Increase B+ supply

[Printer2]

I picked up an old console radio and would like to use the power transformer but it is a little low in voltage. Using a full wave circuit with a 5Y3 rectifier the center tap of the power transformer would normally be going to ground. I want to put another transformer between the center tap and the ground to elevate the voltage by about 50V. I do not see a problem with this but I may be missing something. Anyone care to comment?

[Frank Berry]

If you replace the 5Y3 with a 5AR4, the output voltage will increase by nearly the amount that you need.

[Miles Prower]

Originally Posted by Printer2

I picked up an old console radio and would like to use the power transformer but it is a little low in voltage. Using a full wave circuit with a 5Y3 rectifier the center tap of the power transformer would normally be going to ground. I want to put another transformer between the center tap and the ground to elevate the voltage by about 50V.

If you want to increase the DC by some 50V, then don't use a 5Y3. Use silicon diodes instead. The much lower forward voltage of Si diodes will add that 50V pretty easily. Given that the PTX came from an old console, don't use a large reservoir capacitor. Even if the Si diodes can gracefully handle the higher Isurge, the PTX probably won't like it.

I do not see a problem with this but I may be missing something. Anyone care to comment?

You can always connect DC supplies in series if they are floating. Adding AC to the center tap won't do that. It makes an in-phase linear splitter instead, and would add just 25V to each side. Rectify the AC and you have a DC core magnetization problem with the xfmr connected to the center tap.

[Printer2]

I wanted to use a tube rather than SS diodes as my other amps are SS. This is the only transformer I have with a 5V winding so I wanted to give it a shot. The 5AR4 would give me a lower voltage drop but when the amp is pushed hard will not sag as much either (for a guitar amp but since it is really power supply related I did not post it in the MI section).

The dc core saturation I saw coming so I did not go that route, never thought the AC would split and I would get half, not a big deal just use a higher voltage. I did not want to go to stacked dc power supplies just to keep things simple.

[Frank Berry]

If you want the voltage to sag, add a resistor between the 5AR4 and the first capacitor.

[Printer2]

Originally Posted by Frank Berry

If you want the voltage to sag, add a resistor between the 5AR4 and the first capacitor.

I do not have a 5AR4 but I have a 5Y3 and a range of low voltage transformers I can use.

[Frank Berry]

A 5Y3 has too much voltage drop. Purchase a 5AR4 and, if necessary, add a 100 ohm/10 watt resistor on the output of the tube to give you the 'sag' that you desire.
You could do the same with ss rectifiers. Just add a resistor on the output of the diodes.
I'm not sure how you could use the low voltage transformers to accomplish your goal.

[DF96]

You could full-wave rectify the low voltage transformer and add this to the high voltage centre tap. Seems a messy way to do it, when SS diodes would be simpler and you need SS for the low voltage anyway.

[Printer2]

Originally Posted by Frank Berry

A 5Y3 has too much voltage drop. Purchase a 5AR4 and, if necessary, add a 100 ohm/10 watt resistor on the output of the tube to give you the 'sag' that you desire.
You could do the same with ss rectifiers. Just add a resistor on the output of the diodes.
I'm not sure how you could use the low voltage transformers to accomplish your goal.

No point buying another tube that has less loss and put a resistor in series might as well go SS. The way the additional transformer gives you a higher voltage is the AC potential of the low voltage transformer (say a 48V transformer) adds to the voltage of the secondary that is conducting at the time. So if the transformer was a 250v - 0 - 250v, and the low voltage transformer was a 50V there would be 300V (rms) on the rectifier. At least that is how I saw it.

[Printer2]

Originally Posted by DF96

You could full-wave rectify the low voltage transformer and add this to the high voltage centre tap. Seems a messy way to do it, when SS diodes would be simpler and you need SS for the low voltage anyway.

Not to the center tap as you would be putting DC through the transformer and reducing the capacity of the center tap transformer. To do it this way you would stack the two dc supplies on each other.