Here is a quick comparison of 12AX7 and 5751 tubes in my Diva Audio M7 tube pre-amp.
For those unfamiliar with the Diva Audio M7, it is a fairly popular tube pre-amp designed and built in Singapore by Mr. T.S. Lim. It is based on the Marantz 7 circuit and uses 3 12AX7 tubes.
Here are the observations of rolling the first tube position. Electroharmonix 12AX7 gold is used in the second tube position while the RCA 5751 blackplate (double mica) is used in the third tube position,
1. GE JAN 12AX7WA (1986 date code)
The tonal balance is dense with decent midrange and treble detail. However, dynamics seem a bit flat.
2. Golden Dragon 12AX7
The tonal balance is lightweight. A bit of a paradox, but the tube avoids brightness despite quite a lot of treble extension. Midrange and treble seemed smoothed out but the highs seem to go a lot further. I say it it a paradox to me, because this kind of extended response usually results in a fairly bright presentation. The lightweight sound probably comes from the lack of bass. This tube was a lot better than what I expected.
3. Sylvania 5751 gold pin (tripe mica)
My favourite tube of the lot. A lot of detail with both extended bass and treble. Very good air with good control of sibilance. Tonal balance is close to neutral.
4. RCA 5751 blackplate (double mica)
Dark tonal balance. Very smooth and creamy but compared to the other tubes, some midrange and treble detail is lost. I suppose its quite ideal in bright systems, but match with care. That being said, I've had very good results for this tube in other equipment.