ampllifier design

if you were designing a tube amp, what tubes would you use as input, driver and output ?

my personal favorites include the venerable 6cg7, as input and driver and el 84 as output.
Why do you like those tubes? Any particular brands? Also, what kind of sound are you trying to achieve with your "ampllifier?" Finally, what kind of circuit design do you prefer -- and why? I like the 6922, 6H30 and 6550C. Execution of design probably makes as much difference as the tubes, don't you think?
The 6DJ8/6922 family of tubes, which includes the Russian 6H30 extension, are not designed for audio, and while quite linear, tend to have a microphonic signature that is considerably more pronounced than in other tubes designed for audio, like the 12A* series, 6SN7 or the like. The 6CG7 is the same thing as a 6SN7 but in a miniature style package. It tends to have more microphonics than the 6SN7 does as a result.

Although less efficient, power triodes otherwise will run circles around the best pentode power tubes in every way, including longer life.
If you want to use EL84s as output tubes . . . they're a good candidate for traditional Ultralinear loading, operating class-AB1 with cathode bias - you'll get a solid ten watts with very respectable distortion.

Choice of driver tubes of course depends on the driver circuit topology - you could do it Dyna-style with a pentode driving a split-load phase inverter . . . look at a schematic for a ST-35.

Personally, although I'm a Dyna fan . . . I prefer a long-tailed-pair phase inverter, and it's really hard to improve on a well-implemented 12AX7 or 12AT7 in such an application. For an input tube, if you wanted high loop gain, then an EF86 pentode would work great, and what you end up with is basically a Mullard 5-10 circuit. But for a modern amp (especially if driven from an active preamp), then the gain is not as necessary, so I'd probably use a plain ol' 12AX7, and share it between the both channels. There's a good reason why the 12AX7 is so ubiquitous . . . if correctly implemented, it kicks butt, and it's a reasonable assumption that they will be easily available long after we're all dead.

No matter what you choose, I'm a big advocate of well-implemented feedback amplifiers, and with EL84s, there's no reason why you should need more than two driver stages, and the input stage should be DC coupled to the phase-splitter for minimum phase shift. If well designed, a traditional EL84 amplifier can be of reasonable size and price, good stability, long tube life, good speaker compatability, zero maintainance, low distortion, and very good sound quality.
my reason for selecting the 6cg7, especially tungsol and rca, circa 1960's:

i have found that of the three tube types, 6922, 6dj8 and 6cg7, the latter, in my experience has , subjectively been experienced by me as most balanced in frequency response.

as has been stated, the circuit is most important and i would attempt to create as balanced a frequency response as possible.
Mrtennis: From your previous posts, I thought you preferred rolled-off highs -- caramel coloration? Do you now look for balanced frequency response? What kind of sound do you seek?
hi 9rw:

i seek a balanced frequency response. in the past i have found that many components exhibited an emphasis in the 1000 hz to 3000 hz region. recently i auditioned two, the audionote kits l3 line stage preamp and an implementation of a sabre 32 bit dac chip, which i found balanced in frequency response. both components created resolution and lack of a euphonic signature which i found compatible with many of my problem cds. i can configure my stereo system to sound "soft" and subtractive in the upper mid/treble. when reviewing i attempt to create as uncolored a perspective as possible.

in terms of personal listening, i have, in effect two stereo systems. one is obviously colored, while the other is open, dimensional and relatively balanced in frequency response. the quad esl 57 (quads unlimited version) represents the "linear" sounding stereo system, while replacing the quads with magnepan 1.6 and replacing the vtl deluxe 120 monos with consonance monos creates the "color" .

you can read my review of the audionote preamp on the sabre dac chip has been set up in dac form, including a linear power supply and is currently under review. i welcome your feedback.

you can read about the sabre dac chip on .
I likewise have good associations with the 6CG7, it's the long-tailed-pair phase-inverter tube in my Marantz model 2s . . . it's used effectively for this role in the 5 & 8 as well, and as a cathode-follower in the 9.

But if you're using EL84s as outputs, then the 6CG7 is kinda overkill as a driver tube, and you might have a problem making it work at its best as a long-tailed-pair from the lower B+ plate supply (Marantz had a higher B+ available because he was using EL34s as outputs). And for an input tube, the 12AX7 is a better choice because of the higher mu.

I personally don't feel that different tubes themselves have any "sound" at all - it's the circuits that they're used in that have certain "sounds" . . . and it's just that some tubes are traditional choices for certain circuits, and thus we have these associations. I also think our associations of NOS tubes as "sounding better" is because their manufacturing tolerances were much, much more consistent, so it was/is possible to make more precise engineering decisions about critical circuit parameters. Added to this is the fact that most of our tube data is NOS as well.
Interesting post, Atmasphere.

It might have been yet more interesting had you noted that you yourself used the 6DJ8 and 6CG7 in earlier versions of the MA-1, and then explained how you later came to the realization that these tubes did not provide the best performance.
Hi Jimjoyce25- bingo! We got great linearity out of them, but they would often ring like a bell. Handpicking them got the microphonics down, but never to the point that you can get with a 12A* series tube or 6SN7. I'd always been a fan of the 6CG7, but when I realized that it and the 12AU7 were evolved from the 6SN7 geometry, I decided to try the 6SN7. There was no looking back at that point.