I've checked out a bunch of tubes for the Woo WA6-SE.
Mine is extensively modified so my results might not be typical, but I'm pretty sure they will apply to the stock amp (more or less).
The winner is an RCA 6DR7 I got from Jim McShane. It has good presence, dynamics, excitement, PRaT, and is not unflattering to less-than-perfect recordings.
I learned that with these old tubes, you must remove the oxidation from the pins to get anything like good performance.
Here are the tubes I tried:
- 6FD7 I got from Woo. Not sure of the brand.
- Two types of 6DR7: "Pinnacle" brand (Pinnacle is almost certainly relabeled from something else) from Woo, and RCA from Jim McShane.
- Two types of 6EW7. Raytheon" purchased from Woo, and "Tung-Sol" purchased from Jim McShane.
- Two types of 6DE7: an RCA brand from Jim McShane, and GE brand (I forget where I got that from)
Descriptions in order from bad to good:
- Both 6EW7 were bad: not sure how to do describe it, but an unpleasant tonal balance. Resembles the sound of scraping glass. I know that doesn't make sense, but not sure how to describe it.
- The GE 6DE7 had no dynamic impact. Crescendos couldn't bloom.
- The RC 6DE7 was just very bland. No PRaT or excitement.
- The 6FD7 might be to some peoples' taste. It's a lush tube and very flattering to all recordings. I ended up not liking it because there's no rhythmic excitement.
- The Pinnacle 6DR7 is very good, extraordinary dynamics and PRaT, extraordinary microdynamic resolution, but not flattering to bad recordings. It has a very prominent upper midrange which can become "edgy" in a bad recording.
- And the winner is the 6DR7. It has the good qualities of the Pinnacle 6DR7 but tones things down just enough to avoid that unflattering upper midrange.
I can imagine that some people would say the Pinnacle 6DR7 is the winner because it does have more rhythmic excitement and microdynamic resolution than the RCA 6DR7. For myself, I need to tone things down a bit, so I'll go with the RCA.
What is clear is that both 6DR7 are heads and shoulders above the others. I ordered two more brands of 6DR7 from Woo: Sylvania and Westinghouse. I'll report on those after they arrive and burn-in.