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tube amps "bias" tubes???? (1 Viewer)

Judy Y

Stunt Coordinator
Feb 27, 2000
I really REALLY want a tube amp or hybrid. The New Tube Company will "match" tubes when you buy them. But, I guess, this is different from "biassing" tubes? DO I HAVE THE SKILLS to own a tube amp? Is it going to be so difficult to replace tubes that I really don't want to mess with it? (The reason I want one is because I heard one years ago and fell in love with it...... of course the 2,000 dollar cd player and 2500 dollar bookshelf speakers it was hooked up to were nice too........)

Anyone out there own a tube amp and change their tubes? How hard is it? I don't want to get into this if it's more trouble than it's worth.

I make my own cables but balked at doing much of anything special to set up my turntable-if this gives anyone an idea on my skill levels. I am, strangely enough, the one all my friends call to help set up their new televisions or home theaters, or universal remote controls, but I usually end up doing these things after much research HERE.

Thanks to any and all who take the time to respond!

Tim Hoover

May 27, 2001
Tubes are usually matched at the factory. While all tubes that are sold are obviously in factory tolerance, the matching process finds a pair (or quartet) whose specs are almost identical. This should give them almost identical operating characteristics.

As far as biasing tubes, this is only done when changing tube types. Replacing a 12AX7 with another 12AX7 will not require a bias change. In addition, some amps are more sensitive to bias changes than others. For example, suppose you want to replace that 12AX7 with its lower-gain cousin, the 12AT7. Some amps will require a bias adjustment for this, and some won't.

In a nutshell, sticking with the same type tube won't necessitate a bias adjustment, except for maybe every five years to ensure that the bias hasn't drifted off.

If you do want to adjust the bias yourself, it's not too difficult. All you need is a voltmeter and a screwdriver. A tube's "bias" is really the plate voltage. The spec sheet on the tube will indicate its operating plate voltage. You measure that and turn a little screw until that voltage is within range.

Brett DiMichele

Senior HTF Member
Sep 30, 2001
Real Name

I am hardly a tube expert I guess that would be better off
left to people like Lee Scoggins, Scott Oliver and even
Chu Gai! :)

But what I understand is this.. Tube Matching is when they
take a pair or quad of tubes and burn them in at the factory
and they check the specific outputs of the tubes and then
match them within a certain variance.

Biasing tubes comes when you have more than one tube in the
output stage. Biasing makes the dual or more output tubes
have an equal amount of voltage flowing into the heating
plates. If you overbias you can burn the tubes out (I just
did..) Some amps have auto bias built in.. Others do not..

I don't think Biasing is hard if you have a good multimeter
that can read in the range they will need to be biased in
(I didn't.. I do now!)

Changing tubes is easy you just pull the old out, and stick
the new in.. (and re bias if the amp has manual biasing)
it's not hard.. And tubes that are properly biased should
last at least a couple thousand hours, some tubes like the
6L6's will last 10 thousand of more hours.

Scott Oliver

Aug 30, 2000
Well, before you even think about a tube amp, you need to consider your speakers first:

High efficiency speakers are good for tubes, this one is pretty dependant on how many watts your amp produces.
No super low dips in the impedance curve are good for tubes (

Brett DiMichele

Senior HTF Member
Sep 30, 2001
Real Name
I have to second what Scott said about everything.

Pick a tube amp based on your speakers. If you don't have
reasonably sensitive speakes then you will need similar
power requirements to a Solid State amp and this can be
very costly in the "tube world" if you need 100+ watts per
channel you might as well bend over now because there is no
such thing as an "Affordable" 100+ Watt tube amp.

Like I said over in the DIY section when I responded to
your post. If you have reasonably sensitive speakers that
are 89+ DB Sensitivity then a good 15-30 watts per channel
will get you very good listening levels. Before I fried my
tubes (through my own stupidity) when the tubes were working
correctly on my 92Db Sensitive speakers I could acheive
more than 110Db before distorsion would rear it's ugly head.
To me that is more than acceptable listening level if not a
tad unbearable. I generaly listen at 75 to 80DB in which
case I am using 1Watt or less of power.

Like Scott said (I should have stated this) never touch a
tube with your fingers. A tube is much like a lighbulb and
anywhere you touch any tube the oils in your skin will stick
to the tube and cause a hot spot on the tube and this is
not good. Install tubes with a clean cotton cloth and use
the same to dust them with.

Also make sure you isolate the tube amps from vibration. A
tube is a very microphonic device (some worse than others)
and any vibration can make it's way into the tubes and sent
back through the output into the speakers.

Biasing should be done with every new tube installed reguardless
if it's the same type or not. And as Scott stated yes the
bias should be checked as the tubes age. The heater's are
coated with materials that in time can wear down and I would
assume this could change the biasing and for best sound I
would check it and adjust if needed.

To check bias on a non auto bias amp it's still not hard as
long as you can read a Multimeter. For example my ASL mono's
call for 330 Millivolts on the dual output tubes. You hook
the amp up to a pair of speakers and turn them on and you
check the output taps with a multimeter and adjust the bias
potentiometers till you get a 330Mv (or .3 Volt) reading.

NEVER run a tube amp without speakers connected!
Tube amps are unlike solid state in that they need a load.
If you run a tube amp without any load the output tubes can
go ballistic and burn out prematurely. I am not certain but
this could also decrease the live of the output transformer.

Also never get inside a tube amp unless you really know what
you are doing. Tube amps operate at very high voltages and
have large capacitors that store enough to stop your heard
numerous times over. So touching anything inside is a no-no
unless you know how to discharge capacitors.

Judy Y

Stunt Coordinator
Feb 27, 2000
Thanks for all the good advice. I have a multi meter, (for checking home made cables), I guess, one way or the other, I can figure this all out.

I didn't realize there were so many people on this board who used tube amps!

Thanks again (and, I am sure, I'll be back for more help later!)

Brett DiMichele

Senior HTF Member
Sep 30, 2001
Real Name

Yep it's not that complicated. It's just that we're all
way too comfortable with set it and forget it thanks to
solid state :)

Tubes are much like Vinyl sure they both do have thier
merrits but a lot of it is also nostalgia and being different
than the mainstream.

Tubes definatly sound different and you have to be the judge
if that is "better". To me, I could go either way.. I like
both S.S and Tubes hell I love it all! :)


Stunt Coordinator
Jan 3, 2002

I have no additional "how-to advice" to add, as Tim, Brett, and Scott did a good job.

However, based upon your posts, you undoubtably have more than enough skill to handle any situation a tube amp may pose in its lifetime.

Enjoy the auditioning process! I'm certain, that with patience, you will end up with an excellent amp.


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