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Old September 14th, 2009, 08:21 AM   #1 (permalink)
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12ax7a 12ax7 12ay7?

I know little to nothing about tubes. I have tried to research but only come up with 12ay7 are a little less "gainy". I have an amp that is running 12ax7a pre-tubes and even has the sockets labeled "12ax7a". Are ALL 12ax7 type tubes safe to put in there? I am a classic "F' with it to see what happens" guy and preamp tubes are relativly cheap. And also how much does it really affect the tone of the amp?

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Old September 14th, 2009, 08:26 AM   #2 (permalink)
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Kind of a broad question there.
Yes you can try the other 12A*7 types.
How it affects the tone is subjective based on the circuit and the position you try.
V1 being the first in the chain has potentially the most influence.
Try it and see.

What amp?

Last edited by pchilson; September 14th, 2009 at 09:34 AM. Reason: typo
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Old September 14th, 2009, 09:05 AM   #3 (permalink)
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Different gain factor............

12ax7/12ax7a..........................100
5751.......................................70
12at7......................................60
12ay7.....................................45


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Old September 14th, 2009, 09:07 AM   #4 (permalink)
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Well, all 12AX7s are interchangeable in guitar preamps. You will occasionally find tubes labelled 7025, which were military-spec, high-gain/low noise tubes, but in modern tubes means next to nothing. Different brands will all effect tone differently, but like beauty, it's in the 'eye' of the beholder.

Other tubes can be used in guitar preamps, as well. The 12AX7 triode (of which there are two per tube) has a gain factor of 100. The other tubes follow in this pattern:

5751--70
12AT7--60
12AY7--45
12AU7--19

There are lots of other tubes in the 12xx7 family, but I don't recall ever reading about any of them being much use to guitarists. There are a few out there which have one triode from one type and the other from a different type.

The 12AT7 and 12AU7 are driver tubes. They were not designed for audio amplification, as the X, Y and 5751 were, and are said to sound kinda muddy used in guitar amps, but there's virtually no risk of trying them out in a guitar preamp, so have at it. They ARE useful as Phase inverters and Reverb drivers, though--a common practice to try and tone down really gainy amps (eg-find some more headroom) is to substitute a 12AT7 as the PI and back off the gain hitting the power tubes a bit.

The first tube your signal passes through is the one which will have the greatest impact, so find a layout/schematic of your amp and someone will probably be able to tell you which one that is. In most amps it's labelled V1, but it's not ALL amps.
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Old September 14th, 2009, 09:19 AM   #5 (permalink)
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Originally Posted by pchilson View Post
Kind of a broad question there.
Yes you can try the other 12AX* types.
How it affects the tone is subjective based on the circuit and the position you try.
V1 being the first in the chain has potentially the most influence.
Try it and see.

What amp?
Yeah, broad question. I grabbed a bugera v22 and the gain channel is a little too fizzy/raspy? I just dial back the gain and push the volume a little to get the desired grit. I may try a 12ay7 and see what it does.
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Old September 14th, 2009, 09:39 AM   #6 (permalink)
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Without knowing I would bet that amp has some transistors in the preamp section. I don't know if changing the tube will help, maybe, maybe not. I would try it anyhow if I had them on hand just to know for myself. Probably the best bet is just what you are doing, use the gain knob...

What are you playing through it? Multiple guitars? Do you get the same "fizzy/raspy" on different guitars?

Could be you just have to dial it in for each instrument.

Someone on here a week or so ago post a recording of his Bugera V22. Sounded very good. Can't remember who but you should be able to search it easily.
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Old September 14th, 2009, 10:07 AM   #7 (permalink)
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Whats the difference between a 12AX7 and a 12AX7a ?
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Old September 14th, 2009, 10:46 AM   #8 (permalink)
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I've read a few folks who feel that tweaking preamp tubes is a waste of time in a MV amp, as you're probably just going to dial all that gain back out somewhere down the line in the circuit, and the same folks (all internet dudes, so they must be right!) doubt the practicality of using high-grade NOS tubes in high-gain modern circuits, again, as there is so much other stuff in the circuit that the advantages are dialed out. In fact, many high-gain guys prefer the Chinese 12AX7s, as they have shown to be some of the highest gain AX7s available on the market.
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Old September 14th, 2009, 12:31 PM   #9 (permalink)
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Whats the difference between a 12AX7 and a 12AX7a ?
Originally, it was the low-microphony, low-hum version of a 12AX7. Same thing as a 7025.
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Old September 14th, 2009, 05:00 PM   #10 (permalink)
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Same thing as a 7025.
Correction: the original 12AX7 could only be used in parallel filament circuits. The 12AX7A was an improved version that allowed it to be used in circuits where filaments were in parallel or series. Not a problem in most of our amps, as even the original, "limited"-use 12AX7 will work in most audio amp applications - certainly all modern amps (no series filaments).

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Old September 14th, 2009, 05:11 PM   #11 (permalink)
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I didn't word that clearly -- I meant that the 12AX7A was equivalent to a 7025.

That's interesting about the filaments. I thought the 12AX7 was designed for series-heater use from the beginning, as the current ratings never changed...

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Old September 14th, 2009, 06:12 PM   #12 (permalink)
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I didn't word that clearly -- I meant that the 12AX7A was equivalent to a 7025.

That's interesting about the filaments. I thought the 12AX7 was designed for series-heater use from the beginning, as the current ratings never changed...

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Well, the 12AX7A is technically not the same as a 7025. The 7025 was made with a spiral filament, as a low-noise variant. Granted, that distinction is not followed these days by tube companies that call their products whatever they want, as a sales gimmick.

There are early black-plate RCA 7025, from the '50s, that were produced before the 12AX7A was produced, so I wouldn't call them "equivalent." I would say that, in a parallel-filament circuit, the 12AX7A and 7025 are interchangeable.

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Old September 14th, 2009, 06:33 PM   #13 (permalink)
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Ah, I get it. Looking back through my tube manuals, RCA kept the 12AX7A and 7025 separate, even though they listed "controlled hum and noise characteristic" for both. I had thought that 12AX7A was the "civilian" designation for the 7025, an excuse to use two different labels on the same improved tube.

Thanks for setting me straight -- it seems that back when tube designations mattered , the 12AX7A was the upgraded 12AX7, but the 7025 was still the better tube.

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Old September 14th, 2009, 07:02 PM   #14 (permalink)
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the 12AX7A was the upgraded 12AX7, but the 7025 was still the better tube.
What about the 7025A?










Just kidding... we've taken this far enough.

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Old September 14th, 2009, 07:13 PM   #15 (permalink)
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I have read that the "a" suffix in non power tubes was origonally used to mean that the heaters were built to a higher QC level to permit series conection. I beleve that convention was not closely followed from the 80's on.
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