12AX7 vs. 12AT7

[westondc]

So I'm looking for a bit more clean headroom in my (1) 12AX7 and (1) El84 amp. At a high volume level it's about perfect for my sort of jammin but when I put my TS9 and Fuzz in front the distorsh is too much. Will I lose volume if I put a 12AT7 in the preamp?

[Dana-L]

In my experience, there's not really much you can do to truly increase an amps 'clean headroom' (here defined as the highest sound pressure level you can achieve before distortion).

Discounting the speaker, clean headroom is mostly determined by the type of power tube you're using, the voltage you're running it at, and how it's biased.

Swapping preamp tube types can have a huge impact on your tone, but all it does for clean headroom is change how far you can turn the volume knob up before you get distortion.

For example, running a 12AY7 in place of a 12AX7 in my Fender Champ lets me turn the volume knob up to 10 without much distortion - but it's only about as loud as it is with the 12AX7 with the volume set at about 4.

Cheers,

-Dana

[westondc]

thanks for the info. kinda what I was thinking.

[Phil_S]

So I'm looking for a bit more clean headroom in my (1) 12AX7 and (1) El84 amp. At a high volume level it's about perfect for my sort of jammin but when I put my TS9 and Fuzz in front the distorsh is too much. Will I lose volume if I put a 12AT7 in the preamp?

Speaker can have a big affect. Try something else.

Also, try raising the value of the cathode resistor. Assuming you have a 125 or 130 ohm in there, try 180 ohms

[Franky]

Tone (using 12at7/12au7)
An advantage of using 12at7 or 12au7 is that the preamp hasn't that much gain anymore
You have to turn your vol. pot up to get the same volume as before (you all know that)
Here's the point
--> turning the vol. pot up means less resistance to the grid and more brilliant tone
--> your power tubes have to work more to achieve the same volume (different sound)

Summing up you get different tone when using 12at7/12au7.
(And these tubes sound different, of course)

Clean headroom (using 12at7/12au7)
You need to drive the power tube to it's limit where it still sounds clean and loud, of course with the preamp section being clean, too!
!!!So if your preamp tubes start to distort before your power tube starts to go into overdrive!!! you'll need to do something about your circuit / tubes.

For this case I recommend less preamp tubes and/or lower gain tubes in preamp section (5751, 12at7, 12au7)

But I have to agree with Dana-L that there can usually not be done that much about clean headroom by changing tubes; but it depends on your circuit.

Try it yourself since 12at7/12au7 aren't that expensive and you'll never know what the effects of using these tubes (12at7, 12au7) is till you tried it.

[DonR]

Along with 5751's, try 7189's or the russian military 6b14n power tubes-you can run a bit more b+ to them, and that will get you some more clean head room. A red Fang will help as well.

DonR

[zaphod_phil]

An advantage of using 12at7 or 12au7 is that the preamp hasn't that much gain anymore

The classic 18W preamp has very little gain in the first place. With just a single amplification stage before the PI, it's one of the lowest-gain preamps out there. Most of an 18W's distortion comes from the power stage. To increase the "perceived headroom" of an 18W, the usual trick is to reduce the two 470k grid leaks on the EL84s to 220k. (Edit, the resistor value was missing)

Summing up you get different tone when using 12at7/12au7.(And these tubes sound different, of course)

And they both bias differently than a 12AX7. Also many 12AT7s (including most NOS ones) have been shown to put out significant levels of odd harmonics, including some higher order ones, like 9th harmonics. This can make them sound pretty blah, as I've found in some of my own tests. If you want to use lower gain tubes in the preamp or PI, I would suggest using 5751s instead. They bias the same as 12AX7s, and have a nice fat tone.

You need to drive the power tube to it's limit where it still sounds clean and loud, of course with the preamp section being clean, too!
!!!So if your preamp tubes start to distort before your power tube starts to go into overdrive!!! you'll need to do something about your circuit / tubes.

Like I said, in the 18W most of the distortion comes from the power amp (unless you've got a TMB 18W), and the power amp always distorts first. Using lower gain tubes in the preamp and/or PI just delays the onset of power amp distortion, as you turn up the volume control. Reducing the grid leak resistors also does the same thing.

HTH

[Franky]

So I'm looking for a bit more clean headroom in my (1) 12AX7 and (1) El84 amp.

What kind of amp are you talking about to make things easier?

I could never archieve to get my 12at7 into heavy distortion therefore it's good to know that they aren't very muscical in this area.

[marshman]

I also have a (1) 12AX7 & (1)EL84 amp that I like. And having experimented a lot with tubes, I have found that running an old 12AU7 in there gets me the tone I like most. There is truth that in general it just moves the point at which distortion sets in further around the dial ( a 12ax7 sets into distortion @ 4 on the knob, the U sets in @ , but the 12AX7s just seemed extra fizzy to me.

I was told that the tube I went with is an RCA blackplate triple-mica-something-or-other 12AU7. All I know is that is has a little 5814A in an octagon on it.

The 84 was supposed to be Tung-Sol old stock, as well, though it's unmarked.

Thinking about it, I don't really care as it sounds sooo killer in it's current home. There's still no headroom to speak of with my Les Paul, but my tele just drips goodness in this set up. I love this SE thing...I can pick up any old unmatched hamfest tube and try it out without all that matching stuff to worry about. I just need to build one that will take octals and I might be done for life.

[westondc]

So I'm looking for a bit more clean headroom in my (1) 12AX7 and (1) El84 amp.

What kind of amp are you talking about to make things easier?

I could never archieve to get my 12at7 into heavy distortion therefore it's good to know that they aren't very muscical in this area.

I'm using a a EVJ drop in board with the Bad Cat Mini mod I've done. Getting a taste for the tube amp building hobby... while on a budget. Plan to experiment with tubes and such and decide on the particular sound/tone I'm looking for. I really like the tone I'm getting from this but I might need something closer to 10-15 watts. Can someone give me a rundown on differences in power output tubes like their tone, power and such?

[bdlrelo]

I have found that running an old 12AU7 in there gets me the tone I like most. There is truth that in general it just moves the point at which distortion sets in further around the dial ( a 12ax7 sets into distortion @ 4 on the knob, the U sets in @ , but the 12AX7s just seemed extra fizzy to me.

i totally agree! i bought a JJ EC83S tube that, as i was told, was a great preamp tube. well, since i built my Lite IIb out of an old Hammond organ i had a lot of extra tubes lying around: 12AX7s, 12AT7s, 12AU7s, 12AY7s... so i started trying new ones out and holy crap! that old RCA 12AU7 sounded amazing! with the EC83S in there everything sounded too fizzy, great high gain sounded, but just a little fizzy. with the 12AU7, it sounds AMAZING now! i'm definitely going to experiment more with preamp tubes now that i've had such great results.

[Franky]

I've build a EL84 SE power amp only (my 18watt project isn't finished).
Yes, it's pretty cool that you can easily swap tubes. I got a very cheap Telefunken EL84 output tube .

I know, that you can change EL84(12watt) to 6V6(14watt) tubes without having to change OT impedance (6V6 4kohm to 5kohm <--> EL84 4kohm). 6V6 are fatter sounding is what I hear a lot.
It would install a bias pot to find the right biasing for a certain tube (also nice to get different tone).
That's what I gonna do with my shinrock 18 watt RadioSpares OT (6V6-EL84).

My EI 12at7 sound to thin, so I thought it's a problem that all of these tubes have.
bdlrelo and westondc, your tubes sound just great; I wonder if yours sound thiner than normal tubes, too. Or may it be that I'm simply used to the fat GE5751 tone???

Unfortunately I don't know the EVJ circuit and couldn't find usefull info about it. Maybe you can post a ciruit!?

[marshman]

EVJ= Epiphone Valve Junior. Plenty of copies of that schematic floating around. I modded mine a few months ago and failed to properly record the changes, so I've no idea what I really did at this point. Mine was an early model, and I do know I added a few filter caps, removed one resistor and changed the value on a few others,as well as replaced a few caps with Orange Drops. I also added a big honkin power resistor in there for some purpose I can't recall. As I said, I'm not really worried about it, I'm enjoyin' the hell out of it.

[marshman]

EVJ= Epiphone Valve Junior. Plenty of copies of that schematic floating around. I modded mine a few months ago and failed to properly record the changes, so I've no idea what I really did at this point. Mine was an early model, and I do know I added a few filter caps, removed one resistor and changed the value on a few others,as well as replaced a few caps with Orange Drops. I also added a big honkin power resistor in there for some purpose I can't recall. As I said, I'm not really worried about it, I'm enjoyin' the hell out of it.

[Pressed_Rat]

Well, I just tried this last night and it was too late to really crank it, but..

I had recently tried a 12AX7 as the first tube in a 5F6-A clone - seeing how it would sound. Much higher gain than the original 12AY7, but not very good IMHO, too gritty gnarly, etc. So I then modified the first stage cathode RkCk to that of 1987. Really nice sound now with the 12AX7, and each channel has more divergent tonal properties - which is why I assume Marshall did this.

Back to the trials last night. Out of curiosity, I swapped back to the 12AY7. Lower gain for sure, but I got back more of the original Bassman tone; might have been the highs. I'm still evaluating this change, but I really like it. Cool sound . Sort of a cross between a 5F6-A and a 1987. Note that I do not have aggressive Celestions, etc. Running some Legend 105s in a 4 x 10" cab.

Man, this is really fun .

[zaphod_phil]

I really like the tone I'm getting from this but I might need something closer to 10-15 watts. Can someone give me a rundown on differences in power output tubes like their tone, power and such?

You would need the 125ESE OT and a bigger power transformer to run the bigger octal tubes at higher power. Have a look at the P1 eXtreme on AX84.com to see how it's done.
EL84 5W bright tone
6V6GT 6W warm tone, good crunch
6L6GC 14W very clean
EL34 12W warm tone, good crunch
KT66 11W warm tone with some edge
KT88/6550 19W warm and very clean

You can run most of these tubes with the stock VJr PT, but you'll only be getting about 8W or 9W from the bigger ones.

EVJ= Epiphone Valve Junior. Plenty of copies of that schematic floating around. I modded mine a few months ago and failed to properly record the changes, so I've no idea what I really did at this point.

It's a badly flawed design, but can sound really good after some mods. Further details are in the dedicated VJr threads on this site, and the soon-coming new SE amp & VJr daughter site.