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Transformers! Wow, what an enigmatic item they have become. Ignorance of how
this chunk of iron works seems to be rather wide spread, as evidenced by some
recent events.
A short time ago, I read an article about building a single-ended 2A3 amp.
The builder said he had a transformer that was 8-ohm on the secondary and 5k-Ohm
on the primary. So in order to present the recommended 2500-ohm to the 2A3,
he said he would use just one-half of the primary. The 2M does like a 2500
ohm load, but he will have 1250 ohm if he uses only one-half of the primary.
Then along came this article “Audio Uses of Transformers”. I am activated!
I will offer some observations about item numbers 7, 8, and 9 that he mentions.
**OUTPUT TRANSFORMERS **
While I think everyone would like to avoid that evil black chunk of iron,
it’s best to learn some facts about this mysterious heavyweight by reviewing
some of the laws and rules of inductance and iron-cored transformers.
1. Transformers only work with alternating current.
2. Their inductance varies as the square of the number of turns of wire wound
on the iron core; i.e., if you cut the number of turns in half, you reduce
the inductance to one-fourth of what it was.
3. You determine the impedance ratio of a transformer by squaring the turns
ratio of the primary to the secondary or the reverse in order to get a whole
number and not a fraction.
Now you are going to say, “I don’t know anything about the number of turns
on the transformer, what else can I do?” Well, for iron-cored inductors, the
turns ratio is the same as the volt age ratio of the two windings involved.
Use your ohmmeter to determine which winding has the highest DC resistance.
Get out your AC voltmeter and a source of AC voltage, such as a Variac or
another transformer with about 50V out. Put this AC voltage on the highest
resistance winding. Mea sure the input voltage and the voltage across one other
winding on the transformer. Perform division of the volt ages in order to get
a whole number and not a fraction. Square this number and you have the impedance
ratio of the transformer for the windings involved in this measurement.
Now this transformer is not so much of a mystery. Suppose when you had 50v
on the high resistance winding, you had 3V on the other winding. Fifty divided
by three = 16.666. If you square 16.666, you have 277.76, which is the impedance
ratio of this unit using only these two windings. The impedance ratio will
be different if you use other windings.
If you want to put an 8-ohm speaker on this smaller winding, consider a 2222.1-ohm
(277.76 x 8) impedance to a load. If I put a 4-ohm speaker on the smaller winding,
I will only present 1111-ohm to the load.
Note how much the primary impedance changes when you go from 8-ohm to 4-ohm.
Many people indiscriminately hook up various speakers with different impedances
on their amplifiers and expect them to perform equally well.
For another example, I have a transformer with a 10k-ohm primary. The secondary
is 4, 8, and 16 ohm. Using the 16-ohm winding, I divide 10,000 by 16 ohm and
get 625. This is the impedance ratio of this transformer using only the 16
secondary winding, if I put an 8 speaker on the 16 ohm winding, the primary
will look like 625 x 8, or 5k-ohm.
Now, how much power will a transformer handle? Tough question. I generally
figure 3 lbs = 25W, 6 lbs = 50W. This is not always a good guide, as some primaries
are limited in that they will only carry so much current.
If you have two transformers with the same specifications, and one has a larger
iron core, by all means use the one with the larger core. It will be less likely
to saturate from the DC going through. It will also have a better frequency
response.
Audio enthusiasts will value the fundamentals discussed here in this examination
of transformer types.
Above: Fig. 1: FULL-WAVE CHOKE INPUT.
Above: Fig. 2: FULL-WAVE CAPACITOR INPUT.
Here is an interesting fact: an ordinary 6.3V filament transformer has a voltage
ratio of 19.75. Square this and you have 389.7, the impedance ratio of this
transformer. Put an 8-ohm speaker on the secondary, and the primary will look
like 3117-ohm. You might use this on a single-ended 2A2 and the low frequencies
will sound good. The highs won’t be too good.
If you are a single-ended fan, there is another consideration to look at.
A transformer that has linear taps can provide some more options.
Let’s say the linear tap is a 40% one. This means that the transformer has
a tap 40% of half the primary away from the center point of the whole winding.
Actually, that 40% is only 20% of the whole winding, if you add 20% to 50%,
you have 70% of the winding.
Now, square .7 to get .49. if the whole primary has 5k-Ohm, then .49 x 5000
gives 2450 as the primary impedance. This is the primary impedance using one
40% tap and the opposite end of the winding.
These statements hold true if an 8-ohm load is put on the 8-ohm tap of the
secondary. If an 8-ohm load is put on the 16-ohm tap of the secondary, the
primary will look like 1225-ohm.
Most builders of tube amps think that only one specific transformer will work
on the output of their project.
What I have just said might change that thinking. Most transformers are quite
versatile. I have a Hammond 16500N output transformer that has been adapted
to many different tube loads with great success.
**CHOKE**
A choke—at least the kind you use in the audio world—is an iron core with
only one winding on it. A choke is rated in henries and current-carrying capacity.
Its DC resistance is so low that it’s generally ignored.
A choke’s value lies in its ability to do two things.
1. It can store energy. DC current flowing through the winding sets up an
electric field that acts like a storage battery. When the current is shut off,
the choke actually tries to keep it flowing. If the current flow subsides,
the choke tries to keep it at its original level, if a bump of current comes
along, the choke absorbs it in its magnetic field, thus preventing the bump
from going further. It’s a smoothing device. As such, it’s a very useful component
in power supplies.
In power supplies, the minimum value to use is given by the following formula:
inductance in henries = the output voltage divided by the load cur rent in
milliamperes. A 300V supply with a 100mA load should have a choke of 3H or
more, A choke’s second ability is its resistance to the flow of AC current.
However, this resistance varies with both the inductance in henries and the
frequency.
Above: Fig. 3: FULL-WAVE BRIDGE. C = 0.01uF. R = 500,000 ohm.
If the inductance is fixed at a certain value, then the resistance will be
much higher at high frequencies than at low frequencies. Reactance = 2 x 3.1416
x the frequency x the number of henries in the choke. By the same figuring,
if the frequency stays at a fixed value, in creasing the number of henries
in creases the resistance to the flow of AC current.
From a practical standpoint, this means that in a single-ended amp with parallel
or choke feed, the lows will be reduced a bit by the choke. Compensation for
this can be included in the preamp.
If you have an SE amp with choke feed, the load impedance on the out put tube
looks like a very complicated situation. The tube plate is looking at two inductances
in parallel. In many cases, the inductance of the choke is equal to the inductance
of the primary on the transformer. They are in the circuit in an almost identical
manner. Both have one end tied to the tube, and the other ends go to ground
through a large capacitor.
Now for the differences that exist. The choke is carrying a large DC cur rent,
which will have no reactive effect on the tube if its iron core is big enough.
It is, however, carrying a relatively small AC current to ground from the tube,
the amount being governed by the reactance the choke presents.
**TRANSFORMER**
The transformer’s primary may show a very high reactance if its secondary
is open. But as soon as a load is put on this secondary, you have changed the
situation substantially.
The primary impedance of this transformer is being controlled more or less
by its secondary. In the case of a transformer with an impedance ratio of 325,
8-ohm on the secondary is reflected in the primary as 2600 ohm, 50 ohm on the
secondary is reflected as 16250.
Thus you can say that the transformer is the most important factor governing
the load of a tube. Its impedance is very low compared to the impedance of
the choke, and it’s in essence the only load.
At this point, someone is going to bring up the fact that the inductances
of the choke and transformer are in series with a capacitor and could be in
resonance at some audio frequency. Forget it. With the value of components
you use at audio frequencies, you will never be bothered with that problem.
Power transformers are rather commonplace items that you often take for granted.
You either buy one and hope ii fits the job or you pull one from the junk box
and make it fit the job.
In spite of what you pick, it never seems to be big enough. So right now,
resolve to use the biggest transformer you can obtain for the job at hand.
Forty to 50% over what you think you need will always work out fine. A bigger
transformer will have larger wire on the secondary. This means a lower resistance
secondary, which is a definite plus.
**RECTIFICATION**
You can use either half-wave or full- wave rectification. But don’t even
consider the half-wave route because it just can’t cut it. Too much hum f any
audio purpose. I also caution you not to work from the power mains directly.
It’s too dangerous. Use a transformer.
There are three ways to get full-wave rectification.
1. Use two diodes with the center tap of the secondary on ground.
2. Use four diodes in a bridge circuit.
3. Go the voltage doubler route. Yes, there is a full-wave doubler circuit,
which is used more often than you might think. It just takes a lot of capacitance
(see Figs. 1,2,3, and 4).
The number one route is the preferred one to take. If you use choke input,
the transformer will love it. Output voltage is generally 0.9 x one-half of
the total voltage out of the secondary. Using number one to capacitance input
is harder on the transformer and may cause it to heat a little. With capacitance
input, the output voltage is about 1.1 x one-half of the secondary voltage.
Going the second route, with four diodes, on a transformer intended for use
with the two diode system, causes a little problem. If the transformer is rated
at 200mA, you should not try to get 200mA out of it. It will run hot be cause
the whole secondary is in use all the time. I would de-rate the transformer
about 30%.
The bridge circuit provides two volt age outputs. One is the same as in the
two diode system, and a second voltage twice that of the two diode system.
You can alter both voltages by using either a choke or capacitor input. If
the transformer secondary has no center tap, then only the higher voltage
will be available.
With the voltage doubler circuit, the same caveats from the bridge circuit
apply. The whole secondary is in use all of the time. The doubler system is
a good one to use when you have a low voltage transformer without a center
tap. You should be prepared to accept poor voltage regulation with a voltage
doubling circuit. If your amp is running class A, there is no problem, as the
load has very little variation.
A good transformer for use in the voltage doubling circuit is an isolation
transformer that has a 120V primary and a 240V secondary. One rated at 100
to 150 VA would be suitable.
**TRANSFORMER TIPS **
You can alter the voltage obtained from any of these systems either up or
down by using spare filament windings on the main transformer or by using a
separate filament transformer. I would not recommend raising the transformer-
rated primary voltage by more than 6 or 7%. To do so stresses the transformer
too much. You can reduce the transformer primary voltage as much as you care
to without damage.
The spare windings are used in series with the main transformer primary. You
can wire them in aiding or opposing the supply mains. There is an unusual way
to pick up an additional 45 to 115V on top of the main transformer supply if
you have an isolation transformer with two separate primaries and two separate
secondaries ( Fig. 5).
Above: Fig. 4: FULL-WAVE DOUBLING CIRCUIT.
Many times, the problem is not one of voltage, but of not enough current to
do the job. You can parallel identical transformers without trouble, but don’t
try it with unlike transformers.
If you have spare transformers around or can pick some up at a flea market,
some of the configurations mentioned previously can enable you to put together
a good supply at a reasonable cost.
Diodes sometimes can generate noise in the power supply. You can eliminate
or greatly reduce this by the use of bypass capacitors and resistors as shown
in Fig 3.
I would like to add a thought here about what a power supply looks like to
an amplifier. It makes a big difference in sound if the amp is looking at a
low impedance supply.
With the average supply, you generally have about 40 to 80 of capacitive filtering.
This is not nearly enough to offset the highly inductive transformer and choke.
The addition of 500 to 700 j on the output of the supply will lower the reactive
impedance and make the supply look more like a resistor.
I think that a large capacitive output also acts as a regulator, and sometimes
makes a regulated supply unnecessary. Believe me, it sure improves the response
of the amp.
Many builders use DC on the filaments of their tubes. Here is a little tip
to prolong the life of tungsten filaments: Put a DPDT switch in the filament
line to reverse polarity every 100 hours. It’s a proven fact that tungsten
migrates from the filament with pro longed application of fixed polarity direct
current. You can increase tube life 50 to 75% by switching polarity.
Above: Fig. 5: TWO-TRANSFORMER-FULL WAVE ADDITIVE CONNECTION
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