12AT7 vs 5751

atomheartmother

Senior Member
Messages
594
My LP with hums, semi-hollow with P-90's, and my wah with any guitar seem to overdrive my amp too soon with the JAN GE 5751 in the preamp. The 5751 did make a marked improvement over the two 12AX7's I've used. My amp is an Epiphone Galaxie 10. Would putting in a 12AT7 give me more clean headroom? Any suggestions?

I'm looking at this 12AT7 right now:

http://tubedepot.com/jj-ecc81.html
 

Rich M

Member
Messages
422
For a couple bucks more, you could buy a Mullard CV4024 (military 12at7) which is a killer tube.
 

littlemoon

Member
Messages
857
Originally posted by Old Tele man
5751 is essentially a "ruggedized" 12AX7WA...totally interchangeable...5751 has mu (gain) of 70; 12AX7 has mu of 100...otherwise identical.

12AT7 is slightly different animal...it has different gm, rp and mu values than a 12AX7...
I believe the 12AT7 has a mu (gain value compared to 12AX7) of 60; whereas the 5751 has a mu value of 70.

The only way to know if the 12AT7 will work for your situation is to try one. Theoretically, the 12AT7 will give you marginally more headroom, but your amp may respond to the tube differently than you might expect.

littlemoon
 

Blue Strat

Member
Messages
30,194
Originally posted by atomheartmother
Respond differently how?
Sonically, you won't know until you try one.

The problem with comparing gain numbers (u) for various preamp tubes is that they assume that you're using the particular tubes in the CORRECT circuit for that tube.

12AX7s and 5751s operate correctly in the same circuits with the same cathode bias resistor value. 12AT7s do not.

Looking at circuit operation on a scope, a 12AT7 in a 12AX7 circuit actually breaks up sooner (lower headroom) because the plate voltage is lowered due to different internal resistance of the tube. The same thing happens when using a 12AX7 in a 12AT7 circuit. People who've tried this swap in the PI of a Fender amp know what I'm talking about.

Interestingly, 12AT7s used in 12AX7 circuits often SOUND cleaner. This could be due to the fact that 12AT7s tend to be brighter and hence "cut through the mix" better.

What's the bottom line? Until you hear various tubes you'll have no idea what they sound like in YOUR rig and with YOUR ears.

If there's one thing I've learned in processing over 9,000 tube orders it's that there's no consensus on tone and no one can predict what anyone else will hear.

BTW, here's another vote for the Mullard CV4024.
 
L

Leftee

Ditto Mike's excellent post. Seat of the pants observation: I like the 5751 a whole lot more in a 12AX7 socket than I do a 12AT7. To my ears, the 12AT7 always sounds bright and thin. It seems I never care for where it leaves the amp.
 

Kiwi

Silver Supporting Member
Messages
4,080
I'd guess that changing a 5751 to a 12AT7 would not make that much difference. As noted, it's a gain factor of 60 vs. a gain factor of 70.

I think you're overdriving your amp quickly because you're playing humbuckers and P-90s into a 10-watt amp.

Lower-output pickups (since as single-coils) or a higher-powered amp are your two most probable solutions.

Kiwi
 
L

Leftee

It will make more of a difference than you might think. Electrically a 5751 is closer to a 12AX7 than it is a 12AT7... gain factors aside. See Mike's post above (Blue Strat).
 

Swarty

Member
Messages
1,131
A bit lower gain would be a 12AY7 (gm=40), which is another excellent audio tube (like a 5751 or 12AX7).
 

atomheartmother

Senior Member
Messages
594
Well, I bought a Mullard CV4024 from a fellow forumite. I hope it does the trick and doesn't adversely affect the tone. The amp is actually fairly bright as is, so I hope it doesn't make the tone brittle. Oh well, worst case scenario is I'm out $14. Thanks for the help, fellas.
 
L

Leftee

Hey Swarty!

Yeah, the 12AY7 is a great tube as well. I ran one in V1 of the last Boggy with great success. Fat, rich tone.
 

Jon Silberman

10Q Jerry & Dickey
Silver Supporting Member
Messages
42,079
OK, and now for something completely different ... and DON'T LAUGH!

A cheapo Chinese new stock 12AT7 can sound wonderful in V1 to reduce gain and bottom end while retaining (or ehancing) highs. I wouldn't use 'em for any other purpose but for some reason they have worked well for me this way in the past.

I believe Ruby's 12AT7 will work in such applications, too (not sure where they're made but they seem to be a good choice to reduce overdrive/increase perceived headroom).
 

Blue Strat

Member
Messages
30,194
Understanding all the terminology and technical facts may be interesting (yawn) but virtually useless.

Your ears are the deciding factor. No one else's tonal opinion or description is valid.
 

John Phillips

Member
Messages
13,038
Originally posted by Leftee
To my ears, the 12AT7 always sounds bright and thin.
+1.

'Cold' and 'gutless' would be two other terms I'd use to describe them - in a gain stage, anyway. I really like what they do in place of a stock 12AX7 in a lot of phase inverter positions - especially Mesa amps, but not Marshalls though.

You could also try a 12AU7. That's a very much lower gain tube (20), but actually sounds quite nice and full. I was recently messing about with a little Peavey Classic 20, which is ridiculously hot with the stock 12AX7 (no clean sound above 2 on the volume control, and very buzzy when right up, even though it's qute fun) but sounded much nicer - more glassy and open, and no breakup until about 7 or 8, quite Fendery - with a 12AU7.

But, like Mike says, use your own ears! Preamp tubes are cheap enough to try several types.
 

Jon Silberman

10Q Jerry & Dickey
Silver Supporting Member
Messages
42,079
Originally posted by Leftee
To my ears, the 12AT7 always sounds bright and thin.
+2 ... and that's EXACTLY why it works so well for V1 in the right applications.
 

Jon Silberman

10Q Jerry & Dickey
Silver Supporting Member
Messages
42,079
Originally posted by John Phillips
You could also try a 12AU7. That's a very much lower gain tube (20), but actually sounds quite nice and full. I was recently messing about with a little Peavey Classic 20, which is ridiculously hot with the stock 12AX7 (no clean sound above 2 on the volume control, and very buzzy when right up, even though it's qute fun) but sounded much nicer - more glassy and open, and no breakup until about 7 or 8, quite Fendery - with a 12AU7.
I achieve similar results with a 12DW7 in V3 of my Tone King Continental's lead channel.
 

Blue Strat

Member
Messages
30,194
Originally posted by Old Tele man
re: "Understanding all the terminology and technical facts may be interesting (yawn) but virtually useless."

quote: "When you can measure what you are speaking about, and express it in numbers, you know something about it. But when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind." -- Lord Kelvin, Popular Lectures and Addresses, 1889.

Granted, two people may not be able to talk about something as esoteric as "sound," but they can compare and contrast quantified "numbers" representing sound!
...for what it's worth.

Speaking of numbers, any numbers to explain the differences in tone between various brands of the same tube type?
 




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