# Help me learn about mah tubes!



## Stratocaster (Feb 2, 2006)

Hi, I am the user of a (very sweet) tube amp. But I am ashamed to admit - I know nothing about tubes . I know my amp sounds suweeeet compared to all the solid state amps I've owned before, but I don't know exactly what tubes do lol.

Here are a few questions

1. What do tubes do?
2. I found out my amp has 4 6L6 output tubes, and 8 12AX7 Pre amp tubes. The hell does that mean? lol.
3. If one of the tubes go bad, how can I tell which one it is? Or do I just change them all?
4. In the picture below, only 4 of my tubes are clear, and the other 8 look like they have a metal shell around them? What's with that?








5. What's the fuse do? Does it ever need replacing?


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## keeperofthegood (Apr 30, 2008)

Well...

I may be in error here, but I think I can offer a couple things to start this off.

_ 1. What do tubes do?_

In reference specifically to tube amps: there is no difference between tube amplification and transistor amplification in the "amplification" sense (this being what I think you intend in your question). But in the hearing sense there is a difference. Its in what are called the harmonics. I always get this reversed BUT I will edit if I need to. Transistors amplify the primary signal and the even harmonics and tubes amplify the primary signal and the odd harmonics. The human ear feels better with hearing the odds rather than the evens (unless I got that backwards, but the idea is still the same). You can get this if you use FETs instead of transistors but I think there is issues with FETs that I am forgetting off the top of my head.

One definite difference between tubes and transistors is how they clip sound. Tubes are called soft clippers whereas transistors hard clip. Soft sounds a lot nicer.

The other thing about tubes is; they are wonky. Wonky in the sense they are not stable. A tube will behave differently as you load it or overload it and also will behave differently if you load it fast or slow. By loading I mean making it work. Transistors on the other hand are very stable, changing little from partially on to fully on no matter the speed of turning them on. You lose all that character with a transistor (or silicon diodes).

In a tube amp where tubes really do show their character is in the power supply as a rectifier, and in the output stages where you get the harmonics added.


_2 I will leave alone for now  I once had a 43 volume set introduction to the physics of tubes by McGraw Hill, 1 and 2 inches thick per-volume. Just roll with "these are made for amplification". If you are really interested in tubes a good program to start learning with is http://www.duncanamps.com/tdslpe/index.html even if he has not updated since the passing of his brother  it is really handy to have on your computer. One heads up for these two, they were named at a time naming conventions were being attempted and the first number is filament voltage, 6L6 means it is a 6.3 volt filament and 12AX7 means it is a 12.6 volt filament but the 12AX7 has a midpoint connection so it can be 2X6.3 volt filaments too.

3. If one of the tubes go bad, how can I tell which one it is? Or do I just change them all?_

Tubes are often wired in series, you would need to test them all. Changing them all at the same time... Ive no opinion on that  Mind, since there is only 2 types, all you need are 1 6L6 and 1 12AX7 and then leapfrog through till the bad one is out.

_4. In the picture below, only 4 of my tubes are clear, and the other 8 look like they have a metal shell around them? What's with that?_

You can have almost any material to seal out the air. Thats the important thing. Glass is fragile and it is not a Faraday cage and that is where metal comes in (ceramic tubes are another story and not usually seen in home equipment):

http://en.wikipedia.org/wiki/Faraday_cage

The "cage" is used a lot with guitars. Everyone talks about the ground shield inside their pickups and using co-ax cable etc to keep out noise. Tubes are no different, they are just as vulnerable to noise. Some tubes in some applications more so. Although you can get most tubes in both metal or glass, in a noise prone circuit you will need to either use metal cased tube or you need to add on a metal shell over the tube and ground that. Otherwise, internally there is no difference between metal shelled and glass enveloped tubes. Personally it is harder to "see" a bad tube in a metal shell, and I like to see the orange glow. Another thing to, in glass tubes you get to see the getter. The getter is that mirror metal surface. It is microwave cooked when the tube is made after it is initially vacuum emptied and sealed, and will suck up any remaining O2. If it goes white, you've an air leak either through a crack in the glass or a seam failure between the glass and the pins and this is another thing you cannot see in metal tubes.

_ 5. What's the fuse do? Does it ever need replacing? 

_Well... it's a safety thing. Anything electrical can short. Some rain, mud, damn mouse, meltdown inside a component and before you know it, you've got a hot zone inside your equipment. Fuses are there to "blow" when something goes wrong. Metals are also in a class of matter that isn't solid. They (like glass) are super cooled liquids. Over time the fuse wire does thin and stretch and eventually will fail on its own. It happens and usually people will be WTF! and change the fuse and not think about it again. But if you change the fuse and it "blows" again then something isn't working right and it is time for a service call.


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## NB-SK (Jul 28, 2007)

http://tdsl.duncanamps.com/show.php?des=6L6

http://tdsl.duncanamps.com/show.php?des=12AX7


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## keeperofthegood (Apr 30, 2008)

Yup, fairly sure those are linked from the program too (only fairly as its not been updated in a long while). Lots of information, even if he has archived his forum 

A good resource of people into tubes and tube equipment is:

http://www.antiqueradios.com/forums/index.php

There is a lot of tech available there!


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## Stratocaster (Feb 2, 2006)

Thanks for the help - I read it all. 

I can safely say I know a little bit about tubes now.


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## Michelle (Aug 21, 2006)

And keep this one....

www.thetubestore.com


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