Posted by Wayne Parham [ 65.69.120.160 ] on March 08, 2005 at 04:16:45:
In Reply to: Stoetkit - Thinkin' out loud posted by Wayne Parham on March 05, 2005 at 04:02:00:
A couple nights ago, I pulled the bottom case off the Stoetkit to have access to he circuit for my oscilloscope. I wanted to see if I could quantify the level of hum. When I did, I found R5 to be discolored by heat. So the first thing I did was measure the voltage and it was well above the normal operating level, and beyond that which is safe for the bypass capacitor C5.
So I started troubleshooting it, and had a couple of E-Mail exchanges with Rik and Rogier at Heart, even spoke to Sander on the telephone. The thing is that my amp works and sounds nice, and the only reason I have the bottom cover open is that it has a tiny bit of hum in the left channel. Since the demo unit did too, I thought I'd contact the Stoetkit people and see if there might be an ECO or something, an etch that might have been misssed that shorted something and lowered the grid voltage on the output tube. But you know how it is, there is really nothing a person can do remotely - It's just a process of elimination.
This morning, I took some time to lift leads on parts one by one in a process of elimination, to try to find the reason why the circuit didn't add up. I pulled all the tubes and used static resistance values to compare with. What I eventually found was that capacitor C3 was shorted. Not completely, it has about 60K ohms resistance. But that's enough to shift the bias of the tube. The circuit still worked, and it even sounded pretty good. But it drew excessive current on the left side, and put an uneccessary load on he power supply.
The good news is that I found it. When I lift one lead of the capacitor and take it out of the circuit, all the bias voltages become normal. The bad news is it's one of the nice Audyncap coupling caps. So right now, I have a generic polypropylene cap in there, and it's not even the right value. I've ordered a replacement part, and to make sure its matched with the others, I'm replacing them all at once. Since the bypass cap was subjected to higher than spec'ed voltages, I'm replacing the bypass caps too.
It sounds pretty good, even wih the off-value coupling capacitor. I'll start taking some power supply noise measurements soon. But right now, I've got other fish to fry. Continuation of this project will have to wait for another time. For now, I'm listening to Yo-Yo Ma with my bailing wire and glued Stoetkit. 
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