To answer your question about transformer sizing
requirements....
There is no absolute rule on this...It is at the
discression of the designer to choose a power
transformer that will have the choosen temperature
rise and choosen regulation.....
The heater winding should be choosen exactly to
current requirement, since having the proper load
current will bring in the voltage to 6.3V ....If you
over-rate your heater winding then you may not get the
proper AC voltage drop from the winding resistance and
then you will get a higher voltage such as 6.45V for
example.... this depends on the design...
If you choose a power transformer to small....then the
current demand will over-heat the transformer and the
voltage regulation will be poor from the winding
resistance... Failure could result...
If you choose a over-kill HUGE transformer for your
High Voltage winding..then your temperature rise would
be small and your regulation would be very good....
BUT at the expense of SIZE and space as well as more
$$$$$...
It's not really the core size, since that is at
constant flux density...it's the copper capacity that
is key..BUT, you need the larger core to fit the
proper copper capacity.... So if you had ideal wire
with virtually no resistance than the core size would
not matter as long as you have enough turns to
accomadate a descent flux density...
Some of the requirements you need to properly choose a
power transformer is first your Quiescent idle
current...which dictates where your idle DC plate
voltage will be at.....
Then to figure what your full power output RMS
current demand is... I simply go to the plate
curves..make sure you use the proper curves for the
given SCREEN VOLTAGE you are using..then you plot your
load-line.... Then you can see your peak AC current
swing...for AB1 this would be roughly at the 0-bias
curve intersection of the load-line...hopefully this
will be just above the knee.... That will be your RMS
AC current at full power output...don't forget to
include screen currents....which is your DC current
draw... This would just be for the power stage...you
would inlude additional stages... Class AB has
cut-off....Class A would be a bit different...
SInce transformers are selected by VA...You would then
spec the VA for (DC plate volts x DC current) ...
You can calculate the % DC regulation of the
transformer....
Most transformers normally have DC volatge drop at
full power output...and it's that voltage you are
concerned with for calculating the power output...DC
voltage at quiescent is basically higher and not
usefull for any dynamic calculations...
In Hi-Fi amps....most of your waveforms will be
considered AC sine waves, and your not operating at
full output...so from a $$$ and size point of view the
power transformer spec can ne relaxed a bit ...figure
60% to 70% deratted...
In a guitar amp...the output valves are vertually
SQUARE wave overdriven most of the time....therefore
99% of the sqaure wave is at peak current....So in
that case you can't skimp on the current ratting....
Ultamatley TEMPERATURE RISE is what dictates core size
selection... Then voltage regulation will follow
accordingly to temp.....
Chris
requirements....
There is no absolute rule on this...It is at the
discression of the designer to choose a power
transformer that will have the choosen temperature
rise and choosen regulation.....
The heater winding should be choosen exactly to
current requirement, since having the proper load
current will bring in the voltage to 6.3V ....If you
over-rate your heater winding then you may not get the
proper AC voltage drop from the winding resistance and
then you will get a higher voltage such as 6.45V for
example.... this depends on the design...
If you choose a power transformer to small....then the
current demand will over-heat the transformer and the
voltage regulation will be poor from the winding
resistance... Failure could result...
If you choose a over-kill HUGE transformer for your
High Voltage winding..then your temperature rise would
be small and your regulation would be very good....
BUT at the expense of SIZE and space as well as more
$$$$$...
It's not really the core size, since that is at
constant flux density...it's the copper capacity that
is key..BUT, you need the larger core to fit the
proper copper capacity.... So if you had ideal wire
with virtually no resistance than the core size would
not matter as long as you have enough turns to
accomadate a descent flux density...
Some of the requirements you need to properly choose a
power transformer is first your Quiescent idle
current...which dictates where your idle DC plate
voltage will be at.....
Then to figure what your full power output RMS
current demand is... I simply go to the plate
curves..make sure you use the proper curves for the
given SCREEN VOLTAGE you are using..then you plot your
load-line.... Then you can see your peak AC current
swing...for AB1 this would be roughly at the 0-bias
curve intersection of the load-line...hopefully this
will be just above the knee.... That will be your RMS
AC current at full power output...don't forget to
include screen currents....which is your DC current
draw... This would just be for the power stage...you
would inlude additional stages... Class AB has
cut-off....Class A would be a bit different...
SInce transformers are selected by VA...You would then
spec the VA for (DC plate volts x DC current) ...
You can calculate the % DC regulation of the
transformer....
Most transformers normally have DC volatge drop at
full power output...and it's that voltage you are
concerned with for calculating the power output...DC
voltage at quiescent is basically higher and not
usefull for any dynamic calculations...
In Hi-Fi amps....most of your waveforms will be
considered AC sine waves, and your not operating at
full output...so from a $$$ and size point of view the
power transformer spec can ne relaxed a bit ...figure
60% to 70% deratted...
In a guitar amp...the output valves are vertually
SQUARE wave overdriven most of the time....therefore
99% of the sqaure wave is at peak current....So in
that case you can't skimp on the current ratting....
Ultamatley TEMPERATURE RISE is what dictates core size
selection... Then voltage regulation will follow
accordingly to temp.....
Chris