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From: stevesaf@microsoft.com (Steve Safarik)
Subject: Re: Old tube woes
Date: 07 Sep 93 21:10:07 GMT

You are correct that this amp is a push-pull, but any audio tube 
amp is going to be a class A, or possibly AB at high signal levels.

The first thing to do is to get matched output tubes (the 6550's).  
If the push and the pull sides of the amp don't match, then you will 
get a current flowing through the primary of the output transformer.
That current will eventually magnetize the core which will most likely 
cause a loss of bass, and distortion of other components (although 
I haven't actually experienced this).  I'm not sure how you demagnetize
a core, but maybe using a similar method to tape-head demagnetizing
would work.  If a screwdriver sticks to the transformer when the amp
is not turned on, then that's the problem.  

The smaller preamp tubes don't need to be matched since they don't 
usually work in push-pull.  If there is a difference between them, you
will probably see those differences between the left & right channels 
rather than messing up the sound of a single channel.

That's about all I can suggest.  I don't know what differences there
are between the various 12A_7's.  You do need to get your tubes in sync
before looking for other problems, however.

Steve.


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From: malte@TechFak.Uni-Bielefeld.DE (Malte Uhl)
Subject: Re: Old tube woes
Date: Wed, 8 Sep 1993 17:19:48 GMT

Steve Safarik (stevesaf@microsoft.com) wrote:
: You are correct that this amp is a push-pull, but any audio tube 
: amp is going to be a class A, or possibly AB at high signal levels.

: The first thing to do is to get matched output tubes (the 6550's).  
: If the push and the pull sides of the amp don't match, then you will 
: get a current flowing through the primary of the output transformer.
[ .. stuff deleted .. ]
: Steve.

While I agree with Steves statements, I'd like to add a little to it:
A magnetized transformer core has almost nothing to do with matched tubes,
but equally biased tubes; that is - without signal -  all tubes receive
the same amount of plate current, not grid voltage. If there is no way
to bias tubes individually, you may want to add the neccessary circuitry
now. This is much less expensive than matched tubes.

And then, every transformer core is magnetized a little because of an
effect physicists call hysteresis. 

Furthermore, the transformer itself has to be symmetrical. Old transformers
often suffer from damages like bad isolation between the copper wire layers.
If the output tubes plate current differs by, say a factor of more than 2
when supplied with equal grid voltage, this is most likely to be the case.
I once had an amp where the left channels transformer was nearer to the tubes
than the other. Over the years the tubes heat changed the magnetic properties
of the hotter transformer slowly but noticeably. Finally the isolation broke
so that several layers of copper were cut short. I had to replace it.

On another topic: recently several people asked when to change tubes.
There are 2 three ways to decide this: ask an expert or buy a used tube tester
which I highly recommend when you are into tubes. :-)
But seriously, what a tube tester does (amongst other things) is:

	1) apply a ca. 1KHz sine signal to the tubes grid and measure the tubes
	   amplification factor
	2) then apply the signal via a 500KOhm - 1MOhm resistor directly
	   attached to the grid pin and check amplification again

If the amplification drops significantly, the tube is bad, either the vacuum
leaks or the kathode isn't emitting enough electrons. Significantly means:
A good tubes input impedance at low frequencies is about 10 to 30 MOhms. If
the signal sources' impedance is 1 MOhm, amplification should not drop
by more then 1/10th. I'd say a tube is bad if the drop exceeds 20%.

Oh, amplification means ratio between the grids and the plates AC voltage.

Malte

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Date: Fri, 10 Sep 93 10:48:07 EST
From: peterca@ento.csiro.au
Subject: output transformer magnetisation (re: old tube woes)

Steve Safarik writes:
>The first thing to do is to get matched output tubes (the 6550's).  
>If the push and the pull sides of the amp don't match, then you will 
>get a current flowing through the primary of the output transformer.
>That current will eventually magnetize the core which will most likely 
>cause a loss of bass, and distortion of other components (although 
>I haven't actually experienced this).  I'm not sure how you demagnetize
>a core, but maybe using a similar method to tape-head demagnetizing
>would work.  If a screwdriver sticks to the transformer when the amp
>is not turned on, then that's the problem.  
I was recently told that this was a problem with single-ended amps (which
are having a come-back).   It occurs to me in this case that you could
reverse the leads on the primary and secondary on a regular basis to
prevent this.  What do people think of that idea?  A person who makes and
restores valve amps told a friend of mine about this problem and said that
after 10 years a single-ended tranformer was no good anymore.  However, I
thought it could be easily fixed as above.  
Any imbalance in output tubes is a bad thing because it leads to
distortion, reduces the peak power availlable and is best fixed.  However,
this discussion suggests that it may be worth swapping push-pull output
tubes occasionally if you don't have any way to ensure that they have
identical bias current.

Peter Campbell (peterca@ento.csiro.au)
C/O Div. Entomology, C.S.I.R.O.
Box 1700 Canberra A.C.T. 2601    Ph.61-6-2464158 (w), 61-6-2516213 (h)
Australia                           61-6-2464173 (fax)

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Date: Fri, 10 Sep 93 10:54:07 EST
From: peterca@ento.csiro.au
Subject: output transformer magnetisation (re: old tube woes)

I forgot to comment on the following:
Steve Safarik writes:
>...a current flowing through the primary of the output transformer.
>... will eventually magnetize the core ...
>If a screwdriver sticks to the transformer when the amp
>is not turned on, then that's the problem.  
Just a note of caution before you panic.  Many screwdrivers are
deliberately magnetised themselves and will stick to any lump of iron.

Peter Campbell (peterca@ento.csiro.au)
C/O Div. Entomology, C.S.I.R.O.
Box 1700 Canberra A.C.T. 2601    Ph.61-6-2464158 (w), 61-6-2516213 (h)
Australia                           61-6-2464173 (fax)


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