Just a little update on the Mullard M8098 voltage regulator and M8137 combo... Well, oh lordy it's stunning!
It's undoubtedly the best combination of those valves I've ever heard in the Croft. I've no idea why I stopped using an M8137 (as a premium ECC83 equivalent) and started messing around with other makes of valves, as it is quite simply THE BEST.
As good as the La Radiotechnique 12AX7 is I was using before, and it's a superb sounding tube, the M8137 takes things to another level.
Quite simply, it does the classic 'Mullard thing', which is a producing a lovely natural, open, detailed and lucid mid and top end, such that it often makes valves of the same type from other manufacturers sound rather false and tonally 'forced', (zingy/fuzzy) in comparison - and somewhat like using a tone control, where certain aspects of the frequency response were artificially boosted or muted.
The M8137, in comparison, sounds very even-handed and 'linear', top to bottom throughout the frequency range, and in terms of the low frequencies, they're firm and full but not in any way overblown. Detail retrieval has also gone up a notch, with subtle nuances popping out in musical mixes that previously were glossed over. In conclusion, it's pretty obvious why these valves are so sonically sought after!
However, a special mention must go to the M098, which is head and shoulders the best version of the 85A2 I've heard. This one is vintage 1950s 'golden era' valves at its best. For me there's a unique musicality to valves from this era (or earlier), especially military spec ones such as this.
When I received this valve today from Langrex, it came in a lovely original box, carefully wrapped in its original packaging, totally untouched and NOS, along even with its original guarantee certificate, which had clearly never been opened! The whole thing 'felt' special before I'd even installed it and listened to it. However, the most special thing about it is the sound....
Rather uncannily, it seems to have lowered the Croft's 'operating noise-floor', widened the soundstage and also held music in more of a vice-like grip, such that bass is tighter and more extended, with heightened rhythmic alacrity - almost as if a bigger power supply had been fitted - it's just scarily good and hugely addictive!
So... If you've got a Croft preamp (or other amp) that uses this type of voltage regulator, and can find an M8098 voltage regulator valve, then my advice would be snap it up, tout suite, and prepare to hear it like never before.
Marco.