How do I know which 12AX7's and when to replace?

Talk about tubes and their effect on your tone

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1mb
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How do I know which 12AX7's and when to replace?

Post by 1mb » Tue May 01, 2012 3:21 pm

I have a 5:50 Express. Are JJ's good? Should I just use Mesa branded 12AX7's? How do I know when preamp tubes need to be replaced? What is the difference between regular and long plates? Should preamp tubes be matched (I see some sellers advertise matched).
I have been told by an amp guy from Peavey that the first one in the chain gets the most wear. He said to replace the first one and rotate the others forward occasionally. Does that make sense to anyone and does anyone else do this.
Thanks

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Re: How do I know which 12AX7's and when to replace?

Post by topcat0399 » Tue May 01, 2012 3:27 pm

Rotating them like that is a method,

What makes you think you need pre-amp tubes? Pre-amp tubes last a looooong time. Decades even.

What I did was start a collection of NOS tubes and new tubes and changed them one by one until I found the one being the pain.

Turned out it was a power tube....
Me -------> 1973 Gibson Les Paul Standard------->1991 Mesa Boogie Mark IVa/b Combo -------> World.

1mb
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Re: How do I know which 12AX7's and when to replace?

Post by 1mb » Tue May 01, 2012 3:31 pm

What makes you think you need pre-amp tubes?

I bought the amp used. I have new power tubes and wanted to start off with new preamps too. My amp has Mesa's and JJ's in it.
I wouldn't know what bad preamp tubes sound like.

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Re: How do I know which 12AX7's and when to replace?

Post by topcat0399 » Tue May 01, 2012 3:39 pm

I wouldn't know what bad preamp tubes sound like.


Thats why you need known good spares and start rolling them through the amp to see what changes happen (if any).
Me -------> 1973 Gibson Les Paul Standard------->1991 Mesa Boogie Mark IVa/b Combo -------> World.

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Re: How do I know which 12AX7's and when to replace?

Post by benjamin801 » Tue May 01, 2012 3:40 pm

JJ tubes are good. I mean, opinions differ, of course, but they are generally well-regarded tubes. I personally like them and use them a lot.

As far as knowing when you need new ones, preamp tubes tend to last a long time, then fail. When they fail, you may have crackling, loss of volume, volume that dips and swells, lack of gain, or sometimes the amp will refuse to make any sound at all.

"Matched," when referring to preamp tubes, means the two triodes within the tube have roughly the same amount of output. Some people like matched tubes in the phase inverter position (i.e. last preamp tube before the power section). Having tried it both ways, I don't think it makes a noticeable difference.

1mb
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Re: How do I know which 12AX7's and when to replace?

Post by 1mb » Tue May 01, 2012 5:13 pm

This is great so far. What about the long plates. What's that all about? There seem to be two types of 12AX7's and I think I both two types. I'm glad I kept my old tubes because so far you guys have me thinking there is nothing wrong with them.
I think I have GrooveTubes, JJ's and maybe a couple Sovteks and a Mesa. Can I mix them all?

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Re: How do I know which 12AX7's and when to replace?

Post by benjamin801 » Tue May 01, 2012 5:29 pm

1mb wrote:This is great so far. What about the long plates. What's that all about? There seem to be two types of 12AX7's and I think I both two types. I'm glad I kept my old tubes because so far you guys have me thinking there is nothing wrong with them.
I think I have GrooveTubes, JJ's and maybe a couple Sovteks and a Mesa. Can I mix them all?
You can mix them however you like as long as they're all 12AX7 (keep in mind that European makers will call them ECC83; same tube). You will find differences in sound as you swap them in and out: some subtle, some possibly significant.

Long plate 12AX7 tubes sound great in a lot of applications, but due to simple physics, they will be more prone to microphonics than short plate tubes. This means that if you use them in early gain stages of high gain amps, especially combo amps where the tubes are often inches away from a screaming speaker, you will probably experience feedback.

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Re: How do I know which 12AX7's and when to replace?

Post by domct203 » Tue May 01, 2012 5:38 pm

Welcome to the forum.

You can mix and match any 12AX7 or 12AX7A type tube in the amp. I use a 1950's GE LongPlate in my tone stack driver (V3) of my Roadster and find it had the best low to low mid response for me, while a ShortPlate RFT is my favorite in the later gain stages (V2). Try a few different 12AX7's in V1 where the tube has more influence on the tone early in the gain stages of the preamp.

The current Mesa 12AX7 is a re-labeled JJ.

IMO the Mesa STR-440 6L6GC is a good tube, the SED =C= 6L6GC is my favorite.

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Re: How do I know which 12AX7's and when to replace?

Post by swbo101 » Wed May 02, 2012 8:11 am

domct203 wrote:IMO the Mesa STR-440 6L6GC is a good tube, the SED =C= 6L6GC is my favorite.

Dom

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Re: How do I know which 12AX7's and when to replace?

Post by 212Mavguy » Mon May 07, 2012 8:57 am

@benjamin801 as well as others here...

Most of the time guitar amps are built with PI sections that circuitwise are inherently imbalanced. It's not just the PI tube that needs to have balance, it's a pair of sides that each have four components, each complete side can match the other, but rarely is it done. When they do, the result is increased sustain and harmonic bloom within sustained notes. Each side of the power section contains a PI tube triode, it's circuit, a power tube, and an output tranny primary winding.

Starting from the back, the primary side pair of leads from the OT often have unbalanced windings due to the way transformers are often made...the power tubes driving those primaries are often out of balance as well, the circuit driving them is most of the time wired out of balance, and so do you think that if the PI tube is perfectly balanced between triodes that everything is gonna be all right? Nope. But if you get the imbalances working in the right directions, you can balance the power section of the amp better than before you started.

In a two tube push pull power section I will set the amp at max clean headroom, master volume in the top end of performing stage volume sweet spot if you know where that is, otherwise dime it but keep gain so amp is still clean. Then I stick a guitar on a stand and pluck some open strings and let the sustain ring out to just about silence over the amp noise. I listen to all the strings alone and in pairs. Then I put the amp on standby and hot swap swap the power tubes in each other's sockets. A chunk of old auto or truck tire inner tube helps grip the bases, your fingers don't get too burned that way. I leave the power tubes in the position that gives the best sustain. Easier to do amp chassis removed for tube rolling. Then from my collection of preamp tubes I select preamp tubes tested for relative strength between the sides for the PI spot, you start hot swapping and listening same as before, use the standby switch, you might find out that a certain imbalance within that PI tube in a certain direction, one side or the other might provide the best sustain and harmonics.

Your ears as well as others' will thank you.
212Mavguy

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Re: How do I know which 12AX7's and when to replace?

Post by benjamin801 » Mon May 07, 2012 9:13 am

^^^ That's fantastic info, thanks! (For hot tube pulling, I actually use one of my wife's little silicone mitts she has for pulling hot baking sheets out of the oven.)

Also, is it just me, or is Utah surprisingly well represented on this forum for such a low-population state? :lol:

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Re: How do I know which 12AX7's and when to replace?

Post by 212Mavguy » Sat May 12, 2012 8:39 am

HAHA! So you know Mike at House of guitars on 3rd west??
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Re: How do I know which 12AX7's and when to replace?

Post by benjamin801 » Sat May 12, 2012 8:55 am

212Mavguy wrote:HAHA! So you know Mike at House of guitars on 3rd west??
I couldn't say I know him per se, but I've certainly been in there before. :)

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Re: How do I know which 12AX7's and when to replace?

Post by 212Mavguy » Sun May 13, 2012 7:06 am

He's Mesa Certified and has the nasty habit of undercharging his costomers. I have to tip him out in cash for him to get paid a fair amount for his work and that's what I tell all of my friends to do when he helps them... Great guy to say the least.
212Mavguy

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