It might be a balancing cap. I would have to look at my Woo.
I haven't listened to the Woo for about a week, due to work listening sessions with the portable unit I have. Today I am using the Woo. It is easy to forget, at least for me, how good the Woo is. I wish I had a stock version to compare to mine so that I could now more adequately tell the differences but in my modded Woo I hear some very nice music. It is fast, open and carries the note on a dynamic ride.
I added some images to the review. These have been posted in other threads but I thought they would be nice with the review. The round end seen between the Solen Fast Caps are two pairs of Black Gate nonpolar used for the output caps. I do not bypass Black Gates normally as they can ring and in my experience sound better unbypassed.
while I liked the Woo3 with the stock tubes I bought it with (GE 6AS7 and Sovtek 6922), I really wanted to try some tube rolling to take some edge off my Beyer DT880.
Hence not even a month after receiving my (used) Woo3 I purchased a 1962 NOS Sylvania 7236 to replace 6AS7. The effect was not subtle! The Woo3 (never sluggish) now sounded as fast and punchy as my Headroom Supreme. Unfortunately it also sounded somewhat harsh, in a way more solid-state than the Supreme itself. Replacing the headphile copper IC with a Magwire standard IC did not really change anything.
More tube rolling was in order, so I went on ebay and after several unsuccesful bids I nabbed a pair of Mullard 6922 (UK made, not sure about the year). Boy did they work! Not since I replaced a Meta42 with the Supreme, I had felt so overwhelmed by the sound of my rig. Violins and female voices are now sweet as sugar and still so vivid, so detailed. The Mullards seem to sweeten the mids without rolling off the highs. In particular they don't spoil that (holo)graphical instrument 2D placement that I think is one of the strenghts of the Woo3 compared to the Supreme. It took me almost a year, but finally I tamed the DT880!
The Woo has a solid state rectifier not a tube rectifier. But the rectifier can change the sound. The signal rides on the DC as an AC signal. The better the DC the less the distortion and the better the musical outcome. The rectifier is much more important than once believed.
Opppss, I should have looked at the drawwing first! John I see the diode bridge on the drawwing. I was just thinking that the large tube was a rectifier, not the power tube. Man I hate it when this happen's...... So when you retube you get all three. Time to start watch'n more ebay.
I did replace the stock bridge with a very high quality quiet one from Michael Percy. It cost about 25$ and is fast with little overshot or ringing. At a twenty five amp rating it is overbuilt but does a fine job.
It is interesting to note that while the 421A is a very fine tube it could not keep up with Joe Satriani's "Flying in a Blue Dream", so I switched to the 7236. Interesting that my feeling about this tube quick and defined and that is how it handles this disc. The sound comes through clean and fast. The 421A is beautiful with classical and many other genres but for the ultimate speed, 7236.
Now as long as I don't ge deaf listening to this.. . .
I was looking for some NOS tubes to start in the W3. I read this "The early type of CCa has a slightly different structure compare to the later types. If you look at the plates between the two plate insulators, make sure every plate is grey with NO shiny metal plate on the two sides support the upper structure." Then saw this for sale-