Tube amp no load/nonimal load/infinite load

Abraham · 2012-07-30T20:51:21+00:00

There seems to be a lot of wrong reasoning when it comes to the load on a t=ube amp.Anyone with real knowledge, both theoretical and experimental, mind setting= me straight?My understanding of it is this:No load:Without any load, e.g., infinite impedance on the secondary, the output tra=nsformer acts like a simple inductor(as if there were no secondary winding)=.=20The inductor, will charge up and eventually act like somewhat of a short. H=ence larger than expected currents will flow through both the primary windi=ng and tube when the tube conducts. Class A and class B amps will have a sl=ightly different response but both may or may not be ruined.Infinite load:With an infinite load, e.g., the secondary is shorted, the output transform=er acts much differently. The shorted secondary allows a back emf to counte=r the driving voltage of the primary somewhat canceling out some of the cur=rent. The effect is to reduce the maximum current in the no load case.----If there is a linear effect between the no load to infinite load one would =expect the infinite load to actually produce an minimum amount of current a=nd the nominal load to be somewhere in between?I've read somewhere that what the real issue is, generally, is voltage spik=es that can be much larger than the supply voltage. I don't see how this ca=n really happen unless there is some type of resonance... but that seems li=ke it would depend on specific values of the resistance, inductance, etc...= of all the components?

Jul 30, 2012 25 Replies

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Jim Thompson 13 years ago

I had given it no thought, since the last tube amp I built was 52 years ago, but... pondering my navel, everyone one of them had a dummy load. Thus no arcing, frying transformers, etc. ...Jim Thompson

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PeterD 13 years ago

You did save me the trouble of typing all the reply.

I'm never going to grow up.

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whit3rd 13 years ago

amp.

straight?

In general, an amplifier's output terminal(s) are capable of voltage and current excursions, and the output capability defines a 'load line' curve for any given amplifier. The load has its own characteristics, and crosses the 'load line' to create a solution for output with no particicular input to the amplifier. That is called the 'operating point'.

There is no such thing as 'infinite load' or 'zero load', the load has always some well-defined physical characteristic (often a resistance). You need to know the operating point, so you NEED TO SPECIFY A REALISTIC LOAD, before any useful analysis of normal amplifier operation can be performed. You always start such analysis, classically, with determining the operating point.

The only reason to short or open-circuit an amplifier would be to determine fire-hazard kinds of (abnormal operation) characteristics. Both short and open outputs are zero-power-delivered conditions.

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Phil Allison 13 years ago

** Can is the operative word - such events are rare.

** They do not "blow up".

** Same always was and is true for most consumer electroncis.

But nowadays, the hacks are all speaking in Chinese.

.... Phil

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josephkk 13 years ago

on a tube amp.

setting me straight?

current

any given

create

is called

always some

know the

always

determine

and open

Should i ever ask you to design an amplifier for make sure i require you to include these and other worst case analyses. I worry what i might get with the way you worded your reply.

?-)

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Phil Allison 13 years ago

"whitless turd"

** Fraid they are both common events occurring in the "field" and on the test bench.

Any amplifier that fails immediately upon either event is not fit for sale to the public.

** So what ?

An audio amplifier NEEDS to be able to deliver at least 50% of its normal, rated output current into a short circuit.

I do such testing regularly ( using a 0.1 ohm dummy load ) and a differentiated, 100Hz square wave input to avoid over dissipation in the output devices.

.... Phil

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