guitar speaker/was Re: Tube challenge!

Paul R. Higgins higg0008 at tc.umn.edu
Thu Nov 25 19:22:57 CET 1999


> While it's well known that an *open* output can cause some damage on a tube
> power amplifier, I nver heard of a constant load resistor instead of a
> speaker would cause any problems. Is this really true ? And if so, why ?

> Perhaps some types of output configurations cannot be run like this (Class A,
> etc), but my push-pull amp works fine in this case.

I have been designing and building with tubes for many years, and I can say for 
certain that a properly rated load resistor on the output of a tube amp will 
work just fine and will not cause any damage to the output stage/power 
amplifier.  That is, it won't cause any damage that wouldn't have also been 
caused by the speaker being connected.  Some amps, especially Marshalls, are NOT
adequately conservatively rated for the kinds of playing conditions they're 
often put in by musicians.  

As an example of this, take a look at a 100W Marshall (or Fender) output 
transformer; then compare it to a 30W tranny made by a good manufacturer like 
Hammond.  The 100W Marshall/Fender tranny will be the SAME size as a 30W 
Hammond!!  The amp companies deliberately use underrated trannies because 
they're much less expensive than using the right part for the job.  Now, a primo
amp builder like Mesa/Boogie WILL use properly rated parts and conservative 
design because their amps are made for professionals (read: expensive) and have 
to hold up under real-world conditions.   

The reason you don't hear as much about people blowing up their Fenders as you 
do about Marshalls is because Fenders have been traditionally used to make, 
shall we say, "quieter" music than Marshalls.  It is my opinion (along with many
other tube designers, I might add) that "normal" operating conditions for a 
Marshall actually constitute abuse: running the amp constantly at redline 24 
hours a day, pushing the power amp into saturation and near-square-wave 
conditions, etc.  One of the most severely affected components by this kind of 
abuse is the output tranny.  Check out a Marshall some time, and you'll see that
its output tranny is almost always the same size as a Fender, despite the fact 
that a Marshall is put under far more demanding conditions.  Read any guitar 
magazine, and you'll hear professional players talking about how they love their
Marshalls, but they go through them like water.  

There have been many power-attenuator devices over the years, many of which are 
purely resistive.  One of the best known is the Scholz R&D "Power Soak", which 
is little more than a lot of power resistors and a stepped attenuator.  It works
fine, but be careful about redlining that Marshall!  The thing that some players
complain about is that the sound and "feel" are different than a real speaker, 
because of course, a speaker is a reactive load.  Hence the rise of the 
"reactive load simulators", one of which I know for a fact is a copy of a 
speaker equivalent circuit in the "Radiotron Designer's Handbook".  You could 
probably build a reactive load of your own for far less $$$. 

Also, I have never seen an amp that couldn't be connected to a power-soak device
merely based on its amplifier class (A, AB1, AB2) or design (single-ended vs. 
push-pull).  However, lower-powered single-ended and Class A amps usually don't 
need a power soak; you won't go deaf or get evicted by cranking a little 3W 
Fender Champ.

Hope this clears up some of the confusion on this topic.  There is indeed a 
plethora of snake-oil and voodoo salesmen in this field.

-PRH

_____________________________________________
Paul Higgins
email: higg0008 at tc.umn.edu
University College, University of Minnesota
_____________________________________________




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