Why 2 transformers in real mctube

[Ashurbanipal]

Why are there two transformers used in the real mctube and other similar tube power supplies I've seen? The higher voltage transformer simply converts the 12.6v back into 120v. So why not just have one transformer to get the 12.6v and tap the main AC for the high voltage?

[Mark Hammer]

Why not?  Because Fred Nachbaur was a great guy but no one is in a big hurry to join him in heaven.  That's why.  The back-to-back configuration allows the builder to have easy access to both a 12v supply for the tube heater/s and to a higher voltage supply for the tubes, without the need to buy an expensive high-voltage PS transformer or expose oneself to the electrical risks of direct wall AC.  Bad enough the damn circuit uses high voltages.  The transformer isolates the user from the wall.

[JimRayden]

Why not?  Because Fred Nachbaur was a great guy but no one is in a big hurry to join him in heaven.  That's why.

That was the best darn explanation ever.

---------
Jimbo

[Ashurbanipal]

So, it's not that the thing wouldn't work, it's that a second transformer is safer. So the transformers fry before enough current gets into somebody?

[bancika]

not exactly. Transformers are usually rated more than enough to kill you. 6mA through heart should do the trick, while transformers can take much more. My transformer for tube preamp is rated 20m@220V, for instance.

Maybe I'm wrong, but I think that's correct 

[brett]

Hi
here in Oz, the primary of all mains transformers are rated at 1200V AC.  Presumably, almost all noise on the line is less than 1200 V.  But plenty of spikes are 400V AC or more, so the dampening/regulation effect of a transformer is important.
But it is a good question, coz it's not obvious why we use the added complication of 2 transformers.
You can also wind a single transformer that goes direct to the required voltage, but for most people it's more hassle than using step-down and step-up.
cheers

[JonFrum]

You always want a transformer to act as isolation between your wall supply and your circuit wiring. In the old days, they ran series string wiring for the heaters in tube radios and phonos. The voltage required to run the tube heaters in the unit would sum up to standard wall supply voltage - Google "All American Five" for an explanation. The problem was that by simply taking the wall supply and wiring the heaters straight from the house wiring, anyone who touched that wiring got full house power. As a safety matter, series string wiring was eliminated, and power transformers were built with separate filament wiring for heaters. In cases where you find a vintage amp that used series string wiring, you should really add an isolation transformer at the front of the power supply to limit your risk.

As stated above, Fred's design gives you both filament and B+ wiring with commonly available parts, and you get isolation in the package. You could do the same thing with a dedicated power transformer, but the specs are not commonly used in tube amps.

RIP Fred