Why Are 12A*7 Tubes So Bad?

[TubeHead Johnny]

On this site I often read about how bad 12A*7, and other 9 pin miniature preamp/driver tubes out there. (mostly due to their non-linear properties). And worshiping most things octal. Most hatred seems to come from the 12AU7. If these tube are so bad, why did so many audio people from the "golden age", like McIntosh and Heathkit, use them?

~John~

[Psychobiker]

Cheap, easily available. They're primarily guitar tubes to drive overdrive circuits

L

[tubewade]

This is my opinion. Some will disagree.

I think it's not really that these are so bad, but that there are many times other valves that can do the job better. There were many regarded amplifiers of earlier days that used these valves and they sounded very nice, in spite of the ugly duckling valves, but upgrading them today can make them sound even better. Today, for new construction, it only makes sense to use what has been found to sound best in that particular function. And that usually is not a 12AX7 or 12AU7.

There are some nice sounding 9 pin small signal valves. Two that come to mind are 6FQ7/6CG7 and 12BH7. There are lots of mixed opinions on 6DJ8 and 12AT7.

Wade

[ashok]

IMHO you should listen to the preamp without looking at the tube type. Compare completed amp circuits rather than just the tubes themselves.
Some designers use certain tubes based on what they like and if the end result sounds good , so be it .

I don't think it make sense just to pull out tubes and put in others of similar type unless they are all working in the region the designer intended to. That would mean that some/most resistor values around it might have to change.

Want to compare tubes......... try them in their own competently designed circuits . Easier said than done !

Cheers.

[tubewade]

Ashok,

Well said!

[SY]

It is utter horsepoop promulgated by people who wouldn't know linearity if it bit them on their hindquarters. Or can't do a competent design and blame the tube.

Except the 12AU7 which really is nonlinear.

[tubelab.com]

I have always stated that there are very few tubes that are "bad", only nonlinear sets of operating conditions. Granted there are tubes that were not designed to be linear, like those that were designed for "gain controlled" (AGC) amplifiers. Most other vacuum tubes can be biased for very linear operation. Often those conditions are very different than the "commonly used" conditions, and often they vary considerably from tube to tube.

The 12AX7, 12AT7, 12AV7, and 12AY7 and their industrial offspring all have operating points useful for audio amplifiers. The 12AU7 might have a set of useful conditions, but I haven't found them yet. I am not saying that the 12AU7 is useless for audio (yet), but there are better tubes to consider (6CG7, 6FQ7, 12BH7, 7044, 5687, 7119 etc) with similar gain specs.

[kmtang]

What's good or bad?? I truly believe it's your personal choice.

Through A/B comparison, it would be easy for anyone to pick the one they like.

I am glad we are the DIYers that we could build some amps which we could spend as much as we can for the best result.

My friend built 9 Marantz 7C line amps with different components and architeture. He is going to build the last one that he call it the ultimate version M7.

Last night, my friend invited me to listen to a rather famous Japanese factory made line amp and compared to his clone Marantz 7C. Surely, I would say the DIY M7 is way superior.

12A*7's are bad??? I would say hamburgers are great!!!!

Cheers,
Johnny

[MartyM]

This forum is great. I have learned to appreciate rare things like diyAudio.com while they're here-never it take for granted.

Back on this subject:

Is there somewhere perhaps you can refer me to for finding drop-in replacements for tubes? As an excellent example, perhaps for the 12AX7?

Thanks a lot.

[Psychobiker]

Duncan Amps has a fantastic Tube Datasheet Locater with substitiute parts etc etc. It's apples and oranges really - some like them, some dont. All I've seen them do really well in is guitar amps/distortion based circuits