In the late 1980s I tested as many push-pull transformers as
I could get ahold of. The test bed was a modified Heathkit W-6
amplifier with push-pull 6550s. The goal was to get a relative
comparison of the quality and characteristics of what was available
at the time. Copies of these test results have been floating around
for years, and are now reproduced here. I was unable to dig up
the original Lotus 1-2-3 files, so these are scans of the last
version I made, from December 29, 1990.
Tests page 1 (PDF:
Tests page 2 (PDF:
Tests page 3 (PDF:
Tests page 4 (PDF:
396Kbytes) (includes sample waveforms)
There have been some misunderstanding on these measurements.
The following is the text of a letter I sent to an inquirer in
1991 regarding the tests. It pretty clearly states the methodology
and goals for these tests.
- I am sorry I haven't gotten back to you sooner,
but I have been busy, between finishing various projects and
traveling to Asia for three weeks. Enclosed is the latest update
of the transformer test listing. I have knocked-down the transformer
test set-up, since the chassis it was built on was needed for
other purposes, so I won't be measuring more transformers soon.
I am still interested in characterizing output transformers,
but will likely build a somewhat different test set-up, since
I would like to test the transformers in a circuit with feedback.
The characterizations summarize some, but not all, key parameters
about output transformers. An important one is the basic high
frequency response, along with the square-wave waveform characteristic
- this indicates the attention paid to reducing parasitic capacitance
and leakage inductance in the winding. Also important is the
minimum frequency that full power is possible - this indicates
the amount of primary inductance and the quantity and quality
of the core material. The Z p-p and Ultra Linear ratio verify
the design of the transformer. Note that a discrepancy between
the primary impedance between a 4 ohm load connected to the 4-ohm
tap, and an 8 ohm load connected to the 8 ohm tap is often due
to the transformer actually being designed for a 3.2 ohm tap.
There is also some error due to inaccuracies in making the voltage
measurements, but these were reduced in the measurements marked
with a *.
When measuring an unknown transformer, particularly when it has
been removed from its amplifier, I often had to try to deduce
what the intended impedances of the secondary windings were,
by either using an ohmmeter, or looking at the relative output
voltages. In some cases, I may have guessed wrong; this may be
why the Fisher 50A transformer has such strange specs. Running
a transformer into the wrong load will alter its high-frequency
roll-off and square wave response.
I have been criticized for driving all the transformers with
the same pair of 6550s, since each transformer should be driven
from it's correct driving impedance. Thus a transformer with
a 10K primary that is driven by the lower impedance of the 6550s
will be overdamped. This complaint is correct, but I was looking
at testing the transformers in a uniform environment. The relative
differences between transformers of a given impedance are still
valid. The primary errors caused by an impedance mismatch are
the frequency response and amount of squarewave ringing. If the
transformer is used in a circuit with any feedback, these parameters
are changed anyway. An area of future research is to see how
well transformers behave under feedback. One of my preliminary
findings is that transformers that have a smooth (not kinky)
squarewave response, even if the -3 dB frequency is low, (such
as the Marantz 9 and Acro TO-600) often behave better under feedback
than fast transformers with kinks in the squarewave response
(such as the Brook 12A or the UTC LS-55).
Related to the issue of squarewave response, my use of the "A,
B, C..." ratings to describe the squarewave response has
been nearly universally misused and misunderstood. When I first
started these tests, I took scope photos of each transformer's
response. As the quantities of transformers tested increased,
and people asked that the results be put in a tabular form, I
dropped the use of individual scope photos, since I found that
most transformers fell into generic classes of waveforms. The
enclosed sheet showing example waveforms was deemed sufficient.
I started testing the better transformers first, so assigned
them A, B, etc. Later I got to the worse ones, which tended to
get the E and F ratings. These letters are not a grade, but are
arbitrary assignments. People have misinterpreted these letters
as a pure quality rating, placing "A" transformers
at a much higher position than "D" transformers. In
fact, Acrosound transformers typically get the "D"
assignment (an underdamped ringing, with no kinks), yet do well
in feedback amplifiers. As mentioned before, the squarewave response
and -3 dB high frequency cut-off are only relative indicators
of quality that are easily altered by how the transformer is
used in a circuit.
Areas for future research include: measuring core distortion
and losses, primary winding coupling (for class B applications),
tests with feedback, and last, but not least, listening tests.
I have been doing more work on trying to understand what exactly
influences the sound of amplifiers, but am often surprised that
amps with transformers that don't test well (such as the Brook
12A and many 6BQ5-based amps) sound so good. Often the transformer
is not the dominant influence on the amp's sound, but it would
still be worth finding out what influence they have. An amp that
I am currently building will have provision for mounting four
different 60-watt transformers (one at a time); it will be interesting
to swap them and hear the differences.
I hope this helps. I am still interested in gathering people's
experiences with transformers, in order to learn more about what
makes tube amps sound good.
- John Atwood