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Oct 2000

High
Frequency Anomalies in Output Transformer Tests

John Atwood

This
Issue

We're back. Sorry for the missing issues. I know
this journal has a small, but extremely devoted following that needs its
monthly dosage of new tube circuits. I would like to blame the Macarthur
Grant selection committee, but then they probably believe that this
journal makes money. Actually, time was against me. I have spent the last
three months working on a tube manual program that will go on sale soon.
The program uses rich text formatting, filters data effectively and
displays and prints tube curves. It is a big program, but the topic is
even bigger. The tubes… there are thousands and thousands of types. And
there are headaches everywhere: the same tube with different names;
different tubes with the same name; pentode routinely referred to as
tetrodes; conflicting specifications from one manual to another; and
informational blank spots that cannot be filled.
In this
issue, output transformer testing is covered nicely by John Atwood and I
tackle how to make tube amplifiers safer.
Remember, if
you have a request or suggestion of your own for either an article topic
or circuit explanation, please e-mail:

Editor

Having tested a lot of audio transformers, I
thought that I had the procedure down. But when a friend's transformer
high frequency response measurements were significantly different than
mine (for the same transformers), I decided to get to the bottom of the
differences.
The transformers in question were
single-ended ones meant for shunt-feed, so the discussion will focus on
this type, although the conclusions also apply to conventional
single-ended and push-pull output transformers.

The Set-up
The ultimate test of
a transformer is how it performs in the actual circuit it is intended for.
However, when designing and testing a transformer for use by many people
in many circuits, standard tests that can be done on the test bench with
audio test equipment are needed. I have been using a test set-up as shown
in figure 1. Over the years the sine-wave oscillator and meter used have
gone from Heathkits to Hewlett-Packards to a Sound Technology 1700B, and
now an Audio Precision System 2. For output transformers intended to be
driven by a triode, Rs is set to 1/2 the rated transformer output
impedance and RL is set to the rated load impedance. This is the set-up
recommended by Reuben Lee in his Electronic
Transformers and Circuits, 2nd ed., fig. 113.

In This
Issue

1
7
17
11

Output Transformer Tests
Safe
Amplifier Design
E-mail
Publishing
Information
Glossary
of Audio Terms
Glass-Ware.com
Articles
Classic
Magazine articles

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