Most of the tube circuits require anode voltage in the range between 50V and 400V and sometimes those transformer's could be expensive or simply not available. This simple project is to show the way of reusing old power adapters to create anode power supply for the vacuum tube projects.
This simple trick involves 2 similar transformers connected back to back in order to get 120V AC on the secondary side which can be rectified and filter t get 150 V DC. I pulled two 12V 500mA transformers from my junk box pictured below.
I selected two with the similar ratings 12V@500mA.
Opening the case could be tricky on some of them and be sure not to break transformer wiring by doing so.
Next, I connected the 12V of one transformer (T1) to the 12V side of the second transformer (T2).
It does not matter which side you choose as the primary side unless you are using two transformer with different current ratings. If you are using different current ratings then use one with the higher current as primary side. For example:
If you have T1 as 12V 1A and T2 12V 500mA then use T1 as the primary transformer.
The important thing to remember is the power scaling. If the first transformer is rated as 12V500mA his rating will be around 6W. If you chose the second transformer with the same rating, then the output power will be less than 6W due to the losses in both transformers.
With the configuration above I was able to get 145V dc with the 9.5 mA of current which should be plenty for two 12AX7 tubes, assuming you provide heater voltage from the separate source.
Higher voltage could be achieved using one 120V to 12V transformer and one 220 to 12V transformer where 220V one is in place of T2. This should give 250V DC using the full bridge rectifier circuit.
I did many tube circuits in the past using this technique and they all work well.
Always calculate total power you plan to supply with this circuit and pick transformers based on that calculations.
Use extreme caution when playing with this circuit because you will be exposed to the lethal voltage.