# Smoother Tubes



## Steadfastly (Nov 14, 2008)

I have a Kustom 5H amp head that I would like to have a little more clean headroom. In reading some reviews, someone said replacing the 12AX7 preamp tube with a 12AU7 tube would clean up some headroom on it. So, amp gurus, please, what are your thoughts on this. BTW, the power tube is an EL84.

Also, in your opinion, what are the better quality tubes if I change them? Thanks, Steadfastly


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## dtsaudio (Apr 15, 2009)

If the 12AU7 works, you will get considerably less gain so it might seem you have more headroom. You're going to use a lot more of the volume control. You could try 12AT7 or 12AY7. They might give you what you want.


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## Steadfastly (Nov 14, 2008)

dtsaudio said:


> If the 12AU7 works, you will get considerably less gain so it might seem you have more headroom. You're going to use a lot more of the volume control. You could try 12AT7 or 12AY7. They might give you what you want.


Dan, thanks for you reply. I know next to nothing about tubes. Would you mind explaining the difference between the 12AX7 and the 12AT7 & 12AY7?


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## julienpier (Aug 7, 2009)

I put 12AY7 in my Traynor Yba-1a. It doesn't give more clean headroom, it only reduces the gain factor which makes the amp top its output at 6 or 7 instead of 3. Which means that you have to turn your amp to 7 to have the same volume than what you would have at 3 with a 12AX7. The only way you could have more clean headroom is by buying a louder amp that's gonna give you a higher dB versus Overdrive level. 
I only say that based on my limited level of knowledge.
May if you use a bigger cab with higher value speaker (ex: 2 x 150w 15" cabs instead of 2 x 50w 12" speakers), it's gonna push more air per watt.
This is how I understand it and it seems to work for me.

This guy explains it way better than I could ever do.
http://www.thetubestore.com/Resources/Guitar-Amp-Info/Gain-Factor

Hope I helped.


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## Steadfastly (Nov 14, 2008)

Julien, yes, that was very helpful. I can see the relationship between the different tubes now. Thanks very much for your post!


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## GTmaker (Apr 24, 2006)

I tried the 12AU7 on my Fender Super Reverb RI....
Horrible experience....that tube sucked the living tone out of my amp...
I am using 5751 in the 1 and 2 slots and it works for me...

Here is a cool chart that I made when I first started on the tube swapping thing...

Lets say that the 12AX7 represents 100% of the power that a tube has to offer.
the 5751 type tube has 30% less power then the 12AX7
the 12AT7 type tube has 40% less power then the 12AX7
the 12AY7 type tube has 55% less power then the 12AX7
the 12AU7 type tube has 80% less power then the 12AX7

Although the numbers are intereting to look at, I still think that its the sound that counts once you substitute a tube type for another.
Either you like it or you dont...
good luck with your quest.

G.


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## Steadfastly (Nov 14, 2008)

Okay, I made the change.......in my mind. I'm leaving it the way it is. I do have an RP-255 so I can get it as clean as I want. I just wondered if I could do something to the amp without the FX unit.


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## Wild Bill (May 3, 2006)

Voltage gain is not so easy a factor to change the tone of your amp. You see, the total gain of the preamp stages is calculated by MULTIPLYING the gain of each stage along the way!

So if you have 4 preamp 12AX7 tubes in the preamp string, each of which has two separate triode amplifiers inside, means 8 gain stages. Now while a chart might show a voltage gain of 100 for a 12AX7 this is rarely seen in the real world. The actual circuit of resistors and capacitors has a big effect on what happens. Still, for purposes of argument let's assume the gain is indeed 100 for each stage. The total gain feeding the power tubes will be 100 x 100 x 100 x 100 x 100 x 100 x 100 x 100, or 100 multiplied by itself 8 times.

I tried to work that on my 1982 solar powered Texas Instrument pocket calculator and before it overloaded it screamed a total of " 1 BAZILLION!"

If we substituted tubes of a lower voltage gain of 70 does anyone think that it would make any difference? By the time the signal gets to the power tube inputs it is still far more than the tube needs to deliver full power.

So it would appear that once again there is some psychological mojo stuff happening here. Nobody ever seems to set up a true scientific test. They just change a tube themselves, chug a beer, hit a few chords and say to themselves "Yep! Sure sounds different to me!"

If they had someone ELSE changing tubes and could not see which tubes, if any, had been changed they would find noticing differences to be a different story!

Now that all being said, you CAN get SOME useful tonal difference by substituting tubes. You just have to choose a tube that has a big enough difference in voltage gain than the usual 12AX7. A 5751 won't do it. It was actually designed as a "hifi" 12AX7. The gain difference was mice nuts. The important thing was a differently wound heater, which significantly reduced that last bit of unwanted hum in a MacIntosh or Quad hifi unit.

Try the 12AY7! The gain is around 40 and this will make a significant difference to the point of overload of each stage and the total gain of the entire string. The amp will need the volume control cranked higher to push out the same number of watts but it will stay cleaner much longer over the range.

Incidently, the 12AY7 was Leo Fenders first choice of preamp tube for his earliest models, from the late 40's to the mid 50's. They have a very nice classic tone!

The 12AU7 has the lowest voltage gain of all, at 10. However, there's more to the story. Unlike the other tubes on the chart, it needs completely different resistor values in a circuit, particularly the plate load resistor. So if you just blindly stuff one into a 12AX7 socket it will still work, since the pinout is the same, but the tube will distort much more. Depending on your amp and its circuit and on how hard you are cranking it you might not notice that distortion or perhaps, you might like it! Still, it will be there.

As always, one aspect like voltage gain is seized upon by folks who don't know or understand how things really work and is spun into Gospel Mojo. 

Wild Bill/Busen Amps


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## mhammer (Nov 30, 2007)

Just to add to Bill's excellent summary, note that, even though gain is multiplicative, the product one gets at the end is also a function of how much signal is preserved at each step. So, If I cascade two gain stages of some sort that each have a gain of 10x, *BUT* I insert something between stages 1 and 2 that attenuates in either a wholesale or selective fashion, such that what stage 2 receives is not 100% of what stage 1 has produced, then the output of stage 2 will not be 100x the input to stage 1.

So, for instance, I may have a preamp stage, a passive tonestack, and two subsequent preamp stages. That tonestack will suck out (bleed off) a LOT of signal. Increasing the gain of the stage immediately preceding the tonestack, may improve the S/N ratio, but may play less of a role in overdriving subsequent stages, than if we left the gain where it was, and inserted our goosed gain _after_ the tonestack.

That is, where one inserts greater and lesser amounts of gain can play a role in final tone, by virtue of what comes between each stage.

Plus, there is also the limitation set by the supply voltage and available headroom. If the signal can't go any higher, then it can't go any higher, no matter what the theoretical calculated gain is.


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