This section will help explain how changes in wall voltage affect plate voltages.
Because the primary to secondary voltage is a turns ratio
the voltage on the secondary will change
as the voltage on the primary increases and decreases. If you measured the
wall voltage around the
country you would find differences from place to place. These voltages can
be as low as 118vac to
as high as 127vac here in the US. These differences are due to the amount
of demand on the line
and the acceptable tolerance the power company allows for in their system.
The industry standard in the US is 120vac so I have chosen this for my PT.
If you look at the
chart you will see how the input voltage on the primary affects the voltage
on the secondary.
This chart shows just the relationship between the
primary AC voltage and the secondary AC voltage.
|
AC Voltage on the Primary
|
AC Voltage on the Secondary
|
|
118vac
|
344-0-344
|
|
119vac
|
347-0-347
|
|
120vac
|
350-0-350
|
|
121vac
|
352-0-352
|
|
122vac
|
355-0-355
|
|
123vac
|
358-0-358
|
|
124vac
|
361-0-361
|
|
125vac
|
364-0-364
|
|
126vac
|
367-0-367
|
|
127vac
|
370-0-370
|
Now let's look at how the primary voltage affects the rectified
DC voltage.
If the secondary is 350vac and the voltage on the plates is
450vdc then we can find the
conversion number for the GZ34 tube rectifier by dividing 450 by 350 to get
roughly 1.285.
Multiply 1.285 by 350 to get the rectified DC. If we use the chart above we
can show how
the primary voltage affects the secondary as well as the rectified voltage
that will be
applied to the plates.
This chart shows the relationship between the
primary AC voltage, secondary AC voltage
and the rectified DC voltage.
|
AC Voltage on the Primary
|
AC Voltage on the Secondary
|
DC Rectified Voltage
|
|
118vac
|
344-0-344
|
442vdc
|
|
119vac
|
347-0-347
|
446vdc
|
|
120vac
|
350-0-350
|
450vdc
|
|
121vac
|
352-0-352
|
452vdc
|
|
122vac
|
355-0-355
|
456vdc
|
|
123vac
|
358-0-358
|
460vdc
|
|
124vac
|
361-0-361
|
464vdc
|
|
125vac
|
364-0-364
|
468vdc
|
|
126vac
|
367-0-367
|
471vdc
|
|
127vac
|
370-0-370
|
475vdc
|
Another factor that will change the plate voltage is how
the output tubes are biased.
In short, if you bias the tubes hot the voltage will drop and if you bias
them cold the
plate voltage will increase. This is due to the load or current demand placed
on the PT.
I have designed this PT to give 450vdc on the plates with
120vac on the primary with
the power tubes biased at 40mA. This gives you an operating point of 70% of
maximum
plate dissipation.
All numbers and calculations are approximate values and are given for example only.