Why use power transformer when you can use a voltage doubler circuit?

[tonyg88]

I noticed my 11bm8 little amp is running voltages around 250volts. Instead of a power transformer why not use a voltage doubler circuit to make the 120 to 240 volts?

Wouldn't it be much cheaper? Or is a transformer also doing other things related to current and amps?

 

[whoaru99]

To use a voltage doubler requires, afaik, the transformer and components to handle at least 2x the current because power has to be carried through... twice the voltage half the current or vice versa for a given number of power/watts. Caps I think also take more beating from higher ripple.

 

[tonyg88]

To use a voltage doubler requires, afaik, the transformer and components to handle at least 2x the current. Caps I think also take more beating from higher ripple.

Are you saying with VD circuit you actually need a better power transformer in order to do it?

 

[whoaru99]

Are you saying with VD circuit you actually need a better power transformer in order to do it?

I don't know about better, per se, but different. It would have less windings but the wire would need to be thicker.

Maybe someone else would chime in, but I see it more a matter of what you have on hand to use vs. selection for clean slate.

 

[egellings]

If you mean dump the power transformer and place the voltage doubler across the line in order to get your 300 or so Vdc, there are huge safety problems with that and I recommend never going that route. Take the safety due to isolation from the line that the power transformer offers.

 

[RossW]

There are amps that use voltage doublers to get a high B+ voltage (Sansui comes to mind), but as egellings mentioned, you still need a transformer for isolation.

 

[gadget73]

Lack of isolation from the power line. If you were to use a doubler straight across the line, one side of the line ends up tied through a diode to chassis. Chassis to an earth ground would be basically 60vac at enough current to ruin your day.

Fun fact, switch mode power supplies often do rectify directly from the power line. The DC input is then fed through the switcher circuit and the transformer to get isolation from the mains but there is a spot inside there where its a 1940s AA5 power supply.

 

[whoaru99]

Good catch guys... didn't notice that direct from the line part.

 

[egellings]

The little transformers in those DC switchmode supplies can be tiny because they run so fast, like 250kHz or faster. Each cycle has to deliver just a thimble-full of juice, but you get a lot of them in a short time. At 60Hz, everything just bogs down. Primary inductance has to be high, necessitating more wire & iron.

 

[grindfix]

Old cheaper all wood consoles had those "hot chassis" designs without PT using voltage doubler. As long as user isn't curious to get inside - wood is not a good conductor.

 

[gadget73]

one thing you get from a doubler though is the possibility of more powa! More B+ available can get you more plate to plate swing on the output stage. I have actually worked on one hot chassis vacuum tube voltage doubler setup. Philco used it in the late 40s. It gave an extra 50v or so. It was just a single ended tabletop that didn't really work any better than an AA5. PS had more parts and it used a strange rectifier tube.