, actually the Alex Gendrano version shown below. Just wondering if any of the veteran sages on the forum have any advice as to changes they personally would make to the amp and/or PSU circuits.

Especially interested to hear about filter chokes and viable options. And, if there are reasons to change any component values, etc., it would be interesting to hear why.

Regards, Greg

http://diyaudioprojects.com/Tubes/KT88/
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Old 12th September 2008, 11:22 AM   #2
alexg is offline alexg  Philippines
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Hi, I built that amp.

If you use motor run capacitors (ASC preferably) on the power supply, it sounds way better than the Nichicon electrolytic.

I omitted the 5H choke on the LC filter going to the 6n1p and replaced it with a 100R resistor (on some 6n1p, am getting oscillations and replacing the 5H with a resistor cured the problem).

I also bypassed all the cathode bypass electrolytic capacitors with polypropylene, and it seemed to help a bit.

Another change that I did which I did not submit at Gio's site is I replaced the cathode resistor of the 6n1p with an LM317 current sink, you can also try using two red leds to bias the 6n1p (both seems to sound better than the 1K resistor).

I have not updated the schematic at Gio's site, with the tweaks I did.

Look for Sherman and porkchop's thread on this forum, they described how they built this amp and the tweaks they did.


HTH,

AlexG
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Old 12th September 2008, 12:31 PM   #3
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Here's how mine is currently configured:
My SE KT-88 configuration

More info on my page:
http://webpages.charter.net/porkchop...8_SET_amp.html
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Old 12th September 2008, 11:35 PM   #4
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Just the kind of info I'm wanting.

Thanks to you both, Greg
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Old 12th September 2008, 11:43 PM   #5
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I would put a current source on top of the 6N1P and put an LED under the cathode (with no bypass cap). The coupling cap could also be about 1/2 that size and work quite nicely (saving some money).
-Paul
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Old 13th September 2008, 02:52 AM   #6
alexg is offline alexg  Philippines
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Quote:
Originally posted by porkchop61
Here's how mine is currently configured:
My SE KT-88 configuration

More info on my page:
http://webpages.charter.net/porkchop...8_SET_amp.html
Great idea on the cathode resistor bias of the EL34/KT88!
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Old 14th September 2008, 05:09 AM   #7
mach1 is offline mach1  Australia
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Try connecting a good quality 270k resistor (caddock mk132 are good in this application) between the KT88 plate and the 6N1P plate.

The local feedback loop will lower output impedance, distortion and gain. If the gain drops too much, try a higher value resistor.
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Old 14th September 2008, 03:38 PM   #8
Sherman is offline Sherman  United States
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I built Mikael's schematic but as monoblock power amps. I think you are on the right track with changing the power supply to the 6N1P. I had some motorboating in very bass-heavy passages when built per schematic. I decoupled the 6N1P with a simple RC network (can't remember the values off the top of my head) and the problem was completely solved.

I used Mikael's original component values for everything else. I used Auricaps for the coupling caps and motor run (ASC) caps for the PS filter. All in all it is a very good sounding amp.
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Old 14th September 2008, 05:41 PM   #9
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A choke input power supply might also help.
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Old 14th September 2008, 08:20 PM   #10
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Quote:
Originally posted by mach1
Try connecting a good quality 270k resistor (caddock mk132 are good in this application) between the KT88 plate and the 6N1P plate.

The local feedback loop will lower output impedance, distortion and gain. If the gain drops too much, try a higher value resistor.
Thanks, I may give that a try!
Glenn
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