12AU7 / 12BH7 / 6CG7: What's best?

The 12AU7, 12BH7, and 6CG7 have very similar electrical characteristics and can usually be substituted for each other. When deciding what's best for you, the following notes may be helpful:

(1) A 12AU7 is identical to a 6CG7 electrically and in the circuit it will function identically. However, there is a different heater pin out. So if you are substituting you need to make a change. In addition, the 6CG7 draws twice as much heater current so be sure that this will work in your circuit. (This may be one reason it is not as popular as it should be.) If you need help write me via http://www.oldstockaudio.com/More/contact_me.htm.

(2) The 12BH7 requires no circuit modification to substitute for a 12AU7, but the plate resistance is a little lower (5300 vs 7700). Thus, even though it has a lower u (or amplification factor), the gain may be slightly higher depending on the circuit design. [This is mainly true for circuits that use an anode resistor load.] It will also draw a little more current, but the 12BH7 is a beefier tube and can handle it. For most circuits this should be well within the tolerances of the design, but you can ask the manufacturer to be sure.

With respect to audio reproduction there is a fairly large difference between these three tubes. The 6CG7 (or 6FQ7 which is the same exact tube) is identical in internal construction to the 6SN7. The 6SN7 has the reputation of being the most linear of the medium mu triodes and the 6CG7 follows suit. Also, the 6CG7 line is remarkably similar in audio performance across various manufacturers of the same vintage or age.  It is a good tube for early or middle stage amplification and has the added bonus of a grounded shield between sections, which reduces hum and increases channel separation.

The 12BH7 can lend a wonderful life to the tone that sounds great for jazz or vocals. For audio listening,  this tube can temper the harshness of CD's. There is, however, a wider variation in the sound of 12BH7s than 6CG7s, so some tube selection or rolling is in order. You can find very neutral 12BH7s (watch for a coming blog comparing them) . If you don't want to modify the circuit and are unhappy with your 12AU7--this is a good choice. 

The 12AU7 family has the widest variety of tonal characteristics which is probably why this tube is rolled so often by audiophiles. I personally find most 12AU7s a little tinny in comparison with the 12BH7 and 6CG7, but there are exceptions. The RCA clear plates from the 60s are more like 6CG7s in their sound, for example. There are lots of blogs and posts about the difference between 12AU7's so I won't waste your time repeating everything. My favorites are Mullard and Siemens, with RCA clear tops and Tung Sol a close second. A lot of people like Amperex, but they work best in a system that is not forward or bright as they add life to the midrange. My Lowthers are already bright in the mid-range, so I stay away from Amperex 12AU7s.

Part of the fun and magic of tubes is rolling them when listening to different types of music.  When you do, we hope you will consider our vintage tube offerings at http://www.oldstockaudio.com/ .

Testing the New Phono Amp

Congratulations! Burning the midnight oil in preparation for T.H.E. Show (The Home Entertainment Show, Las Vegas 2010) has certainly paid off for you. I've never before heard recorded music sound better than it would in most seats of a concert hall.

 As both a classical musician and budding audiophile, I was very surprised. (The difference, of course, comes from how sound waves move in each individual acoustic space, and how close you are to the sweet spot. I recently missed a lot of the nuance of a Bartok piano concerto because I was sitting too close to the stage and at that proximity the grand piano overpowered everything but the violin section, which was right in front of me. It sounded like the pianist was banging away a' la Rachmaninoff at a college-level competition, which he wasn't.)

 I do wish we'd listened to more than high-quality classical recordings, as amazing as they were (thank you very much for introducing me to Gary Karr's Adagio d'Albinoni, a purely analog-to-vinyl recording using restored vintage equipment).

When we test the amp again I'd like to expand our listening selections - the purity of Kerr's base and the clarity of the strings on Guaneri del Jesu (with Paganini's original violin!) both argue for a high fidelity to any original live sound. (Was it the Albinoni that we could tell was recorded in a studio and not a concert hall? So much fun to be able to hear the actual acoustics of the original "performance" space!) I'm thinking I want to hear some middle eastern folk, Indian classical or a Piazzolla tango. This is, of course, pure personal greed - this is the most detailed and live sounding audio set-up I've encountered, and I want to hear some of MY favorite music played like that.

12AX7 Tube-Rolling Notes

Last night, while hosting a big blind listening test, we decided to compare a few 12AX7 tubes.

We'd started our evening with old-stock 60's black plate RCA and Raytheon pair. Just for fun I put in a pair of Sylvania Gold Label, Gold Pin tubes with low noise testing from the manufacturer. These tubes are really not available from any dealer, anywhere -they are ultra rare, I just happen to have a few quads. (The lettering is real gold leaf! I think they may have originally been made for Amperex.)  Anyway, we put in the pair and oh my, oh my!! I put on a record of J. Heiftz and Brooks Smith doing a recital at the Dorthy Chandler pavilion and you would swear that the Steinway Grand was sitting in your living room! Wow!

The Mullard 12AX7 long plates have long been my favorite for phono stage, but these Sylvania Gold were definitely their equal.

For phono stages, the GE 5751 receives high recommends as well. It is a sleeper tube equal to the Mullard, but often at a better price.