by Ty_Bower » Sun Feb 08, 2009 8:27 pm
Some people have trouble getting enough negative bias voltage with the circuit as shown. It depends on which brand KT88 you've used. The usual cure is to replace the 4.7K resistor (between the pot and the diode) with a smaller value, often 2k or 1k. Mine works fine as shown, but I will admit I need to keep the bias pot turned nearly all the way counter-clockwise. You might want to just start with a 2k and you won't have to worry about it later.
Some have suggested adding a ceramic disc cap across the diode to snub the switching noise. I didn't put one on mine, but I can't see how it would hurt anything. You can read some here:
http://www.diytube.com/phpBB2/viewtopic ... 8&start=12
Others have recommended adding a resistor in series between the output transformer's UL tap and the KT88's screen grid. I used 80 ohm parts because that's what I had around. The typical value is 100 ohms, but I've seen references to values as high as 1k. You can see some photos of mine in this post:
http://www.diytube.com/phpBB2/viewtopic ... 3&start=10
Read more about the screen resistors in this article, starting on page 3 in the section titled "Simple Solution".
http://www.audioxpress.com/magsdirx/ax/ ... ie2544.pdf
For what it's worth, I really like this amp a lot. The Mk3 is a good solid foundation, and the Poseidon driver seems to work very well in it. Some kind of capacitor upgrade is almost a necessity, and the SDS board is effective. I've put a fair amount of time into mine, and I've gotten many enjoyable listening hours out of it in return. I hope you enjoy yours as much.
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."