Audio and Video Talk > Valves / Vacuum Tubes

How to design a Power Transformer for a Push-pull tube amplifier?

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Mars:
After many years of struggling with low HT and filament voltages and over heating power transformers, I have decided to look into the various factors the make a PT tick.

What I want is a power transformer that is silent, runs cool and delivers the filament voltage spot on at 6.3V and the specified HT AC voltage. If a tube rectifier is used, the 5V winding should also deliver 5V spot on. Can this be done? I think so; and this is how I do it. (I reserve the right to dump my method for a better method at any time :thinking:)

But first some  :coffee:

alternativeroute:
 :)

Looking forward to the discourse... It's all Greek to me but maybe something sinks in...  :nutter:

Mars:
I have to design a PT for a 35 + 35 watt stereo amplifier with the option to use a tube rectifier/s.

My OPT's are 4000 ohm primary impedance and rated at 35 watts. Thus what voltage must I have on the tubes to deliver 35 watts into 4000 ohms? Well Power(watts)=Current(Amperes) X Voltage(Volts) and from Ohm's Law Current=Voltage/Resistance(Ohm); therefore

Power=Voltage^2/Resistance

See the diagram below for clarity:


Volts = Sqrrt/(Power X Resistance)
         = Sqrrt(35 x 4000)
         = 374V DC

This is the voltage the tubes must see. Voltage losses across the cathode resistors is probably about 40Volts or so. So the HT should hang around 420V on the Anodes of the power tubes. Now if you were to use a tube rectifier, another 50V or so voltage drop should be anticipated; so we need 470VDC.

A thumb guess at what the AC voltage should be is 470V/1.2=392VAC

and

Current = 374/4000
             = 0.094A DC

Two of such amplifiers are in a stereo amp; so the PT must deliver 0.188 Amperes DC.

My guess would be a center tapped PT delivering 400-0-400 VAC with a current capacity of 200mA.

So let us check what Yve's Power Transformer Designer says the transformer should look like.


handsome:
Dont forget the kind of rectification (half wave/full wave/bridged) plus the kind of PSU (capacitor input or choke input) will also determine the amount of current the transformer must deliver.......

Mars:
Does anybody want to learn of the different power transformer topologies (center tapped, bridge, diode, tube rectifier, choke/capacitor input etc.) ? I do not mind doing a quick survey of the territory.

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