16 August, 2012

Transformerless tube power supply

In 2012 bulky power transformers that work directly from the power grid at 50/60 Hz have mostly disappeared. My objective here is to modernize the power supply for a one-tube transmitter in the same way.

The circuit is based on an electronic transformer for LED or halogen lamps. Electronic transformers usually have a minimum power rating, below which they will not start. This one can tolerate a lower load than most and gives out 12 Volts for a load from less than 10 W and up to 60 W.

My target circuit is the AA8V/W8EXI 6CL6 one-tube transmitter (5 Watts or so). It needs 6.3 Volts for the filament (0.65 A), about 350 Volts DC for the plate and a regulated voltage of 200 Volts DC or so for the screen.

First I was inspired by DL2YEO and his Power Supply for small Tube Amplifiers. His approach was to redesign and rewind the high-frequency transformer (hmm, also a transformer, albeit a tiny one) of the circuit in order to get the desired high and low voltages. It takes some research to find the number of windings per volt and then to fit two secondary windings on the transformer which originally only had a single one. I tried this, but ended up with a burnt, shorted circuit. I concluded that it is too easy to make a mistake in this way.

My second approach was to reuse the small transformer from the burnt supply. I had to wind the original secondary back on it. The transformer is now reversed so the old secondary is used as the primary winding in order to step up the 12V AC. Then with a voltage doubler and filter capacitors I was able to get a DC voltage of 330 V. A series resistor and two 100 V Zener diodes in series gave me the regulated screen supply.

The filament voltage is obtained from the 12 VAC, which is rectified and regulated with a LM317 voltage regulator to give 6.3 VDC. This is shown in the lower part of the veroboard where a load resistor that simulates the tube's filament is attached to the green terminal block.

Because the whole circuit runs at about 45 kHz, only fast-recovery rectifiers can be used, not the ones that are used at 50/60 Hz. The electronic transformer is from the Nordic retailer Biltema (part no 46-273, sorry no English web page).

I only hope now that this power supply won't generate a lot of noise for the receiver.


  1. Hi! Interesting approach indeed. However, do you have any schematics?

    I need to provide 220VDC 60 mA to my OTL head amp (Cavalli Jones). Should I wind the step-up transformer to match that requirement, or should I use fewer windings together with a multiplier? What do you reckon?

    How do you ground (safety ground) a thing like this?

  2. Hi Robin. I planned to wait with publishing of the schematics until I had tested the power supply in a real application.

    But it is very straightforward. The high voltage part is a bridge (Delon) circuit as in the Wikipedia article with 220 uF/400 V capacitors (from an old switch mode power supply) and 1N4937 diodes. I haven't tested it yet with respect to how much the voltage varies with load.

    In your case, I would guess that a straightforward rectifier might give too little voltage and a doubler too much, so a doubler with a changed winding ratio could perhaps be the way to go?

    Safety ground, what's that? I haven't really thought much about that.

  3. OK, I see. Thank you for the further details on the circuit. I look forward reading your results.

    I will try and see regarding the step-up transformer. Did you do any calculations or did you just try?

    Maybe protective ground is more correct to say. I want to connect the amp's metal case to ground, so if any high voltage cable in the amp get lose it will trigger/burn the fuse. Otherwise there is a risk that one touches the amp case and something that is grounded, creating a closed circuit.

  4. I may have been a little unclear on the step-up transformer, therefore I revised the text a bit above also. This transformer is exactly like the step-down transformer. It was taken from the first burned out PCB. Since I had been experimenting with the transformer I had to rebuild it to the original state.

    I would guess you can just connect the minus to ground like with other power supplies.