Well, I've got about 50 hours on this amp now and it sounds great. Highs are very crisp, the mids are smooth, and the bass is mellow but very detailed. As I said above, perfect for soft vocal jazz.
There is one thing I don't quite understand yet. After about 6 to 8 hours, the Hammond power transformer becomes almost too hot to touch. I would estimate it's external temperature at about 120˚F to 125˚F. The first cap after the 5U4 is only a 32µf so the conduction angle should be long enough that I don't get this much core heating. The chokes and output transformers are nice and cool. Now that I think about it, the Hammond power transformers on my headphone amps do the same thing.
Does anyone else have this experience with Hammond power transformers? Is this just typical of their transformers? Any experiences other people have with their power transformers would be greatly appreciated.
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The initial thoughts on DC heaters are pretty much valid. In my experience they are not needed for projects with a S/N of 80 db or less. 80 BTW is rather quiet. I generally design to hit -90 db so in that case they do help. All the effort can be wasted if the particular tubes and and brands of tubes don't hold up their end. I just sent back 6 new production Mullard 12AX7s as they were noisier than the standard JJ ECC803S. The difference was between 2.5 and 3 db per tube. Maybe a bad batch... then again maybe not.