Posted by Wayne Parham [ 64.216.179.215 ] on April 01, 2005 at 00:26:03:
In Reply to: Re: Stoetkit Jr. Impressions posted by matth on March 31, 2005 at 22:56:03:
The 0.22uF caps and the removal of R21 remove the 8kHz switching artifact completely. It's what you hear in so many amplifiers, that tiny little buzz. I think it's what is probably most objectionable, and it's really pretty easy to fix. I'm not sure why it is so often overlooked, but maybe because most speakers are about 10dB lower than what I'm used to, so it might not be a big issue to some people. But it does seem to me that the tube amp crowd would use high efficiency speakers as a rule, so the omission of switching caps is probably a big oversight. Seems like almost every tube amp with SS diodes is done that way. Add the caps and remove the resistor and you'll be dead silent, black as night.
Do a little experiment this weekend. Listen to some music on CD and notice the silence between songs. Listen for the ramp up in hiss just before the song begins and the ramp down right after the song ends.
Then do the mods I've suggested and listen again.
What you'll notice is that before the mods, the silence between songs is barely noticeable. The stock amp's noise floor is higher than that of the recording. But after the mods, the amp is now much quieter than the recording noise floor. You're really notice the ramp up and ramp down noise. Between songs, you'll have dead black quiet.
As for tubes, I think each of the ones I have are pretty good. I'm not sure there's that much difference between them. But I think the Svetlana tubes I got from BOI Audioworks are matched better though, because the amp has less 120Hz hum when they're installed. They do a better job of cancelling common mode noise.
One nice thing is that a set of tubes is very inexpensive. It is no problem to tube roll a few different ones. You can get a set of good NOS tubes for fifty bucks. So you might as well try several, and that way you'll have spares too.
I need to listen a while longer, to compare differences between tubes. I might make some measurements sometime too, to quantify any differences. I purchased eight Svetlana 6BM8's from BOI Audioworks, and when they arrived, I put the four closest matches in the amp and never took them out. So I haven't given the RFTs and the Sovteks a good comparison. Last time they were installed, the amp was still open and with a junk electrolytic bypass cap on one of the tubes. One of these times, I'll go back and do some comparisons, but right now, I'm enjoying the amp too much to fool with it.
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