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In Reply to: Harshness and WE harmonic equaliser posted by arend-jan on February 18, 2006 at 04:22:31:
If this double posts - my apoligies. Thanks for your observations and comments. I think my harshness was a result of poor implementation in the driver stage. When I built the amp originally, each // 6sn7 had its own cathode resistor and cap. These were separated by a few inches on the ground bus bar - this was an easy way to set it up, and gave a lot of room for changes/experimenting. What I didn't think about until later was the fact that the cathodes for the output tubes was tied into the bus bar between them. I changed the two 470r cathode resistors for one 220r, tied downstream of the output stage. All returns for the driver stage are now at this one point. In addition, instead of tying the WE cap directly to the cathodes of output and driver stages, I placed a resistor in series (30r on output, 100r on driver). The harshness is gone. The results for both amps were the same. These things sound really good - easy to listen to for hours.
Good thing you sorted it out. This kind of thing can drive you mad :-D
It's easy to make such a mistake with bus-bar grounding. that's why I prefer per-stage star grounding, especially with IT coupled stages sincy you already broke the ground loops between each stage.
Anyway, could you comment on the WE cap? Can you hear a difference with and without it and if so, under what conditions and how does it show?
Sometimes things like this are probably 50% reality/50% imagination, but the biggest change I think I noticed is the amps have a little "blacker" background. Very complex passages seem a little more discernable. They were very quiet before, fraction of mv at speaker terminal, they are a little quieter still. I really should have made changes one at a time to hear the impact of each change rather than listening to a combination. The midrange is very good/smooth. Overall detail, especially low level (things like lightly tapped cymbol) is better than my 300b se. I think the bass may be a little cleaner/deeper with WE caps. I should state that while I have lundahls in input/splitter and driver position (1676, 1660s), my OT is a hammond 1615, which will be upgraded eventually, although once you get into the upper bass and higher, it is difficult to imagine how much better a more expensive ot will actually be. The hammond does start to roll off about 35hz, and the wave form looks really flattened by 25hz. When I built this, I was concerned about using the //n7's, as I had read comments that // tubes sometimes smeared detail. I already had the holes and sockets in place, so I decided to try anyway. I had also heard ecc99 (which I bought for this!) had a tendancy to sound a little harsh. I am glad I used the n7's. I am going to take some measurements in a couple of weeks to see what it looks like in the upper treble region - but for now - I am enjoying the music.
I assure you that other output transformers will make a very positive difference in the mid-band sound, especially the Lundahl iron amorphous core outputs!
I can't count how many times I have been brought up short by just the sort of expedience as you describe! Thanks for sharing the result.
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