I have a Zoe, and have acquired the following tubes thus far:
Sovtek 6H30pi
NOS Mullard 12AU7
NOS Siemens 12AT7 (early '60's)
NOS Sylvania 6BK7B
The Sovtek was warm, smooth and too weighty, and lacked some focus, detail, and dimensionality. I wouldn't say it's accurate.
The Mullard was a fantastic all-rounder. Rich & nice balance, but I was looking for more punch and midrange clarity. I'm now trying the Siemens. It's not yet broken in (20 hours or so). I'm very impressed so far. Very quiet, more 3d, solid bottom, and slightly more air, although w/ some grain and not as rich as Mullard's midrange (maybe after a few more days?). I've not yet tried the 6BK7B. About the others, I'm not sure if 6BZ7 is worth a try (the only one I can find is $8 a pair though), and I've read that the 12BH7 and 6922 are not low in microphonics and were not originally designed for pre-amp use... so I'm leaving them for later.
One odd thing I noticed when changing from 12AU7 to 12AT7 was that I had to turn the volume dial 40% higher with the 12AT7 to achieve similar volume levels. I was under the impression the 12AT7 tube has about 40% more gain than the 12AU7. So shouldn't it mean I should be turning the volume dial down instead? Anyways, I'll let you know when the Siemens has fully broken in.
Also don't forget the Zoe is a phase-inverting pre-amp, so you should reverse the polarity of your speaker cables (on one end) for optimum sound. Cheers.
Sovtek 6H30pi
NOS Mullard 12AU7
NOS Siemens 12AT7 (early '60's)
NOS Sylvania 6BK7B
The Sovtek was warm, smooth and too weighty, and lacked some focus, detail, and dimensionality. I wouldn't say it's accurate.
The Mullard was a fantastic all-rounder. Rich & nice balance, but I was looking for more punch and midrange clarity. I'm now trying the Siemens. It's not yet broken in (20 hours or so). I'm very impressed so far. Very quiet, more 3d, solid bottom, and slightly more air, although w/ some grain and not as rich as Mullard's midrange (maybe after a few more days?). I've not yet tried the 6BK7B. About the others, I'm not sure if 6BZ7 is worth a try (the only one I can find is $8 a pair though), and I've read that the 12BH7 and 6922 are not low in microphonics and were not originally designed for pre-amp use... so I'm leaving them for later.
One odd thing I noticed when changing from 12AU7 to 12AT7 was that I had to turn the volume dial 40% higher with the 12AT7 to achieve similar volume levels. I was under the impression the 12AT7 tube has about 40% more gain than the 12AU7. So shouldn't it mean I should be turning the volume dial down instead? Anyways, I'll let you know when the Siemens has fully broken in.
Also don't forget the Zoe is a phase-inverting pre-amp, so you should reverse the polarity of your speaker cables (on one end) for optimum sound. Cheers.