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Output Transformer Impedance
=20
Determining Output Transformer Impedance
When replacing the audio output transformer on a radio, the =
replacement=20
should match the impedance of the original as close as possible. If =
the wrong=20
transformer is used, the results can be low output and loss of tone =
quality.=20
Universal output transformers are available which have multi-tapped =
primary and=20
secondary windings to match a wide impedance range. =20
It is not uncommon for the vintage radio collector to have various =
output=20
transformers laying about that have been pulled from parts sets, or =
have been=20
obtained at swap meets. Often times, the primary and secondary =
impedance=20
information is not available for these units, and it would be nice to =
have that=20
information to be able to use these in a set that needs a new output=20
transformer.=20
The average beam power output tube, such as the 6V6 requires a load =
of around=20
5,000 ohms, and the average speaker voice coil can vary from as low as =
1 ohm to=20
8 ohms or more. So, how do we determine which of our output =
transformers will=20
match that impedance range? With some simple test equipment and ohms =
law, we=20
can calculate the impedance of an output transformer, but first lets =
look at=20
the function of an output transformer and how it works.=20
Function And How It Works
A tube is a =
high-voltage/low-current=20
(high-impedance) device, while a speaker is a low-voltage/high-current=20
(low-impedance) device. The function of the audio output transformer is =
to=20
transform the high impedance of the output tube to match the much lower =
impedance of the speaker. This is necessary to get an efficient =
transfer of the=20
audio signal to the speaker. The output transformer as an impedance =
matching=20
device, works on the principal of reflected load. To help =
explain this,=20
refer to figure 1 below.=20
To keep the math simple, lets assume an output tube is supplying a =
100 volt=20
ac signal to the primary of an output transformer with a 10:1 winding =
ratio,=20
and the secondary is feeding a 10 ohm voice coil (see figure 1A below). =
With 100=20
volts across the primary, there will be 10 volts across the voice coil=20
connected to the secondary. Using ohms law, there will be 1 amp of =
current=20
flowing in the voice coil.=20
I =3D E/R=0A=
I =3D 10/10 =3D 1 ampere=0A=
For further simplification we will=20
assume 100% efficiency in the transformer. Since we have a 10:1 ratio, =
the=20
current flowing in the primary will be .1 ampere ( 1 amp in =
secondary=20
divided by 10). With 100 volts across the primary, ohms law tells us =
that the=20
primary looks like a 1,000 ohms impedance load to the tube.=20
Z =3D E/I=0A=
Z =3D 100/.1 =3D 1,000 ohms=0A=
Now if we decrease the=20
impedance of the load, what happens to the impedance in the primary? If =
we place=20
another 10 ohm voice coil in parallel with the original one, we now =
have a 5=20
ohm load (see figure 1B below). Using ohms law again we see that the =
current in=20
the secondary is now 2 amperes.=20
I =3D E/R=0A=
I =3D 10/5 =3D 2 amperes=0A=
This means that the current in the=20
primary also doubles to .2 amperes. Again using ohms law, the =
impedance=20
of the primary is now 500 ohms.=20
Z =3D E/I=0A=
Z =3D 100/.2 =3D 500 ohms=0A=
This is called the reflected=20
load. A 10 ohm load reflects back a 1,000 ohm impedance, while a 5 =
ohm load=20
reflects back a 500 ohm impedance. The reflected impedance is a function =
of the=20
turns ratio of the transformer. Notice that the ratio of the primary =
impedance=20
to the secondary impedance is the square of the turns ratio, or 100:1. =
In other=20
words, a 10:1 turns ratio will give an impedance ratio of 100:1.=20
Figure 1
|
=
TABLE>
Checking An Unknown Output Transformer
We can now use =
this=20
knowledge to determine the impedance of any unknown output transformer. =
All we=20
need to do is determine the turns ratio of the transformer, and with =
that=20
information we can calculate what impedance will be reflected back to =
the=20
primary with a given load on the secondary. The test equipment to do =
this is=20
quite simple; an ac voltmeter and a variable source of 60 Hz ac is all =
we need.=20
To determine the turns ratio we apply an ac voltage to the primary, =
and=20
measure the voltage in the secondary. The voltage on the secondary will =
be
stepped down by a proportional amount determined by the turns ratio of =
the=20
transformer. Figure 2 below shows the schematic diagram of the test =
set-up. A=20
variable auto-transformer (sometimes referred to by the brand name =
Variac) is=20
used to apply the variable ac to the primary (see note below). An ac =
voltmeter=20
is connected to the secondary to measure the output voltage. To make =
the=20
calculation of the turns ratio easy, the input voltage is increased =
until the=20
voltage on the secondary reads 1 volt. With the secondary reading 1 =
volt,=20
measure the input voltage to the primary. Since the voltage on the =
secondary=20
is set to 1 volt, the voltage measured on the primary will be the turns =
ratio.=20
For example, if the voltage on the primary measures 25 volts, the turns =
ratio
is 25:1 as illustrated in figure 2 below.=20
Note: An auto-transformer does not provide isolation from the =
ac line.
For safety reasons, the auto-transformer should be used in conjunction =
with a=20
1:1 isolation transformer.=20
Figure 2
|
=
TABLE>
Now, armed with the turns ratio, we can calculate the impedance ratio =
and=20
the impedance that will be reflected to the primary with a given load =
in the=20
secondary. Remember we said earlier that the impedance ratio is the =
square of=20
the turns ratio. With our 25:1 turns ratio transformer in figure 2, the =
impedance ratio is the turns ratio squared or, 25 X 25 =3D 625:1. So if =
the=20
transformer is working into an 8 ohm load, the impedance that will be =
reflected=20
to the primary will be the impedance ratio (625) multiplied by the load =
impedance (8 ohms), equal 5,000 ohms. If the load in the secondary is =
changed=20
to a 4 ohm load, the reflected impedance in the primary would be 625 X =
4 =3D=20
2,500 ohms.=20
So it can be seen that the turns ratio of the transformer determines =
what=20
impedance will be reflected to the primary by the load impedance of the =
secondary, and that an improper load on the secondary can have several =
effects:=20
- if the load impedance is too low this will increase the current in =
the =20
transformer windings. This also reduces the primary impedance that =
the=20
output tube sees which increases tube current flow.=20
- if the load impedance is too high, this reflects a higher than =
normal
impedance in the primary for the output tube.=20
- poor sound quality and lack of volume.
The impedance =
load seen by=20
the tube and output transformer is not constant. The frequency of the =
audio=20
signal will vary over a wide range. The inductance in the windings will =
have a=20
different impedance at different frequencies. At a certain frequency an =
8 ohm=20
voice coil may have an impedance of 10 ohms or at low frequencies it my =
have an=20
impedance of 4 ohms. This varying load impedance is reflected back to =
the=20
primary, so the tube, and output transformer must work into a varying =
impedance=20
range.=20
Chosing A Replacement Output Transformer
Now lets =
assume we=20
have a set that has an output transformer with an open winding that must =
be=20
replaced, and the output tube is a single 6F6. The schematic will =
sometimes give=20
the dc resistance of the windings but that doesn't help us know what =
impedance=20
the transformer should be. First we look at the chart below and see =
that a 6F6=20
should work into a load resistance of approximately 7,000 ohms. What =
about the=20
speaker voice coil impedance? If we do not know the voice coil impedance =
we can=20
get a fairly good estimate by measuring the dc resistance of the coil =
and=20
multiply by 1.25. Lets say this one measures 6.5 ohms, multiplied by =
1.25 =3D=20
8.125 so we could call this an 8 ohm voice coil.=20
With this information we can determine what the turns ratio should =
be. By
dividing the secondary load of 8 ohms into the 7,000 ohms required by =
the 6F6,=20
we get an impedance ratio of 875:1. If the impedance ratio is the =
square of=20
the turns ratio, then the turns ratio is the square root of the =
impedance=20
ratio. Taking the square root of 875 gives us a 29.6:1 ratio, so an =
output=20
transformer with a turns ratio in this range should work. =20
By using the test procedure outlined above we can test our spare =
output=20
transformers to see if we have one that will meet our approximate 30:1 =
turns=20
ratio requirement.=20
Wattage Rating
Output transformers are rated in =
wattage. A good=20
rule of thumb is that the replacement transformer should be about the =
same size=20
as the original. If the replacement has the same size core or larger it =
should=20
handle the wattage OK.=20
Chart Showing Approximate Plate Load Resistance
For =
Various=20
Output Tubes
Primary Load Impedance
| 18,000
| 14,000
| 10,000
| 8,000=20
| 7,000
| 5,000
| 4,000
| 2,000 |
Single Ended Class A=20
| 1F4 1F5=20
| 1J5 1LB4 1T5 10 12A 12A7 38=20
| 6G6 19 41 49=20
| 1C5 1G5 1Q5 1S4 3Q5 3S4 6K6 6A4=20
7A5 7B5 12A6 14A5=20
| 2A5 6F6 12A 42 47=20
| 3B5 6AQ5 6L6 6V6 7C5 12A5 14C5 50=20
71A=20
| 6AC5 12A4 43 45 117L7=20
| 2A3 6Y6 7A5 25L6 35A5 35B5 35L6 =
48=20
50A5 50B5 50L6 |
Push Pull Plate to Plate=20
|
| 6K6 47=20
| 6AQ5 6F6 6L6 6V6 42=20
| 6V6 12A5 50 71A=20
|
| 2A3 45=20
|
| |
Note: The load resistance will vary somewhat depending upon =
the=20
voltages applied to the tube and the type of bias (fixed or self-bias). =
Consult=20
a tube manual for actual load resistance under different voltages and =
bias.=20