Can the Power Transformer Take It?

In
a word, yes. It is true that we are
increasing the amount of output power from the amplifier 77%, adding
another 54-watts of output, so that takes more from the power
transformer. On the plus side, we are relieving almost 10-watts of
rectifier heater power. The solid-state rectifiers are more
efficient, dropping less voltage than the GZ34, so that saves power.
If you’re squeamish, you could consider sizing the new heater
transformer to power all of the 7591 heaters, relieving the main
power transformer of another 20-watts.

Perhaps
most importantly, one must bear in mind that there is typically a
14dB peak to average power ratio in music. That is a factor of 25X
in power. So if you set the output for the maximum power (both
channels) of 124W at peaks, average power out will be less than 5W.
The power transformer will not be called upon to supply maximum
power, except for brief periods.

Moreover, originally,
Andres operated the four new 7591 heaters on the existing power
transformer. That, along with the GZ34 which was there at the time,
drew about 30W more from the power transformer and it operated okay
that way. So it has had its margin test. In its final configuration,
after several hours of strong listening tests, the power transformer
did not get noticeably hotter than usual. So we conclude that the
power transformer can take it.

What About
Distortion, Frequency Response and SNR?

Distortion

Andres does not have
a distortion analyzer and since he lives in Venezuela, we cannot
assist. So the power output measurements had to be made with respect
to the visual clipping point, rather than a distortion figure, such
as 1%. As far as distortion at lower levels, we would expect those
to be similar to results from an ST-70A,
without the AC Balance mod.

However, distortion
performance will be dependent on the setting of the Negative
Feedback adjustment (R37). If the tubes and circuit are perfectly
balanced AC-wise, and R37 is set for suitably low gain, distortion
performance should be similar to the ST-70A. For best distortion
performance, the AC balance mod from the ST-70A
article should be included.

Frequency Response

Since this project is
all about developing the maximum power from the ST-70, frequency
response has not received much attention, at this point. There is no
reason to think that it would be impaired, given that the Negative
Feedback adjustment is not set so low in gain, that the amp is
marginally stable. At very low gain (high negative feedback), one
would expect peaking at high frequencies (perhaps in the 50-100kHz
range) and maybe at subsonic frequencies, as well. Either of these
could lead to a rise in response at the extremes of the audio band.
However, if the negative feedback is set properly, this should not
occur.

Signal to Noise Ratio (SNR)

This project did not
deal with the preamp sections of the ST-70 and those are almost
certainly the main contributor to noise in the amp. We addressed
those issues in the ST-70A. Andres included the line amp and power
amp mods that allow lower power-amp gain and improve SNR. Given that
the Negative Feedback adjustment is done properly, noise should be
similar to the previous excellent results.

Verifying the
Results

With the performance
of the project being so surprising to experts, we felt it necessary
to pursue a series of verification tests, to give some assurance
that the measurements are correct.

ST-70 Output
Transformer Power Test

David Gillespie
kindly volunteered to conduct a lab experiment to measure the
maximum undistorted power, which the ST-70 transformer can handle.
We were particularly interested in the performance at 20Hz, since
low frequencies are where saturation effects are most prevalent.
That is because the inductive reactance of the transformer is
proportional to frequency. At low frequencies, the reactance falls
and magnetization current (drawn by the equivalent parallel
inductance) of the transformer increases. That increased current
tends to saturate the core, causing distortion and reducing the
output level, if the drive source is susceptible to loading.

The lab test that
Dave ran is illustrated below:

The idea is to drive
the transformer in reverse and measure the maximum undistorted power
that it can handle. From the point of view of the core, it doesn’t
matter much, which way the power is flowing. The ST-70 transformer
(seen below left) normally works with a plate
impedance of 6500-ohms. To represent the half-impedance operation
used in this project, a precision load of 3250-ohms was placed on
the plate winding side, which is serving as a secondary in this
test. Since the DH-220 power amp has a very low impedance output,
the output of the transformer will only limit when the core becomes
fully saturated. This represents the maximum amount of power which
it can handle.

The generator frequency was set to 20Hz.
Output was driven to the onset of saturation as viewed on the output
of the plate winding. There was no clipping as is classically
viewed, but a very abrupt change from a normal looking sine wave, to
a strong spike, typical of that which crossover distortion produces.

At just below this
point, a measurement of output voltage was made with an HP-3466A
DVM. This showed 506.6-Vrms across 3250 ohms, or 79-watts RMS. That
demonstrates that the ST-70 output transformer is capable of
supporting the undistorted power claimed by Mr. Fontanills, given
sufficient drive.

Our next steps in
verification involved working with Andres to check the following
aspects of the measurement process:

The quality of
visual clipping used to establish maximum power.

Accuracy of the
load resistance.

AC meter frequency
response flatness.

Mr. Fontanills
patiently followed the procedures that we agreed upon and provided
documentation of the results. He included photos showing meter and
oscilloscope results together, with settings visible. This was very
reassuring. When the camera shutter speed compromised the 20Hz
waveform photos, he went to considerable effort to obtain better
ones.

Just Below Visual
Clipping

One person’s visual
interpretation of “just below clipping” might differ from another’s,
particularly at very low frequencies where tube amp limiting is
often indistinct. That is why maximum power is commonly measured at
a specified amount of distortion, as measured on a harmonic
distortion analyzer.

As mentioned though,
Andres does not have access to one. In lieu of that, he has provided
images of the waveforms (shown at right) at which the
maximum power readings were taken. These were all measured into an
8-ohm load. As you can see, the waveforms do not show a lot of
distortion at the maximum levels chosen, so those figures are
conservative. [In case you are wondering why the negative peak on
the right of the 20Hz waveform is lower, there is a small amount of
trace rotation in the scope, so all of the waveforms are slightly
tilted.]

Accuracy of the Load
Resistance

At
left, Andres verifies the 8-ohm load used for power
measurements.

AC Meter Frequency
Response

We wanted to make
sure that the TekPower DT9602R (above) which was being
used for AC measurements was indicating accurately at the frequency
extremes. To test that with precision, a
procedure was used which uses the DC-coupled oscilloscope, without a
probe, to maintain the flatness of the generator. By expanding the
scale vertically and centering the top of the trace on a graticule
line, the level of the generator can be adjusted at each frequency
to a precise level. The results at right show very
good flatness over the audio band.

With all of these
verification tests completed, we are satisfied that the power
measurements are reasonably accurate. Of course, since we can’t be
there to see for ourselves, we cannot guarantee that. But hey, we’re
talking about hotrodding here, not space engineering! It
looks to us like Andres has done a great job of documenting his
creation.

How is This
Possible?

For those who have
worked with vacuum tube amplifiers at low frequencies and have
experienced the effects of saturation, what we are presenting here
might be hard to believe. An ST-70A’s transformer output drops from
35W at 1kHz to 19W at 20Hz, with standard push-pull drive. It seems
like the 19W is a limit resulting from core saturation. How can it
be able to deliver 53W at 20Hz with push-pull-parallel drive and
half-impedance operation?

There are three
answers to this question:

Saturation is a
soft rather than hard limit.

As long as
sufficient current is available, secondary voltage will track
primary voltage, well into the saturation region.

When a transformer
is operated at half impedance, reactance of the core (including
saturation effects) is scaled up by a factor of two, relative to
the load.

To clarify this, let’s look at the
equivalent circuit of the ST-70 output transformer shown at
right. First, the primary and secondary resistances usually
aren’t significant and neither are the leakage inductances, at low
frequency. That leaves us with just the core loading and an ideal
transformer. By “ideal,” we mean that it losslessly transfers power
between primary and secondary (according to the turns ratio) and
doesn’t put any loading across the signals.

Notice that, with
just the core loading and the ideal transformer, there is no
mechanism for voltage loss between primary and secondary, at low
frequency. Hence, secondary voltage tracks primary voltage, in spite
of core saturation. What happens is simply that the core loading
just gets worse and worse as we go lower in frequency and higher in
power, as a result of core saturation. So, if you can drive it with
a stiff enough source, low frequency performance can be greatly
enhanced. This explains the results that David Gillespie measured in
the lab. When we operate at half the normal impedance, the effect of
the core loading is halved.

However, the core
loading is nonlinear and the tube output stage has significant
impedance. To minimize distortion, we need to maximize the negative
feedback around this. That is done by the ST-70A
mods. Extra development effort was needed to maintain both high
and low frequency stability, while accommodating 30dB of negative
feedback.

Naturally, there are
limits to this transformer model, particularly with our assumption
that leakage inductances and winding resistance effects are small.
Ultimately, saturation will become so severe that these are no
longer valid. Nevertheless, the empirical fact that the ST-70 output
transformer supports 79W at 20Hz, is a testament that it goes a lot
further than many thought.

Possibilities for
Future Improvements

AC Balance Mods

As mentioned above,
the AC Balance mods (at right) of  the ST-70A
article were not used here. Having those would reduce distortion and
make the performance less dependent on perfect tube matching. It
would be implemented exactly as was done in the ST-70A. The fact
that each former output tube now becomes a parallel pair, does not
affect the mod.

Increasing Negative
Feedback at Low Frequency

In the ST-70A, we
increased negative feedback from the stock value of 20dB, to 30dB.
While much effort went into optimizing the compensation networks to
maintain and actually improve stability, there are tricks left in
our engineering toolbox which might allow us to squeeze out more
feedback at low frequencies. If that is possible, it could benefit
this project, reducing distortion from nonlinear operation of the
output transformer cores. Stay tuned.

Delaying the High
Voltage?

Conventional wisdom
once held that applying plate voltage to a tube before the heater
has warmed-up could degrade the cathode. Unlike some of their vacuum
tube counterparts, solid state rectifiers develop full B+ voltage
almost instantly. As a result, many vacuum tube enthusiasts advocate
providing a means of delaying application of B+, when solid state
rectifiers are used.

However, the thinking
about this issue is changing. It begins with the question: If
tube cathodes are harmed by applying plate voltage before the heater
warms up, why aren’t rectifiers affected? When we asked vacuum
tube amplifier expert, David Gillespie, about this, we received the
following reply:

The RCA transmitting tube manual
specifically warns against applying plate voltage to mercury vapor
and high vacuum type rectifier tubes until the filament has
reached full voltage for a rated period of time. But by way of
parenthesis however, they specifically exclude receiving type
rectifier tubes from this requirement. None of the RCA tube
manuals warn against applying B+ to receiving type tubes before
their cathodes are fully heated, even basically endorsing the idea
of switching the filament voltage only in battery portables as a
means of turning the radio on and off.

It seems that, while
transmitting tubes with thousands of volts of B+ might benefit from
delayed B+, it is doubtful that it helps tubes in normal hifi
applications.

Listening Tests

Note: This is not
easy for Andres (due to the language barrier) but he has done his
best to get across to us, his impressions of the sound. Slightly
edited for clarity:

The ST-70 has a
beautiful sound, but with this modification, now the sound is more
than beautiful, it’s great (you should hear it). Believe me, the
improvements are many. I have listened to many songs (and albums) on
vinyl, but the ones with which, I have been most impressed with the
sound of the new Supermods are:

Behind the
Mask
Fleetwood Mac (Warner Brothers Records, 1990)
“In
the Back of My Mind” (This song is especially important for low
frequency notes at the beginning of the song.) The impressive sound
of the drums and the “mysterious” sound of a voice that seems to be
right in front of you...

Take Five
George Benson
in concert, Carnegie Hall (CTI Records, 1975)
You can hear very
clearly, all of the notes of the guitar.

Ummagumma
Pink Floyd,
1972
“Several Species of Small Furry Animals Gathered Together in
a Cave and Grooving With a Pict” In my opinion, the greater clarity
of the “sounds of silence” in a cave, can be felt in this song. The
various high frequency tones of these strange animals can be heard
clearly.

Sgt. Pepper’s Lonely Hearts Club
Band (CD)
Here, the power of the amp lets me raise the
volume! Very impressive sounds of the Indian instruments, violins,
cellos and acoustic guitar...

Conclusions

We have reported on
Andres Fontanills’ Supermods™ technique and it doubles midband power
of the (already enhanced) ST-70A, with one channel driven. It also
increases total power (both channels driven) by 77%. Perhaps even
more amazing is the fact that 20Hz (single channel) power is
increased by a factor of 2.8-times.

The really big
news is that this technique is applicable to almost any push-pull,
fixed-bias, hifi, tube amplifier, using the original
transformers! (Given that there is space for the new tubes.) The
essential elements of the invention are:

Half-impedance
operation of the output transformers.

Doubling-up the
output tubes (in parallel).

Increasing
negative feedback, using sophisticated loop compensation.

Use of a solid
state rectifier, filament transformer, and doubled
first-filter-cap.

The only caveats we
have found are: (1) Andres does not have a way to measure distortion
and (2) the mods operate the power and output transformers beyond
their specs. However, distortion at lower levels can be comparable
to the ST-70A and at higher levels, the waveforms look pretty good
on a scope (shown above). No ill effects on the
transformers have been noted (but please proceed at your own
risk).

One can only wonder at the (perhaps more
modest) gains which might be achieved by modifying standard
push-pull (as opposed to push-pull parallel) amps for beefier output
tubes and half-impedance transformer operation.

Congratulations to
Andres for demonstrating something that experts said was impossible.
Drilling steel with a spade (wood) bit is no small wonder, either.
<wink>  Happy
hotrodding!

Acknowledgments

Our thanks go to
Andres Fontanills of Venezuela for his fantastic Supermods™
technique and for answering a seemingly endless series of questions
and requests for measurements. Thanks also for the fine
pictures.

Special thanks go to
Dave Gillespie, who expeditiously performed and documented the lab
test on the ST-70 output transformer. He showed that it is capable
of supporting 79W of power at 20Hz. That went a long way in
motivating a deeper look into this project.

Please see the
previous page of this article for reader comments.

Copyright © 2011 by
Stephen H. Lafferty. All rights reserved.