Here's some tube talk from the point of view
of a repair and hotrod guru. I hope you find it useful and entertaining,
in the true spirit of the web.
The Driver Tube;
The driver tube is the least understood tube in an amp. If you
are putting in matched sets of output tubes (and you really should
be) then you are wasting your time and money without a Driver
Tube that has been picked for this special job.
Just about every amp with 2 or more output tubes uses a driver
tube. Why it is so important is because this tube takes a single
signal voltage, and splits it into two. There are actually 2 separate
amplifiers in a 12AX7 and in a 12AT7. If each amplifier section
in it are working at the same potential - great! But they hardly
ever do... so, you need to find one.
As far as I know, no one markets a "driver tube" for
musicians, however I do. That's because when I
purchase tubes, I sort them for where in the amp they would perform
best. This symmetry in the driver tube allows the 'matched' output
tubes to see the same input signal voltage levels, and in turn
deliver matching output voltages. When all this and properly biased,
the tubes will sound better and last longer. Be sure to read the
section on matched sets to fully grasp
this vital aspect of your amp. My driver tubes are matched to
within 1%. The others claim 5 and 10%. No joke.
I WANT TO BUY A DRIVER TUBE RIGHT
NOW.
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Preamp Tubes V.S. Output
Tubes;
This is where the tone really happens: in the preamp. Read more
about that in "ABOUT PREAMP TUBES". Most amps don't
depend on output tube distortion to generate the tone. There is
an audible difference in the way preamp tubes distort and the
way output tubes distort.
Preamp tubes are capable of much higher levels of distortion (safely),
and tend to sound like a finer grit of sandpaper than the course
distortion of an output tube. For amps that use EL34's (6CA7),
EL84's (6BQ5) like Marshall and Vox; these tubes are likely to
impart their own tone and or type of
distortion under normal operating conditions. For almost all Fender
circuits you will find the 6L6/5881: this is a workhorse clean
tube. When working properly, the output tubes do a good job of
NOT imparting their own tone, or distortion. In some Marshall's,
you'll find 6550's; these tubes are the big brother to the 6L6/5881.
They extend the treble a fair amount, and by comparison can be
shrill. They also allow you to hear what's going on in the Marshall
preamp circuitry with much more detail. Its a good rig for most
Metal genre's, as articulation is a key component to a great sound.
Its also a great tube for jazz players. The classic Marshall tone
is 3) 12AX7's with 4) EL34's for the Rock Arena, for the blues
and proto-rock glean its the 40 to 50 Watt version (as above but
only 2) EL34's.
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Power Tubes;
Just to clarify, Output Tubes AKA "Finals" although
they do a lot of work, are really not "Power" tubes,
as they are often referred to. I'm too old to try to change the
world, but, Power Tubes are rectifier tubes, and (almost never
seen in audio) regulator tubes. In both cases "power tubes"
refers to tubes that are part of the power supply. Output Tubes
are not part of the power supply, so it doesn't make sense to
call them "Power Tubes".
Examples of power tubes are 5AR4/GZ34, 5Y3, 5V4, 5U4, A03.
Examples of output tubes are 6L6, EL34, 6V6, EL84, 6550.
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About Preamp Tubes;
First off, nobody's tubes sound any better than anyone else's.
Its entirely a matter of taste. From one 12AX7 to another, you
will hear differences in tone, dynamic range, and distortion characteristics,
if you listen to them at clean, low volume settings. If you want
to know what they are really doing behind your wall of distortion,
first give them a critical low volume listening. If you lack the
discipline to do this, then you will never learn what all the
different tube qualities are. Its great that you like to play
loud - whatever, I do too.
To hear the differences you need only one of
each type, and listen to each one in the socket nearest the input
jack. Here the preamp tubes' effect will be the most evident.
Learning the way you can refine the tone of your amp with tubes
is fun, inexpensive, and worthwhile.
I have sufficient experience to interview you about what style
of music, what amp, etc., to be able to advise you on what tubes
are most likely to make you "Wow". I don't charge for
that if you are ordering tubes from me. But who
should you be buying from?
Read "who makes tubes".
I WANT TO BUY
A SAMPLE PACKAGE OF PREAMP TUBES.
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Who should you be
buying tubes from?
I love the web. I have the time here to say all I need to, and
over and over again. And when I say it, I'm not loosing bench
time! This is important so listen up.
First off, lots of people do a great job of
buying whatever tubes are on the market and screening them for
quality. For example; Fender, Groove Tubes, Mesa Boogie, Ruby,
and Marshall.
Secondly, the retailers are ready to serve: Guitar Center, your
local music store.
Third, mail-order offers low prices: Musicians Friend.
Fourth, the web offers worldwide access to retailers, some web-only.
Now lets get to it. The best place to buy tubes is where there is a tube tester! That's it! When you buy a tube you should have the ability to test it! That's the way its been done since the 1930's. When you buy a tube you expect that is a working tube, without shorts or opens. If you're old enough to remember, those tube testers were in Radio Shack to test the NEW tubes as much as for testing your old tubes!
Tubes go bad in shipping. Filaments open, short to the cathode, vacuum leaks, etc. That's not the fault of whoever made or sold the tube, all tubes have a mechanical aspect, and are prone to mechanical failure.
The next best place to buy
tubes is from the place that is servicing your amp.
Are you going to send your new tubes back to foreign country land
when you discover they don't work? How much money did you save
now? When you buy the tubes from your tech, all your warranty
concerns are addressed there; parts and labor. Nobody has to point
the finger at anyone else, there are no shipping delays or costs
to you when a tube fails. You don't have to make multiple trips
between the point of sale and the test machine either.
Yes, that many new tubes are bad. This is the leading reason why people often aren't happy with their new boutique tubes. If you buy tubes online, you can really be asking for it. Where do the tested bad tubes go? For one; unsuspecting individuals trying to save a buck, who may not have access to a tube tester!
Say you buy 3 preamp tubes and 2 output tubes, and 20% of the new tubes in the world are bad. This is realistic, by the way. There's a 100% probability that you'll have to return 1 tube. Then there's a one in five chance you'll be taking that new tube back for replacement, its bad too. Do you want to make three trips or pay three shipping charges? Yes, you will be paying the return freight.
To sum it all up, the absolute best place to buy tubes is from your amp technician who allows you access to one of his tube testers. Your amp will sound better, and you will have saved time and money.
I get that you have integrity, I want to buy my tubes from you.
Tube Failure:
Some tubes are almost impossible to use in a high gain circuit
due to Hum or Microphonics. A tube that is humming is likely to
have leakage from the AC voltage filament supply to the cathode.
This not only introduces hum into the circuit but also throws
up to 12 volts of AC on an electrode that's only supposed to have
less than 2 volts of DC. This means the tube is junk, even if
it works, even if you don't really notice that much hum, unless
you want to cause your amp to break too. A tube with a
short from the heater to the cathode is not uncommon. If it is
an output tube, then you may get lucky and pop a fuse; hope that
you do. Most amps do not have a fuse on the heater voltage. Your
audio and transformers can blow instead. If the short is in a
preamp tube you will not pop a fuse because the cathode
connects to ground via a small resistor (unlike most output tubes,
they usually connect to ground directly) which limits the amount
of current. Thats really not good because you are still overworking
your power transformer. Power transformers are sensitive to "magnetic
saturation", an irreversable condition that prevents full
output. Its obvious to the amp owner there is a problem, "it
just doesent have the nuts it used to", but is very tricky
to spot on the test bench. So all you guys that think the tubes
that came with the amp 10 years ago are fine; you don't know,
you need to get them checked. You are taking a huge risk. You
own a vintage amp? Then you should know how important, expensive,
and sometimes impossible to replace those transformers are. But
getting your tubes checked and possibly getting you amp to sound
better than new is fun - do it. All output (power) tubes
(finals) will eventually form a heater to cathode short. Output
tubes should be checked for merit and h-k leaksevery six months.
My store has one for you to use free of charge.
Microphonic tubes have something loose inside them that, when
gently tapped or otherwise vibrated (like sound from your speakers)
will cause the tube to make its own sympathetic sound. Just past
marginal microphonic failure you will hear a grumble when you
play a bass note. More microphonic tubes wont let you turn up
the treble without tearing out a shriek. Worse still you can't
play lead style distortion without your amp squawking at you.
There are lots of shades of grey in this. If you are hearing any
tones that don't belong then you most likely have a microphonic
tube. It may plague you only under certain conditions. But a bad
tube is good opportunity to upgrade the sound of your amp.
Other failure modes are:
Now wait a minute, you said you just put new
tubes in right? And it sounded great for a while, but now yadda
yadda yadda. Here's what happened. You heard from a friend or
teacher that you need to put in
Brand X tubes to get a good sound, because you were not completely
happy with the sound you were getting. Then you went to the store
and got those puppies. Somehow you knew that there is a high failure
rate in new tubes, and bypassed all mail-order possibilities.
Now, you either made the mistake of putting them in yourself,
or the technician you took it to biased your amp one of three
of the four correct ways that won't work to keep your amp out
of the shop. You didn't let some retail clerk put them in, right?
Again, the technician you took it to biased your amp one four
correct ways, but not the one method that will work to keep your
amp out of the shop. This happens all the time. And by the time
you figure out your amp REALLY sounds bad, the tubes are off warranty
by one day.
I said there are 4 correct ways to bias an amp. One of them is better, so your amp is more likely to stay out of the shop. Unknowing or unscrupulous repair shops will make you a steady tube customer. Who's to blame? Maybe the tubes they are using, maybe the tech, maybe you had something really go south in your amp. Either way, if your back for a third set of output tubes in a year! Go find a tech who knows what the good tubes are, who really makes them, and how to keep your amp out of the shop. Follow his recommendations, and pay a little extra for (probably his) good tubes because you will save money in the not too distant future.
Let your amp cool for 3 minutes, that means
completely off, before you attempt to move it. This will help
keep your tubes from shorting out. They are the same as light
bulbs: a vacuum, a heater. And if you unscrew a light bulb while
it's on it usually blows because of the violent shock taking place
while the hot metal filament is transitioning from it's hot almost
molten like state to it's cold, very hard and brittle state. You
banged it when it was changing from hot and soft to cold and brittle.
Same as a tube.
HOW DO I FIND A TECH?
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How Do You Find A Tech?
Call the amp manufacturer, and get the name of a factory authorized
service center. If there isn't one, ask your friends if the shop
they use does good work. Use the Yellow Pages - Believe it or
Not - This is the heading: Musical Instruments - Repairing.
A busy shop will not have the best turnaround times. A good shop
will always be busy. In my area shops doing quality repairs have
4 to 8 weeks turn.
My shop is open 4 days a week. I charge a premium
of twice the labor for emergencies. I also will go to the customer,
but for a minimum charge. I bring everything under the Sun and
fix it on the spot.
I
WANT YOU TO FIX MY AMP.
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Matched Sets:
There are variations in any manufactured product, this is especially
true in tubes. What matching does is pair up tubes that are equal
in potential. Typically, we're talking about Output Tubes, although
there is a need for matching in driver tubes.
By using matched sets, you get an equal amount of output voltage
swing and current driving capability from each side of the push-pull
audio transformer.
To try to visualize this: picture each of your hands as an output
tube. Put your hands together like you are praying, only with
a sheet of paper in between your hands; that's the speaker. Now
push from
side to side. Notice that the speaker moves to either side of
center. Now what happens if one of your arms is made to push harder?
Your weak tube gets beat up, and the strong one has to work harder,
try it. Lots of people match output tubes and do a very good job
of it. Its expensive to do, in labor and electricity, and its
worth it for you because in the not so long run its cheaper.
Matching tubes can
also entail BURN-IN time. This addresses "infant mortality".
Infant mortality describes the effect of most parts that will
fail prematurely will do so in the first 24 to 48 hours of use.
What burn-in is, is simply putting a device (tube) under conditions
that resemble an actual circuit, and let it sit there and run.
Then you test it to see if still works. If it does, two things
happened:
1) The future reliability of that same tube went way up.
2) The BIAS POINT of the tube drifted. Again, you get a more reliable
tube, because it wont need to be readjusted two days after you
put the new tube in! Read more about bias.
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WHO MAKES TUBES?
Fender now owns Groove tubes. At this time, Groove Tubes actually
does make a few tubes. They make a KT66 and one particular 12AX7,
Both use gold in the construction of the control grid. They have
purchased the original tooling to produce US made 6L6s'. That's
all they make at this time. The rest of their tube line is, as
it always has been, a representation of all the worthwhile tubes
made, mostly currently. They, like Fender, Marshall, Mesa Boogie,
Ruby, TNT, do a very good job of screening out the bad tubes and
putting their own name on them. To the best of my knowledge, Groove
Tubes is the only one on that list that manufactures any tubes,
and that represents only a small portion of what they offer. Fender
is now shipping "Fender Groove Tubes'" in their amps.
All the ones I've seen still carry a third name, the name of the
actual manufacturer: Sovtek.
Marshall has dropped Sovtek from certain model amps, and is currently
shipping Svetlana tubes in
their amps (pronounced Sevitlana). Prior to that they had used
Tesla tubes for a long time, and are currently made also. EI was
apparently on the wrong end of Western politics, and the manufacturing
plant in Yugoslavia fell down, went boom. Its production of tubes
rises and falls to this day. The highly abused Chinese Sino 12AX7
was the tube everyone loved to hate, while at the same time widely
used for the production of new amps. The tooling is said to have
been put outside, and has gone to rust. I guess another form of
friendly fire. So much for the most reliable 12AX7A I ever used.
There is more than one tube manufacturer in China! Lets not lump
everyone together unfairly. Just like there is more than one Russian
tube company, there are few enough choices in tubes to start getting
a National Bias (is this a joking matter?). For anyone who cares,
there is a market for American made tubes, as proven by the gentleman
who purchased the tooling and licensing of the name from Westinghouse
- who is making the 300B in the good old USA. The import 300B's
sell for up to $300, so it's a fair guess the American made Westinghouse
will cost double. By the way, the 300B is it for true hi-fi.
If you want to buy a guitar amp that uses this 42 watt triode
please tell
me.
I
WANT TO BUY EXTINCT TUBES WHILE I STILL CAN.
I
WANT TO BUY 300B'S.
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Bias;
This is for geeks like me. Its misunderstood, there's too much
misinformation out there, and my best customers are the ones know
something, so here goes. I'll keep it simple, and cover some stuff
you wont find elsewhere!
Preamp tubes are in a self adjusting circuit, so they never need biasing. We are referring now to output tubes.
Bias is an adjustment that determines how hot
a tube runs. It also determines what the duty cycle is of a tube
in a push-pull circuit. The amplifier design determines what class
of operation the tube will run
in, and so in turn determines what the correct bias point is.
Regardless of class, there is a range of proper adjustment!
You need to know what you are doing to make
this adjustment or you can seriously damage your
amp, sometimes irreversibly.
Since the Audio transformer design dictates Class of operation, how much voltage V.S. current at idle is also a function of design. Class A runs a lot of current at idle, Class B barely a trickle. Watts equal Amps times Volts, so depending on the design, you may be setting the bias either too hot or too cold when you set the bias to say, minus 52 Volts.
Tubes also vary, and picking an arbitrary bias voltage can only be useful if all your tubes run at the same potential. Its not the same thing but if it helps, think of it in terms of efficiency. Under any given set of conditions different tubes will perform more or less work. Tubes within a manufactured lot vary. From one model of tube to another of the same type, there is variation. There is an even greater variation from one manufacturer to the next.
To build a 6L6 you need to meet the specifications for tube type (beam power, pin basing), for the maximum Wattage rating (ability to deal with heat), maximum plate voltage, maximum control grid to cathode voltage, some inter-electrode capacitance stuff. The point being that there is a lot that's not said about the design. They vary by design and by the fact that they are manufactured (manufacturing tolerances).
I'll bet this is more concise and complete
than anything you've seen. I hope you enjoyed it. Feel free to
print it for you own personal use but do not duplicate it, as
it is copyright protected material.
If you want more information than this, I'll provide it for a
fee.