Workbench Tech Tip of the Week(5)

What to do with a power transformer that shows excessive secondary voltages?
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What to do with a power transformer that shows excessive secondary voltages?

Radio World’s Workbench by John Bisset is an industry classic, one of radio’s most-admired, and most-copied, columns. Find out why in each issue.

This tip is from the Aug. 1 issue.

Although today’s engineer may not have the luxury of learning at the side of a seasoned veteran, the next best thing is the Internet, to which we didn’t have access in the 1970s and ‘80s. The various SBE listservs are great places to get a lot of opinions on a variety of subjects.

Case in point was a friend who was building a tube audio processing amplifier.

Unfortunately, the power transformer he purchased had excessive secondary voltages. For example, the 6 volt AC filament voltage measured nearly 8 volts and the high voltage was nearly 130 percent of the rated voltage in the transformer schematic. Even under load, the high voltage was 550 volts.

A 25 ohm, 50 watt power resistor was placed in series with the primary in order to drop the input voltage to 109 volts. This gave a reasonable filament voltage on the secondary. A 1200 ohm resistor was placed in the HF lead to drop the plate voltage to 300.

But there had to be a better way. Curiously, the manufacturer of the transformer could not offer suggestions.

Going back to electronics theory, Roger DuFault with the CBS Radio cluster in Washington suggested adding a buck-boost transformer on the AC input leads of the original transformer.

For those not familiar, a 24V or less filament transformer is connected normally to the 120VAC line. The 24 volt secondary is then placed in series with the 120 VAC input of the original transformer. Measure the outputs, and if the voltage is again too high, the buck-boost is in phase and must be reversed.

When the phasing of the filament transformer is 180 degrees out of phase with the original transformer, you get the “buck” action of reducing the voltage to the original transformer (wire it in phase, and the voltage is “boosted,” hence the term).

It’s a technique you may recall from Electronics 101, still applicable today. Thanks, Roger, for cracking open the electronics book for a lesson we may have forgotten.

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