Ok, just got in my first set of replacement tubes, the Chatham 6AS7G and the GE 5-star 5751 both came today. I'd been listening to the stock tubes all afternoon and was all prepared to hear a subtle - or maybe not so subtle - improvement in the sound with these.
But no, it was terrible! I may not be well trained in listening to this sort of thing, but there was very obvious distortion after just a few minutes of listening to my reference tracks, especially muddy lows, weird noises in highs around e.g. a violin solo. Also everything is off-balance, noticably heavier in the left ear, like someone tweaked a balance knob without my knowledge. (Of course I changed nothing else in the system, just the two tubes.) There must be something wrong with one or the other of these tubes, maybe mostly (or all) in the right stereo channel, since the distortion appears greater in the right ear - though this could also be because I listen mainly to classical and the lower instruments (cellos, basses) are on the right side of the orchestral stage.
I also noticed I had to turn the volume knob up higher to reach the same sound levels as before, presumably due to the lower gain of the 5751. Not necessarily a bad thing as it seems like there should still be some extra room, but maybe that exacerbates any tube weakness...? Maybe a 5751 isn't the best for me, given that I use low-impedance K701's, and I should go back to a regular 12AX7? Not sure.
Anyway, tomorrow I will try switching out one tube at a time with the stocks, to try to isolate where the problem is occurring. Both these are used tubes, so it could just be that I got a dud. Though they tested well according to the sellers. Fortunately I didn't spend a lot on these two, although they were't dirt cheap.
