12AX7? 12AY7?

[ye olde fretmonkey]

Do I understand correctly that a 12AY7 preamp tube will interchange with a 12AX7, but yield a cleaner tone?

[Tim Armstrong]

The other tubes like 12AT7 and 12AU7 are lower gain than a 12AX7. Here's the gain factors for some members of that family of tubes:

# 12AX7 (gain of 100)
# 12AT7 (gain of 60)
# 12AY7 (gain of 45)
# 12AU7 (gain of 19)

So using tubes other than a 12AX7 will give you less gain, hence less distortion. Much like turning down your guitar...

Cheers, Tim

[zombywoof]

Yup, will reduce gain. It is the Preamp which shapes the Tone!!!!
You could also do other stuff, even mix em up a bit. As example, using 2 x 12AT7 in 2 & 3 will increase the clean headroom. There are numerous possibilities.

[Dacious]

There's actually a little more to it than that. If you go swapping lower gain preamps in, as Tim says, to some extent it's like turning down the volume on your guitar, or using lower-output pickups. If the amp clips early, it may stay slightly cleaner up the dial because the preamp tube is not clipping so early. If the amp is clean all the way up the dial to start with, it may just lose some volume.

All these different preamp tubes can have been developed for different things. As well as different gains, they can use different cathode bias caps and grid resistors. That not only produces different gain or volume levels, but also different plate capacitances and resistance will produce different responses to frequencies. You might find a certain tube causes susceptibility to feedback at frequencies the amp is normally stable at.

You should do no harm by swapping in lower gain tubes in the channel posis. But a 12AU7 will sound markedly different in most amp preamp slots - not just like a 50% volume cut, but perhaps also pretty 'dead' sounding in a slot with 12AX7 bias values due to it's different nature. You have to bear in mind what is in front of it, and what is behind it.

I would personally not put a low-gain preamp tube in a phase inverter position intended for a high-gain tube or vice-versa, as you can't be sure today why the amp designer chose that tube to start with. Not without knowing a lot about the amp. Going from a 12AT7 to an AX7 or vice-versa is not a big jump in some respects, but could still have adverse impact.

[zombywoof]

Excellent points provided by ya'll. Yeah, your sound will vary depending on the type of tube, manufacturer (translation-quality) and amp circuit design as well as pickup output. The 12AX7 is high gain and a pretty clean sounding tube. A 12AY7 in the V1 or V2 position, while low gain, might help quiet down a noisy amp or possibly clean up feedback issues. But soft clipping could become a problem. A 12AU7 gives a bit rauncher sound while a 12DW7 tends to lie between the AX and AU and so on.

Point is, to not be afraid to do a bit of tube rolling. There are some excellent manuals available on tube substitution. And as always, if you are not sure what you are doing, check with a tech.

[Stan Martin]

gain factor of 70. An AT7 as a V1 tone tube kind of blows. My opinion. The 5751 is a real good choice to knock off some gain without killing your tone.

[Big Mike Simpson]

Any thoughts on the cross referencing of tubes?
Are they actually the same?
12AX7 =-->7025 =--> ECC83S (JJ)
12AT7 =-->ECC81(JJ)

Any opinions on whether a NOS RCA or Mullard preamp tubes would have a precievable difference in tone over a current manufacture JJ/Tesla tube?

Anyone have a brand preference?

I play through BFSR's mostly and have JJ tubes in my 65. I have also used some NOS phillips preamp tubes and tried Sovtek and Svetlana 6L6's.

[Tremo]

Originally Posted by Big Mike Simpson

Any thoughts on the cross referencing of tubes?
Are they actually the same?
12AX7 =-->7025 =--> ECC83S (JJ)
12AT7 =-->ECC81(JJ)

Any opinions on whether a NOS RCA or Mullard preamp tubes would have a precievable difference in tone over a current manufacture JJ/Tesla tube?

Anyone have a brand preference?

I play through BFSR's mostly and have JJ tubes in my 65. I have also used some NOS phillips preamp tubes and tried Sovtek and Svetlana 6L6's.

Yes, ECC83 = 12AX7 = 7025. Etc....

ECC83 is the Euro number for the identical spec. 7025 was the "low noise" version back in the day, but today it makes no difference. 6681 I think was a ruggedized version. 12AX7A had controlled heater warmup time.

For what sounds good, that's very subjective, and only YOU can determine what sounds best to YOUR ears. My suggestion is to buy up a good variety selection from the various dealers and try them yourself and decide for yourself what you like. What sounds good to me might sound like crap to you. You get the idea.

[JohnnyCrash]

It's not really going to give you any "extra" headroom.

It'll be similar to changing to less hot pickups.

If you got the cash to buy an extra set of tubes - give it a shot... never hurts to try.

[Scott S]

Originally Posted by zombywoof

A 12AU7 gives a bit rauncher sound while a 12DW7 tends to lie between the AX and AU and so on.

Technically speaking, a 12DW7 has two different triodes: one has the 12AX7 spec, the other a 12AU7.

Most of the generalizations above work for preamp tubes, but there are good reasons to install a "low-gain" tube as the phase invertor. If your current phase invertor tube (let's say a 12AX7) is clipping, replacing it with a 12AT7 will give it more headroom (2.5 watts max dissipation vs 1 watt). Just the thing to move the distortion downstream to the power tubes...

Mu alone doesn't tell the whole story.