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Old 26th April 2009, 08:15 PM   #1
flysig is offline flysig  United States
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Default 12A_7 vs 6SN7EH vs EF86?

The 12AX7 gets a bum rap for preamp service, though it seems to be the favorite in commercial use for guitar amps.

How do the 12A_7 tubes compare to the 6SN7 or an EF86 in triode mode? I'm looking for low distortion and very low noise. Low microphonics are a plus but not more important than low noise.

Future availability of replacement tubes is a concern. I don't want to get into something that is sole sourced or that is likely to go out of production or become crazy expensive in the future.

I'd like to get going on specifying the design and buying some parts! This will be the preamp for an AB1 class amp using 6V6 output tubes.
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Old 26th April 2009, 08:51 PM   #2
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12AX7's don't really get a bum rap. But compared to a 6SN7 it will have considerably more gain given circuit topolgies are similar. In order to get gain down in a line stage more feedback would need to be applied, or some kind of pad. Neither is a good option. 12AU7's are closer to the 6SN7
One other aspect of a 12AX7, is that it is not good for cathode follower use or any other area requiring low output impedance and current drive.
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Old 26th April 2009, 08:52 PM   #3
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Quote:
How do the 12A_7 tubes compare to the 6SN7 or an EF86 in triode mode?
How does a pickup truck compare to a convertible? Or a 32 tooth 6" diameter gear with a 128 tooth 2" diameter gear? These are all very different tubes with very different uses. What is it you're specifically trying to do?
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Old 26th April 2009, 10:07 PM   #4
flysig is offline flysig  United States
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I'm trying to build an amp for an acoustic guitar. So it is more of a hi-fi tonality rather than the mid heavy electric guitar amp. Low noise, low distortion, and a wider flatter frequency range than your basic Fender or Marshall style amp. Low end rolloff at 40 hz or so, high end rolloff above 10khz.

The plan is to have two channels, one for guitar and one for a mic. For each channel either paralleled triodes (12A_7) for the first stage of the preamp, or one EF86, running into a Baxandall tone stack., then a second preamp tube for gain makeup. Then mixing the channels into a phase splitter and finally on to a class AB 6V6 PP output stage. Some negative feedback from the output transformer to the PI.
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Old 26th April 2009, 10:31 PM   #5
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OK, that's a bit clearer. For your front end tube, you want lots of gain, so the 12AX7 makes sense. Noise will be quite low. Microphonics will be more a function of tube selection than anything else. The two cautions are:

1. High input capacitance. This may or may not be a problem with your pickups, but it ought to be considered.
2. High output impedance- you'll have to watch what load the 12AX7 drives since it's rather weedy in this respect. Buffering might be needed.
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Old 27th April 2009, 01:28 AM   #6
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Quote:
For each channel either paralleled triodes (12A_7) for the first stage of the preamp, or one EF86, running into a Baxandall tone stack
If by a Baxandall tone stack you mean a James tone stack (sometimes called "passive Baxandall"), then you need a low output impedance. IMHO, either 12AX7 or EF86 would need a buffer to drive the tone stack satisfactorily.
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Old 27th April 2009, 02:10 AM   #7
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Cheaper options are 6N2P, 6J1P
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Old 27th April 2009, 08:06 AM   #8
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Quote:
Originally posted by flysig
I'm trying to build an amp for an acoustic guitar.
How about totally forget a triode input stage and use an EF86 or 6EJ7/EF184.

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