RE: 12AT7's ?

Tom Gorr · Post by **Tom Gorr** » 4 Oct 2004 5:38 pm

I have some different preamps for my electric guitar, and was wondering what effect substituting AT's for AX's would have in the different valve positions...What's the theory here?

I'm not suggesting wholescale switching of all the tubes, just 1 or some of them, at different positions.

eg. What effect on headroom, richness, smoothness during breakup, keeping lows tight, keeping brittleness down - etc...

Thanks !

Jennings Ward · Post by **Jennings Ward** » 4 Oct 2004 9:03 pm

Assuming that they are good tubes, ie, not gassy or microphonic, and they have the same [gm] on a quality tube tester, you should notice little to none difference. Quality should be same. Hope this satisfys your question.......Jennings

Play steel for the angelic sound of it.

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EMMONS D10 10-10 profex 2 deltafex ne1000 pv1000, pv 31 bd eq, +

Steve Alcott · Post by **Steve Alcott** » 4 Oct 2004 9:23 pm

You can substitute a 12AT7 for a 12AX7 in the first tube position in the preamp section-you'll have a bit less gain,but have less potential for overdriving the output tubes.I've done this in a Traynor YBA-3 that I use for bass,and it sounds great.

Dan Tyack · Post by **Dan Tyack** » 4 Oct 2004 9:25 pm

I suppose it would depend on your amp for this answer. In my THD amplifiers the AT7 is significantly cleaner (it distorts at a higher gain level). The other characteristics (e.g. glassiness, tone) are quite dependent on the particular tube (different makes sound a lot different. I use the AT7 instead of an AX7 quite a bit. The AT7 is right in between an AX7 and an AU7 in terms of clean gain.

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www.tyack.com

Jon Light (deceased) · Post by **Jon Light (deceased)** » 5 Oct 2004 1:04 am

Here is the chart on tube gain:

12AX7 / 7025 / ECC83 (100)

5751 (70)

12AT7 / ECC81 (60)

12AY7 (44)

12AV7 (41)

12AU7 / ECC82 (20)

jim milewski · Post by **jim milewski** » 5 Oct 2004 2:35 am

I think the 12AT7 being less voltage gain makes up for it in being able to deliver more current, they are usually used in driving reverb tanks and the power tubes

John Daugherty · Post by **John Daugherty** » 5 Oct 2004 4:04 am

The bottom line in laymans terms is: the 12AX7 has more gain than a 12AT7. The pinout is the same,so they are interchangeable in the same socket.........JD

Tom Gorr · Post by **Tom Gorr** » 5 Oct 2004 7:25 am

The AX's have "more gain" than the AT's...

Does that mean they are capable of a greater dynamic range (headroom) before breaking up? Or simply that they can deliver higher output voltages?

Conversely - Would I be hitting distortion sooner with AT's?

BTW - if AT's deliver more current as JM suggests (thanks!) - that should help me get a better impedance match between pre & power amps which should lead to a "tighter" sound...IS this correct reasoning? I've always wondered why some tubes give me a totally loose and unsatisfactory bottom end..Is this why???

John Daugherty · Post by **John Daugherty** » 5 Oct 2004 7:51 am

You can't supply current to anything!! Current is determined by the applied voltage and the resistance to which the voltage is applied ( I=E/R ). Don't get confused by thinking "current". The current drawn by the tube is hardly affected by the impedance of the device connected to the output. The current is set by the value of the plate load resistor,cathode resistor and grid bias and is normally set on a linear portion of the amplification curve to limit distortion. Preamps are VOLTAGE AMPLIFIERS. If you want to know if one tube sounds different from another in your application, I suggest the old "ear test". We do not need to go into technical jargon to choose a tube for a guitar preamp(bite my tongue..hahaha)Just pick-and-listen....JD

Dan Tyack · Post by **Dan Tyack** » 5 Oct 2004 8:57 am

How about this subjective assessment (leaving aside the technical discussion of gain, current delivered, etc.):

If I substitute an at7 for an ax7 in the preamp tube, I get a cleaner sound at an equivilent volume level. I do know that my amplifier (THD) was designed to be used with many different preamp tubes, so I'm not guarenteeing that this is the same for every amp.

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www.tyack.com

Brad Sarno · Post by **Brad Sarno** » 5 Oct 2004 12:57 pm

I'll use an AT7 or a 5751 (similar) on the input tube of my Twin. By taking the preamp gain down that way, it seems to insure that the preamp stays cleaner making the power section more likely to break up first. Generally makes the Twin run cleaner and less hard sounding.

Brad Sarno

Tom Gorr · Post by **Tom Gorr** » 5 Oct 2004 1:36 pm

Brad - that makes a lot of sense to me. I much prefer poweramp sponginess to the preamp chainsaw. More dynamics and more 'professional' sounding.

So - if I was to gain learnings from this - Changing the preamp buffer (V1) and the master output tubes to an AT would be a good bet from a theoretical point of view.

Now - I will change V1 and Vm - and do the old-fashioned "ear test" - now that I know what to listen for...

Still open to more help !

Tom Campbell · Post by **Tom Campbell** » 5 Oct 2004 7:06 pm

Can you just swap out tubes without addressing the bias setting. I was always under the impression if a different tube was substituted then the bias also changed (but not automatically). It had to be readjusted if the amp allowed for it.

Tony Prior · Post by **Tony Prior** » 6 Oct 2004 3:41 am

ok i'll chime in for what it's worth.

You can swap tubes that are in the same family..no operational issues.

Should you ? thats not for me to say.

You own an amp that was designed for optimal performance with the tubes that it came with.

If you are going to join the ranks of those that perform tube swapping do some research before you purchase the tubes and install them. It's highly possible you will spend the better part of a hundred bucks and NOT like what you end up with.

The Forums that we hangout in are a great resource to send folks the wrong direction. Just becasue I like something and brag on it means absolutely nothin' in the schem of things.

I recently made two changes at the same time for my HR Deville with one of my Tele's..BLAH..I hated it !

I replaced all the tubes with exact replacements except for preamp #1..I put in a recommended hotter Chinese tube or something like that..but I did also purchase the exact replacement Sovtek tube as well.Both 12AX7's..I also installed a Texas Special PUPs in the Tele'..I hate it..

I put the Sovtek back in the amp..played it with the Texas Specials..it was better but still not what I really wanted so I put the stock 52RI PUP back in the Tele' and guess what..i'm back..

Know what it is your doing and really have a good idea what it is your gonna end up with or you , like many, will have a whole box full of stuff that you paid for and will never use.

by the way the combination fo the Chinese ( Sino I think) tube and the Texas Specials gave way too much edge for the Tele..for my taste that is..the neck pup chords were just no longer clean..way too much grit. But I did it because others that I respect recommended it..it's good for them..but not for me..

just be aware that things can and will change..
maybe for the better..but maybe not..

This little Tele'/HR Deville excercise reminded me why I DON'T have a box full of tubes for the amps and PUPS for the Tele's...

t

[This message was edited by Tony Prior on 06 October 2004 at 04:47 AM.]

jim milewski · Post by **jim milewski** » 6 Oct 2004 4:04 am

for sure swapping things around doesn't give us what we want in performance often but none the less we learn something we didn't know before, I have stuff here that I'll never use that I thought would be a "step up".

Tom Gorr · Post by **Tom Gorr** » 6 Oct 2004 7:50 am

In general, one would expect that a preamp should be designed for the stock tubes...However, often there is equipment that just doesn't perform as it should.

Take my Mesa Boogie Formula Preamp, for example. If you read reviews on Harmony Central, you would conclude that it was the biggest botch job in the history of MB.

When I bought mine off ebay with the stock tubes, I may have agreed. A gross flabby unuseable sound.

I took out the stock boogie tubes, and after some trial and error, tentatively settled on the 'new' Sovtek AX7LP tubes for V1-V4, and a JJ ECC83S on V5. I can say that it is now my preferred preamp - giving Fender Blackface chime on the rhythm channel, Marshall crunch on LD 1, and classic Boogie on LD2. What more would a person want in a Pre?

However, I never invested *that* much in optimizing the sound. I figure I elevated the performance from 10% to 80%, but I'm sure it can hit perfection. Problem is, I don't understand much about tubes (I know the brand names, but thats about it...), especially how they interact with each other, etc.

I think it should be possible to deduce the 'best' tube schedule, based on knowledge of different tube character, and the internal circuitry of the preamp.

Wishful thinking?

Mark Herrick · Post by **Mark Herrick** » 6 Oct 2004 11:59 am

Can you just swap out tubes without addressing the bias setting.

You own an amp that was designed for optimal performance with the tubes that it came with.

You shouldn't have to adjust bias when swapping preamp tubes. If you swap POWER tubes you should adjust bias (and possibly replace the phase inverter tube) unless your working with something like a Mesa Boogie where you can't adjust the bias and are replacing power tubes based on Mesa's number/color ratings. (This info is based on what I have gleaned from this and various other forums. If I am wrong, someone will undoubtedly correct me.)

I think the fact that you CAN change tubes in amps is one of the beauties of these beasts. You can try different arrangements and see what you get. And the changes are pretty safe as long as you stay within the same tube family.

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Dan Tyack · Post by **Dan Tyack** » 6 Oct 2004 8:35 pm

I've got to agree with the sentiments of Brad and others versus preamp distortion versus power amp. (There are many other terms used, but that's really what's happening). If you use a lower gain preamp tube (especially the input tube), then you can crank the amp and get less preamp distortion, and grind the power tubes more, which gives more of a 'clean grind'. THis is the sound I look for not only for rock and roll/blues/whatever, but also for a clean country sound. If you have the power tubes cranking, you get that smooth natural compression happening (think of Brumley's sound on the Carnagie Hall record, or Lloyd on the Panther Hall record).

For my sound, it's all about minimizing the preamp distortion (=buzz saw) and using the power amp to get the sound you are looking for (minimal drive for clean, maximal for rock and roll).

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www.tyack.com

Tony Prior · Post by **Tony Prior** » 7 Oct 2004 12:59 am

Loyd at Panther Hall..Interesting that this was mentioned. And no doubt that many of us refer back to this as a "Lesson in what it should sound like"..

This past weekend at the N Tenn Jam I spent some time talking to Loyd and he shared the Panther Hall story with me..

He told me about the show, how it went down, all the little things that were going on with the promoters and Charlie, all the things that you will not find on the LP Liner notes.

But..here is what suprised me..He went to the show with his Sho-Bud..and asked that an amp be supplied for him. He requested a Twin with JBL's..and that is in fact what they sent. A local music store sent a brand new Twin with 2/ D120 JBL's..still had all the tags on it from the store.

Loyd told me that he was crazy for not buying that amp as he now reflects back on just how great it was.

He shared many things in conversation with me, this was just one of them..

He's an amazing man..not just for his musical talent..but the total package, quiet, soft spoken,polite, personal and respectful, and man he sure has a great book hidden deep down in his memory bank !

By the way..I couldn't resist asking him which of the LP's /recordings was his favorite..and yes..Panther Hall gets the nod..

so there it was..stock Twin with 2 JBL D120's..

t

[This message was edited by Tony Prior on 07 October 2004 at 02:01 AM.][This message was edited by Tony Prior on 07 October 2004 at 02:02 AM.]