To all users of Millennia Twin Topology and other vacuum tube equipment:
A comment we sometimes hear from customers is that Millennia's tube topology equipment "is too clean -- it doesn't sound tooby enough." Some owners want "thicker euphonic coloration" from their Twin Topology vacuum tube circuits.
In response to these customers, we've recently finished a comprehensive listening test on a modest collection of 12AX7, 12AU7, and 12At7 variety vacuum tubes. We picked NOS tubes which are readily available from specialty dealers and auctions. Here are the results of the test:
12AX7 Vacuum Tubes Under Test
• Sovtek 12AX7WA (baseline, our stock 12AX7 tube)
• Sovtek 12AX7LPS
• Mazda 12AX7 Chrome Plate (1963)
• Sylvania JHS12AX7 (1961)
• Tungsram ECC83LN
• Brimar 12AX7
• GE 12AX7 Large Plate (1963)
• Mullard CV4004 (UK)
• Philips 12AX7WA (11/86)
• Telefunken 12AX7 / ECC83 "Smooth Plate"
• 12AU7 Vacuum Tubes Under Test
• CIFTE 12AU7WAH (1959)
• RT Production 12AU7WA (France, 1965)
• Mullard 4003 KQDD/K
• Telefunken ECC82
For these tests, we used a number of program sources in our editing/listening room (Pass X amps, Dunlavy type 5 monitors) including Bob Katz's Chesky jazz recordings, Bruce Leek wind band recordings, and some of our own classical music recordings.
Many of the 12AX7 tubes tested sounded similar to the new-stock Sovteks we currently use. However, two tubes stood out with a very pronounced sonic signature, adding what we were calling an "aura" around the music. The two tubes that were preferred for enhanced euphonic performance were the Mullard CV4004 (UK) and the Telefunken ECC83 smooth plate (Germany). Notes for the Mullard CV4004 say "adds a bit of extra dimension and body. Very sweet. Not 'hyped' in some negative way." Notes for the Tele say "Wow! Puts an 'aura' around the program. Does some interesting things to the HM and HF. Seems to lose a bit of LF presence, but gains some 'inside detail' of program (?). More immediate?"
On the 12AU7's, there were two NOS varieties that we found "positively euphonic" -- the Mullard 4003 KQDD/K box plate and a French military tube called RT Production from 1965. The RT was noted to "provides a 'sheen' on HF and HM, interesting effect on the HF's." The Mullard "adds a nice sense of brightness, but not too much. Slight loss of LF and LM. Rich in character." The other tubes tested did not sound significantly different than the new-stock Russian or Chinese 12AU7 tubes we now use.
Note that these results were found in NSEQ-2 and TCL-2 test beds. These same results may or may not be achieved in other audio equipment. Note also that we tested additional Mullard, Telefunken, and RT Production vacuum tubes and assured repeatable subjective results. Moreover, there are a myriad of other tubes that should have been included, but for reasons of time, cost, and/or availability, weren't. In the future, we may conduct additional tests if we can gather another modest stable of different NOS tubes. Comments and suggestions for future tests are invited. We also kept good notes on all the other tubes, so send an e-mail if you're interested in what we thought about the other tested vacuum tubes.
Since completing our tests, we've been stocking up on these "colorful" valves wherever we can find them.
• ORIGIN STT-1: Two each 12AX7 (4004) and 12AU7 (4003), one each 12AT7 (4024)
• NSEQ-2: Two each 12AX7 and 12AU7
• TWINCOM TCL-2: Four each 12AU7 and two ea. 12AT7
• TWIN DIRECT TD-1: Single 12AT7 (4024)
What we might have available (call or e-mail for current stock):
• Telefunken (Germany) 12AX7 Smooth Plate, NOS & fully tested used
• Mullard (UK) CV4003 (12AU7) Box Plate, NOS, tested
• Mullard (UK) CV4024 (12AT7), NOS, tested
You can also find these exact tube types via on-line auctions, and from on-line tube dealers such as Upscale Audio. Please call if you have any questions. Thanks for your interest! It was a fun test.