Tube and Opamp Rolling.
In my tube rolling quest, I found out that the stock RCA Clear Top tube is actually very good performer. It has a very balanced presentation, with a slight emphasis on the midrange. The clarity is very good, and it strikes a good balance between a mellow lush sound to an energetic and dynamic sound. The soundstage performance is likewise very good. As it is, the RCA Clear Top ranks very high on my recommendation list.
Sometimes, when switching to another tube, you get a certain boost in one area, such as a more lively sound, or a more forward presentation, or low end boost, but often you will also suffer changes in other areas that the stock RCA tube does well. So far I haven’t really found one tube that betters the RCA Clear Top on every aspect. Hence, tube rolling should be seen more as a sidestep, or a change in sound signature, rather than an “upgrade”.
There are a lot of different choices for 12AU7, and even a certain brand would have different variations of the 12AU7 tube. For instance, Telefunken has the “smooth plate” and the “ribbed plate” version, and even between those two versions, there are OEM brand version of the Telefunkens. Then you also have the fake Telefunkens, which sometimes can be very hard to spot depending on how well you’ve known Telefunken tubes for. Another popular 12AU7 choice is the Amperex, which has three different versions depending on production year. The earliest versions comes with the Treble Clef logo an are produced in the 50s. Then you have the Bugleboys which are produced in the 60s, and finally the World logo for the 70s production. Other popular brands also include Mullards, Tung Sol, CBS/Raytheon, RCA, Sylvania, and GE. The choice for tube rolling is quite endless, and every one of these tubes will implant their own signature to the EF-5’s sound output. I guess we can be thankful that the EF5 only uses one 12AU7 tube, which makes tube hunting a little easier, since you don’t need to get matched pairs of the tube.
Here are some of the tubes that I tried with the EF5: (special thanks to Poeng and JA for the 12AU7 tube loaners)
- RCA Clear Top (Stock). Slightly warm, good tonal balance from top to bottom, with slight emphasis on Midrange. A little grainy and mellow sounding.
- Telefunken Smooth Plate: Smooth, thick midrange and bottom, weighty bass punch. Forward and grainy vocal.
- Tung Sol Black Glass: Forward treble, slightly recessed upper midrange. Tight bass. Good clarity, solid state like. Crisp, but vocals get sibilant easily.
- RCA Black Plate (Early version): Low end boost. Full and weighty bass. Grainy vocals.
- Mullard Short Plate: Wide soundstage. Full bodied bass and midrange. Weighty vocal, slightly sibilant.
- RCA Gray Plate (Late version): Similar to the Clear Top RCA, but more forward vocals, slightly less mellow, punchier bass.
- Philco Short Black Plate: Midrange oriented. Very intimate yet smooth and non sibilant vocals.
- International ServiceMaster Smooth Plates: Like the Philco Above, but with more weight on the vocals and bass.
- Philips Ribbed Gray Plate: Midrange oriented, clean and non grainy. Slightly sibilant vocal. Clear treble. Full bodied midrange.
If you were to ask me, I really find the stock RCA Clear Top tube to be the best all rounder performer. I personally enjoy the Tung Sol Black Glass, but it makes the EF5 more solid state sounding, and it doesn’t nearly have the mellow and smooth tube sound that the stock RCA has. I suppose Hifiman did choose a good stock tube for the EF5. Tube rollings are fun, however, and you can experiment by trying to find alternate tubes that you can use to change the signature of the EF5. I don’t think that the list of tubes I had put up is the definitive list. Rather, they were the tubes that I had available for the testing purposes. The cost of 12AU7 tubes vary anywhere from $10 to $75 (or more), depending on how rare or how desirable a tube is. The ones or my list are nothing too rare, and most of them are fairly easy to get.
For opamp rolling, it’s important to take note that the EF-5 supplies close to 34Volts to the opamps. I wasn’t aware of this when I started rolling opamps on the EF-5, and as a result, I fried an AD8066 opamp (too bad, it was a good combination with the Tung Sol Black Glass) which maxes at 24V. Another popular opamp like the AD797 are also out of the operating spec. The popular OPA627 opamp, however, is a good match with the EF5, and also highly recommended for an upgrade over the stock OP275 opamp for a more refined and higher resolution sound quality.