Re: Maybe I am just being unbiased...

On Mar 31, 10:20 am, J.P. <jpas...@xxxxxxxxxxx> wrote:
On Mon, 31 Mar 2008 04:36:40 -0500, Rich Koerner <ri...@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.- Hide quoted text -

- Show quoted text -

I am not looking for a career in electronics. I do enjoy tweaking and
restoring my amps tho.
Rich and JPs comments pretty much sold me one getting one (but not
necessarily for biasing).

I have been thumbing my books at home and the one that seemed most
comprehensive was the
book by (Dan?) Torres. He shoots down the oscillocope method by saying
you are putting the amp in
an atypical state (1KHz sine wave with the amps volume turned short of
clipping and into a dummy load)
and expecting it to perfom the same way when you have a reactiv load
(speaker) and and a
signal source that produces a range of frequencys at the same time.I
am not knowledgable enough
yet to agree or disagree.

I get the impression that biasing isnt exact anyway. Tubes are never
perfectly matched.
Static dissapation (?) can vary as components heat up. As long as the
tube
is running hot enough to not have any crossover distortion (for push
pull amp for example) but not so hot to
fry the tubes you are fine. Someone once said "Let your ears be the
judge" (I have to work on that too lol)

Does that make sense? Thanks for your comments Rich, I have seen you
often in the forums and find your
posts enlightening. As for JP.. I reread your post and the last line
convinced me to stop being such a cheap ***
and think more long term ;)

Cheers

Drew
.

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