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Peerless Exciting Current Test

Posted on May 26, 2004 at 15:16:50

Posts: 4322
Joined: June 29, 2000

At the 1950 Audio Fair, Peerless is exhibiting another in a series of tests that shows that “Peerless Transformers are Best by Any Test.”

During the past year, Peerless has challenged the transformer industry to compare performance by square waves of any frequency from 20 cycles to 20,000 cycles.

Now Peerless calls attention to another very important property of transformers as shown by the EXCITING CURRENT TEST.

A transformer’s ability to deliver plenty of power at low frequencies is inversely proportional to its exciting current, and it is this low-frequency P-O-W-E-R that gives the solid undistorted bass that is the goal of every music lover.

In order to acquaint the transformer user with the principle of the Exciting Current Test, we have prepared the following technical explanation:

When a transformer is connected to a generator of AC, which may be the power amplifier tube in an amplifier, the AC current from the generator sets up a magnetic field in the core, the amplitude of the magnetic field varying in accordance with the instantaneous value of the AC current.

If the secondary be open-circuited (no load connected), the AC current flowing into the transformer primary is called the exciting current. If a load is connected to the secondary, such as a loudspeaker, then an AC current will flow through the secondary and the load, which current will set up a magnetic field in the core which at every instant opposes the magnetic field set up by the exciting current. The attempted reduction in magnetic field causes the primary current to increase, the additional component being called the load component. Thus, the primary current contains two components, exciting current and load component.

In vacuum tube circuitry, the output transformer is connected to a generator (tube), which has a resistance called the plate resistance. All tubes have plate resistance, some types higher than others. Thus, the AC current (music, etc.) from the output tube or tubes is fed to the output transformer through the plate resistance. At every instant, the AC current flowing into the transformer must adjust itself so that the instantaneous voltage drop across the plate resistance, plus the back or counter voltage in the primary set up by the core’s magnetic field equals the generated voltage from the tube. Because of magnetic non-linearity of the core material (the flux vs. current relationship is much the same shape as a hysteresis loop) the exciting current may be NON-sinusoidal EVEN THOUGH THE GENERATOR VOLTAGE IS SINUSOIDAL. Hence, the voltage across the resistance (plate resistance plus winding resistance) may be non-sinusoidal as it has the same wave form as the exciting current. In a well-designed transformer, the DC resistance of the primary is smaller than the plate resistance so that most of the distorted voltage appears at the output terminals (plate to ground) of the tube. This is so because it was pointed out above that the primary voltage (same as counter voltage under no load conditions), plus the resistance drop must at every instant equal the generator voltage. The sum of the resistance drop voltage and the original output voltage is impressed across the transformer primary as a distorted voltage, which is transferred to the load.

At low frequencies and particularly at high power levels, the exciting current component greatly increases in amplitude. An increase in exciting current causes the distortion of the exciting current, which in turn causes the secondary voltage to be distorted. These effects occur long before the magnitude of the secondary voltage begins to drop, thus distortion is the limiting factor in output transformer performance long before the “frequency response,” or secondary voltage, begins to drop.

While it is desirable then to keep the wave form of the exciting current reasonably sinusoidal in shape, it is vitally important to keep it low in amplitude, particularly at higher power levels.

The Peerless Exciting Current Test applies 450 volts at 60 cycles to the 5000 ohm primary of a 20 watt output transformer. This gives equivalent flux density in the core to that produced at about 5 watts (or ¼ power) at 20 cycles. In the test, the generator’s internal resistance (equivalent to the tube’s plate resistance) is low and, thus, each of the transformers is operated under conditions far more favorable than in an actual tube circuit. The distortion and variations in amplitude of exciting current are even more marked when higher source resistances are used.

The superiority of Peerless power handling capacity at low frequencies is shown by its lower and less distorted exciting current.

Those who are interested in pursuing the subject further are recommended to the book Magnetic Circuits and Transformers by the staff of Massachusetts Institute of Technology, pages 161, 268, and 281. The book is published by John Wiley.

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Oh da good old days when folks actually...., posted on May 27, 2004 at 15:41:26

took pride in what they built, truly belived in their accomplishments, and put reputations on the line in head to head open invitations.

But I guess that would be suicide today so one had better obscure the important facts and hype up the unable to acertain ones. We call it progress.

Makes you wanna puke and move to Montana or something.

Nice post BTW and sorry for the rant.



Re: Oh da good old days when folks actually...., posted on November 12, 2004 at 09:44:28

Posts: 194
Joined: February 28, 2001
That's not progress. That is marketing and short term profit. The problem is that most people don't have enough of a basic understanding of how things work to ask intelligent questions. If enough people would demand quality and not settle for crap then you would have better quality. Somehow the big marketing firms have convinced everyone that if you want quality you have to pay outrageously inflated prices for it.


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