Tube question

[TommyJ]

I have a ?.
Whats the differance in a 12ax7 and a 12at7,and
can you use a 12at7 in place of the 12ax7?
Thank You..

[Kap'n]

12AT7 and 12AX7 tubes differ in a number of ways.

The one that most guitar folks care about is that a triode in a 12AT7 has an amplification factor of 60. A 12AX7 has an amplification factor of 100.

A 12AT7 also has a higher plate dissipation, which is why you see it used as a reverb driver (it makes a better power amp to drive the springs), and in places like phase inverters.

[NTBluesGuitar]

Kap'n, you have a way with words.

[TommyJ]

Thank You kap'n..

[JAM]

In guitar amps they're mostly interchangeable. You can try them in different locations and see what you like. Forget about technical considerations - irrevelant in guitar amps when your more concerned with what they sound like.

[Kap'n]

Forget about technical considerations - irrevelant in guitar amps when your more concerned with what they sound like.

Not really. If you throw a 12AX7 into a SF Fender 'Reverb' amp as the reverb driver, you'll burn it out quickly because it can't handle to idle current.

Plate dissipation matters, whether you're biasing an output section, or whether it's a preamp tube.

[JAM]

Depends on how it's biased, just like any tube.

[Kap'n]

Sure, but you can't just toss it in there and hope for the best, like everybody does with preamp tubes.

[toobalicious]

Sure, but you can't just toss it in there and hope for the best, like everybody does with preamp tubes.

that is a fact.

both parts, even.

besides, 12AX7 doesnt really *sound* good as a 'verb driver, and though it is recommended by some folks, IMO doesnt work so hot as a PI either. in typical fender circuits, that is.

a

[TommyJ]

Thank's guy's,
The amp I was going to try the 12at7 in is a
little supro amp that only has 1 pre tube,will it damage the amp,
or just lower the pre output?

[Kap'n]

that is a fact.

both parts, even.

besides, 12AX7 doesnt really *sound* good as a 'verb driver, and though it is recommended by some folks, IMO doesnt work so hot as a PI either. in typical fender circuits, that is.

a

that is a fact.

both parts, even.

OK, the first part is subjective. Well, maybe both are.

But they're facts anyway.

[Kap'n]

Thank's guy's,
The amp I was going to try the 12at7 in is a
little supro amp that only has 1 pre tube,will it damage the amp,
or just lower the pre output?

It shouldn't be a problem. In most cases, you can sub a 12AT7 for a 12AX7*. But only your ears can tell you if it sounds good.



*The only exception I can think of offhand is in an oscillator circuit, like the tremolo tube.