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since 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: 

    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


Output Transformer Tests
Safe Amplifier Design
Publishing Information
Glossary of Audio Terms Articles
Classic Magazine articles

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