It's been a very long while, but I've finally completed my shootout of seven rectifiers!
From left to right: Mullard GZ34 metal base, Mullard GZ37, Mullard GZ37 fat bottle, EML 5U4G, United Electronics 596, Brimar 5R4GY and Mullard GZ34 black base. Together with the Tung Sol 6SN7GT round plate round mica and GEC 6AS7, each rectifier spent three days in my WA22 before evaluation with an HD800 with Whiplash TWag cable. Source is a modded Cambridge Audio 840c CDP. I'll split this review into three - one for the Mullard GZ series indirected-heated tubes, one for the 5x series direct-heated rectifiers, and the last for some afterthoughts.
Mullard GZ34 black base
This particular tube has factory/date code X8H, indicating manufacture at the Philips plant at Sittard, UK, in August 1958. It is not the fat base version despite being one of the earlier batches made. After being used to large rectifiers, this tiny tube came as somewhat of a shock when I received it. I had bought it NOS, and after 200+ hours a dark deposit had gathered on the glass below the bottom mica. So if you see GZ34s with such dark deposits, it doesn't mean it's at the end of its lifespan, but might still have a long life ahead.
The first thing that struck me when listening with it was a bright, cool-sounding sound signature, followed by a closed, narrow soundstage. The upper bass/lower midrange seems to be toned down, and treble extends high and bites hard. Bass punches decently enough, but does not extend very low. Of the seven rectifiers, this is the brightest sounding. Soundstage width is the narrowest as well, but not exactly unpleasant. Soundstage depth is not its strong suit as well, reigning in the HD800's expansive presentation. For those who feel that the HD800 is too wide and diffused in soundstaging, this tube can cure that.
However, this tiny tube has an upfront, aggressive presentation with great speed and attack. Trumpet fanfares sound exhilarating, and it seems to suit rock pretty well. Still, the black base GZ34 is not my cup of tea and I would personally rank it bottom on my list.
Mullard GZ34 metal base
I got this tube used and it showed; the base had lots of oxidation and the glass had lots of dark deposits inside below the bottom mica. No matter, some brasso on the base made it shine again, and my experience with the black base version meant I wasn't too worried about how much life it had left. Date and factory code indicated manufacture in December 1956 at the Philips plant in Holland.
So how different is it from the regular black base version, and is it worth the premium? Happily, it sounds like an upgrade. Treble is silker, making it less fatiguing with the HD800. Soundstage is slightly wider, but just slightly. There's a hint more bass volume as well. Overall, it is an improvement, but still not to my liking. I got mine for USD200, which is a good price if you like its signature. But these days they can go for over $500, which is pretty ridiculous when the regular version doesn't perform that much worse at a much more affordable price. If you want one, be patient and wait for a good deal instead of paying horrendous sums for the mystique and rarity.
Mullard GZ37
The GZ37 (or CV378) came in two versions, an earlier fat bottle version and a later skinnier version. This is the later skinnier version, manufactured in Blackburn, UK, in March 1967. I bought it NOS, and after about 600+ hours, a silver ring formed on the glass between the top two mica, and "getter" deposits appeared above the heater elements.
Putting the GZ37 in was a relief after the GZ34, as the warmer sound signature was very welcome. In fact, it seems to be a polar opposite of the GZ34; it's warmer sounding, the treble is reigned in, and the soundstage is quite wide. Also, it is more laid back and not aggressive at all. If the GZ34 were a rocker at a mini concert, the GZ37 would be a jazz crooner at a casual bar.
However, the GZ37 is noticeably more sluggish than the GZ34. Attack is not as fast, and the bass is more bloated. Soundstage depth is also similar to the GZ34. For those craving tight bass or snappiness in rock and techno, this rectifier is not for you. But those looking for something laid back can give this a try, and it is often cheaper than the GZ34.
Mullard GZ37 fat bottle
Is this really the same type of rectifier as the later GZ37? The glass shape is different, the plates are different, even the place of manufacture seems to be different (High Wycombe vs Blackburn). And despite being used and putting in 200+ hours, the glass remained clean and spotless with no deposits. Is the older tube really better?
Indeed. Gone is the sluggish response of the newer tube, the fat bottle sounds more agile and dynamic. Not as fast as the GZ34, but it doesn't lose out too much. Bass punches harder, tighter and deeper, it just might be my favourite tube when it comes to the lower regions. Soundstage width is the widest of the GZ series here, with slightly improved.
One thing that strikes me about this tube is how smooth everything sounds. Perhaps it's the treble roll-off as compared to the other three and the sweeter midrange, but the big bottle sounds very relaxed and refined in its presentation despite being more dynamic that the skinny GZ37. Pair this tube with soothing jazz or slow ballads and you can just kick back and relax. Of the four GZ series tubes, this is my firm favourite.
To be continued...
erformance difference between that and the WA-5. Saw the new DAC and CDP, can't really comment on performance though aesthetically they go nice with the mono blocks. Also got to try out a bunch of headphones and other Woo amps.