Hi all - a basic question, but one which has confused me a little:
For a Class A single-ended amp, the efficiency is quoted as 25%, however I've read that's for a resistive load and the efficiency with an inductive load is 50%.
Therefore, if sizing a power tranny for a 12w stereo, should I be looking for B+ rating of 12x2x2=48W? Adding, say, 10% for transients taking it to ~55W. I'm sure there's some rule-of-thumb out there.
I don't want to undersize, but as the same time double the iron necessary is, well, unecessary.
Thanks in advance!
For a Class A single-ended amp, the efficiency is quoted as 25%, however I've read that's for a resistive load and the efficiency with an inductive load is 50%.
Therefore, if sizing a power tranny for a 12w stereo, should I be looking for B+ rating of 12x2x2=48W? Adding, say, 10% for transients taking it to ~55W. I'm sure there's some rule-of-thumb out there.
I don't want to undersize, but as the same time double the iron necessary is, well, unecessary.
Thanks in advance!
These are theoretical efficiency numbers and valid *only* for the output tubes alone ignoring power supply filtering, rectification, possible cathode biasing, driver and pre stage currents a.s.o.
Furthermore they are only valid for continuous full power output.
Class A Single ended has max power dissipation at idle (efficiency zero) and actually drops with signal (theoretically reaching your numbers at full volume).
It is not advisable to decide about mains transformer first, instead it is necessary to do it the other way round:
Decide about a concept for the output stage, triode, pentode, ultralinear, cathode bias, fixed bias ...
next pick your favourite suitable output tube ...
then design or pick a circuit schematic ...
now that you know which voltages and currents you need, decide about B+ filtering, choke, resistor, caps, rectification, tube or solid state ...
and now calculate the power requirements for the idle case and finally the last thing you do is pick a suitable mains transformer ... and don' t forget the heaters.
Example: say you need 100mA standing current in your outputs, at 500v plate, cathode bias drops 50v, ps filtering swallows another 50 v, so your total B+ DC requirements becomes 120W, add 30W for drivers and pre and other stuff, thats 150W DC. Your transformer is rated AC though, which has to include transformer losses and the fact that rectifiers produce sharp current pulses which cause more losses in the windings. Tube recifiers will make things much worse - power wise.
You will have to pick a 240VA trafo at least, and it will not be oversized.
And it will not include heater power.
Please note that the above example is *not* a recommendation for your case. just a demonstration of the method.
Furthermore they are only valid for continuous full power output.
Class A Single ended has max power dissipation at idle (efficiency zero) and actually drops with signal (theoretically reaching your numbers at full volume).
It is not advisable to decide about mains transformer first, instead it is necessary to do it the other way round:
Decide about a concept for the output stage, triode, pentode, ultralinear, cathode bias, fixed bias ...
next pick your favourite suitable output tube ...
then design or pick a circuit schematic ...
now that you know which voltages and currents you need, decide about B+ filtering, choke, resistor, caps, rectification, tube or solid state ...
and now calculate the power requirements for the idle case and finally the last thing you do is pick a suitable mains transformer ... and don' t forget the heaters.
Example: say you need 100mA standing current in your outputs, at 500v plate, cathode bias drops 50v, ps filtering swallows another 50 v, so your total B+ DC requirements becomes 120W, add 30W for drivers and pre and other stuff, thats 150W DC. Your transformer is rated AC though, which has to include transformer losses and the fact that rectifiers produce sharp current pulses which cause more losses in the windings. Tube recifiers will make things much worse - power wise.
You will have to pick a 240VA trafo at least, and it will not be oversized.
And it will not include heater power.
Please note that the above example is *not* a recommendation for your case. just a demonstration of the method.
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The relative specification requirements of the B+ secondary:
A choke input filter (0.9 x rms Volts) requires more secondary volts than a cap input filter in order to get a specified B+ Volts.
But the I squared heating losses in the secondary are lower than for a cap input filter.
Therefore the required current rating of the secondary is low, and runs cooler.
A capacitor input filter (1.414 x rms Volts) requires less secondary volts than a choke input filter, in order to get a specified B+ Volts.
But the I squared heating losses in the secondary are higher than for a choke input filter.
Therefore the required current rating of the secondary is high, and runs hotter.
A choke input filter (0.9 x rms Volts) requires more secondary volts than a cap input filter in order to get a specified B+ Volts.
But the I squared heating losses in the secondary are lower than for a cap input filter.
Therefore the required current rating of the secondary is low, and runs cooler.
A capacitor input filter (1.414 x rms Volts) requires less secondary volts than a choke input filter, in order to get a specified B+ Volts.
But the I squared heating losses in the secondary are higher than for a choke input filter.
Therefore the required current rating of the secondary is high, and runs hotter.