What do you know and how do you like the RCA 5963 Black Plates?

mulveling, I see. I thought you had all VAC electronics, or this is your second system.

Yes, the 2nd system / office rig is sort of my "sandbox" system. That’s where the ST100 is. I have VAC Master monos and premap in the bigger loft rig. Also got a VAC Statement 450S floating around lol. Speakers have been Tannoy Canterbury GR in the loft for years; recently re-introduced my old Canterbury SE in the office rig.

So, RCA 12AX7 black plates from 50s are not microphonic, I understand, I might want to try them too instead of Mullards, though I would not anticipate the improvement, but who knows ?

So, RCA 12AX7 black plates from 50s are not microphonic, I understand, I might want to try them too instead of Mullards, though I would not anticipate the improvement, but who knows ?

I mean they can be microphonic - all tubes can. But in practice, yeah my RCA 12AX7 (including 1950s black plates) have generally been usable even in phono stage slots. In fact I used black plate RCA 12AX7 in a Rogue Ares phono stage, several years ago. I liked them a lot there. And right now I've got them in the ST100's V1 slots, in front of 5963. It's a good all-around tube. Definitely not bright, and not too warm. Mullards ended up being a bit "thick" for my liking (12AX7/CV4004, 12AU7/CV4003, EL34). And I'm a guy who loves SPUs and Koetsus - so I don't know why Mullards turn me off like that, but they do. Somehow those come off as "too thick" and syrupy, whilst Koetsu and SPUs come off as "full bodied", "natural", "energetic", and "organic". But you really just have to try for yourself. The Mullards are fine tubes to be sure. I just never end up leaving them in for long. 

mulveling, I understand why some might not be particularly fond of Mullards, it is really a matter of taste and system match. I prefer somewhat loose vintage sound with silky highs and full bodied midrange. But perhaps even for me the full Mullard set-up would be too much of that. I will probably just stay with RCA blackplates for 12AU7 in the amp and Mullard lonplates for 12AX7 in the preamp.