Re: Maybe I am just being unbiased...

On Mon, 31 Mar 2008 04:36:40 -0500, Rich Koerner <richk@xxxxxxxxxxxxx>
wrote:



DH82C wrote:

Ok.. more bias questions.
When I was young and impressionable I read the "right" way to bias a
pushpull amp was to use a signal generator and an oscilloscope. Look
for crossover distortion, the most negative grid voltage that didnt
have crossover distortion was the ideal. You could run with less neg
voltage on the grids if you liked the tone but at the expense of tube
life. But that was many many moons ago before Al Gore invented the
internet. Now I see there are more than one way to skin a cat than
just fu*ing it to death.

Question: Does anyone actually use an oscilloscope to bias their amps


Every day.

The only way to SEE what those free electrons are doing is to use a scope.

The scope is the single most important piece of test gear you could have on the workbench.



or do you all use Bias probe/1ohm cathode resistors/


Oh, you mean the paint by numbers method?

Where'd that number come from for the tube that really sit in the amp in front of you?

Some web site with the clairvoyance to tell everyone that... that their *one* number also fits YOUR
tube exactly.

BS!

Shunt Method ?


Never use it.



I am thkinking of buying an oscilloscope (used 150$)
1 Ohm resistors are .50$ ea. (i.e. much cheaper than a silly scope)
Shunt method is free (I have a decent DMM already)

I would love to know what y"all think.

You are blind without a scope.

You are blind without a scope playing the numbers game on one ohm resistors.

You are blind setting output tube bias, when you have no awareness of it's effect on the dynamic
energy flow to the load.

The only bias method mentioned in this thread that addresses the individual tube sitting in front of
you
is the DARKROOM METHOD. Which tells only of the individual tube's current limit. The method is
incomplete as a stand alone bias method. It's just another quick bias method's shot in the dark.

Without the observation of the dynamic transfer of electrical energy though the amplifier to the
load, you are in the darkroom with your bias setting toys.

Dynamic operation comparisons of different output tubes are not possible with the limited
information these bias methods require.

So, as long as a single idle adjustment gets the motor to run, you can get to the store and back.

But, if you want to take it out on the highway..... well?

Now, look at that amplifier, and think about stepping down on it's gas pedal as you are about to hit
highway Rt. #66 with your foot on the floor.

It is unreasonable to think that a single bias adjustment, and/or bias method... will unify all the
variables between the current crop polished turds for tubes, and make any one of them INDIVIDUALIZED
for YOUR amplifier's *maximum* performance.

Your amp's performance level can only be as good as the tubes put in it, and the man who knows how
to get the most out of them.



Regards,

Rich Koerner,
Time Electronics.
http://www.timeelect.com

Specialists in Live Sound FOH Engineering,
Music & Studio Production,
Vintage Instruments, and Tube Amplifiers
How come some people are running from oscilloscopes? They are really a
neat piece of test equipment. And is a BIG part of bench test
equipment. They do look daunting with all of those knobs but the "How
to use a Scope" DVD explains it very well. And a few questions on the
scope Yahoo groups and here can direct you to what to get so you are
not wasting $$.
As far as spending $$...if you cannot find the $$ for a scope then
you likely don't need to pursue electronics as a career..maybe a
hobby. But, if a $250 scope helps save a set of $250 tubes, then the
money is well spent. And being able to see the waves comping from your
amp stages is an educating experience also.
.

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