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Articles — Component Testing —
Output Transformers

Beginner

I built this test amp for quickly testing freshly assembled boards. It also
comes in handy for quickly evaluating different transformers. They can be
swapped in about 5 minutes with only a screwdriver.

I decided to use the “Coke Bottle” shaped 6L6GC’s made by Shuguang for this
testing (amp pictured has EL-34’s) for two reasons. I have previously abused
these tubes to over 40 watts dissipation without failure, and they are cheap ($4
each on Ebay). In order to subject the transformers to the most possible power,
I cranked the bias up to 80+ mA for a dissipation of over 30 watts. The actual
current and dissipation varies slightly since each transformer has a different
DC resistance. Even though the tubes are running above the recommended maximum
ratings, these seem to handle it well. This set has already served about 4
months of tough duty in a Fender Bandmaster, The owner didn’t like them because
they were “too clean”. They have been sitting here cooking away for hours at a
time while I make measurements, without issue. I can get more power from a 6550,
but I can’t get 6550’s for $4, and I don’t want to risk frying any of the ones
that I have.

I ran the usual frequency response curves because that is a common
measurement criteria for OPT’s. This however does not tell the whole story.
There are several small transformers (like the little Edcor) that have excellent
frequency response. Even at full power the low end response goes down into the
20 Hz region. In listening tests it is obvious that the small transformers can’t
deliver wall shaking bass like the big ones can. I spent some time looking for
an electrical test that can quantify this ability.

I found that distortion VS power curves ran at several different frequencies
can do it well. The 1KHz curves are the baseline. In the ideal world the curves
at all other frequencies in the audio spectrum should be identical. In the real
world the iron in most transformers will saturate if pushed hard at low
frequencies and high power levels. This will show up as distortion on the low
frequency curves. Also the high frequencies may be distorted by excessive
capacitance, and magnetic losses. Since the amplifier remains unchanged from
transformer to transformer, you can learn about the relative efficiency of each
transformer by comparing a common point, say how much power can I get at 5%
distortion at 1KHz. The larger transformers eat up more of the valuable power in
magnetic and resistive losses.

I tested several transformers from tiny to huge. Each was run in triode mode
and UltraLinear mode if they had the tap. Each was tested with cathode feedback,
and without CFB. All transformers are tested at 100 Hz, 1 KHz, and 10 KHz. The
large transformers are tested at 44 Hz, this frequency was chosen because it is
the lowest note on a bass guitar. The large Hammonds are the only ones that
handle this frequency without issue.

Hammond 1628SE

The largest transformers tested were the Hammond 1628SE, 11 pounds of iron
and copper here. These are the older 1628SE’s, NOT the recently released
1628SEA. I have the new ones on order, but they haven’t arrived yet.

The Hammond 1628SE in UltraLinear mode at 5 watts without any feedback. The
response is rolled off at the high end. It is 3.4 db down at 20 KHz. The
response is the same in triode mode.

Cathode
feedback improves this to 2.7 db.

Triode Mode

This is a graph of distortion VS power at 4 different frequencies in triode
mode with no feedback. The increased distortion at 10 KHz above 5 watts should
not cause a problem with music. This transformer shows 4 curves that are very
similar up to 5 watts.

This is a graph of distortion VS power at 4 different frequencies in triode
mode with cathode feedback. Cathode feedback improves the distortion slightly
especially at low power levels. The distortion remains primarily second harmonic
up until the onset of clipping (about 4.8 watts).

UltraLinear

This is a graph of distortion VS power at 4 different frequencies in
UltraLinear mode with no feedback.

This is a graph of distortion VS power at 4 different frequencies in
UltraLinear mode with cathode feedback. The distortion at 10KHz is not helped
much by cathode feedback. The other 3 curves are lowered significantly with
CFB.

Cathode Feedback

This graph shows what cathode feedback does for this transformer at two
frequencies. The blue and magenta curves are 1KHz. The yellow and cyan curves
are at 44 Hz. This graph is made from the same data sets as the graphs above. It
just graphically displays the improvements in distortion afforded by CFB.

Edcor CSXE25-8-5K

These are almost the same size as the Hammond 1628SE’s, but they don’t have
as many laminations so they are not as heavy.

The -3DB response here is well centered over the audio band. This plot was
taken in UltraLinear mode at 5 watts with no feedback. Triode mode looked the
same.

Here cathode feedback was applied. The frequency response was improved
slightly.

Triode Mode

This is a graph of distortion VS power at 4 different frequencies in triode
mode with no feedback. The increased distortion at 10 KHz is not as bad as the
1628SE. However the 44 Hz curve is significantly above the 100 Hz curve at all
power levels. Saturation effects could be seen on the scope at power levels
above 7 watts. Note that this transformer offers more power than the 1628SE. The
Hammond allows 5.5 watts at 5% at 1 KHz. This one allows 6.8 watts. This is
because it is a more efficient transformer (less magnetic losses) and it has a
lower DC resistance which allows about 10 more volts on the plate, along with
less resistive loss in the transformer.

Cathode feedback was applied to this transformer in triode mode. It offers
more improvement than it does with the 1628SE. CFB helps lower the distortion
produced in the tube, it can’t do anything for the magnetic saturation effects
seen at 44 Hz above 6 watts. I can now get over 7 watts at 5% distortion. 7
watts in triode mode is a lot for a 6L6GC!

UltraLinear

This is a graph of distortion VS power at 4 different frequencies in
UltraLinear mode with no feedback. Note considerably higher distortion at all
frequencies than the Hammond 1628SE. 44 Hz is particularly bad.

Cathode feedback helps this transformer quite a bit. For the frequencies
prominent in most music (100 Hz to 1 KHz) this transformer will make 13 to 14
watts (at or below 5%) the 1628SE will only do 10 watts. This transformer sounds
louder, by a lot.

Cathode Feedback

This graph shows what cathode feedback does for this transformer at two
frequencies. The blue and magenta curves are 1KHz. The yellow and cyan curves
are at 44 Hz. This graph is made from the same data sets as the graphs above. It
just graphically displays the improvements in distortion afforded by CFB.
Compare the improvements here with the CFB chart on the 1628SE. Listening tests
reveal similar behavior.

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