A question about transformers

[maroonedwizard]

Hi

I am about to built The real McTube from Tonepad (http://www.tonepad.com/getFile.asp?id=43). In the schematic there is one trasformer that takes 120V input and has 12V output that goes at A and C pads on the PCB. Then in parallel from the pads B and D the 12V voltage are going into another transformer that has an output of 120V.

My question is , i have e toroidal transformer with 2 inputs of 120v and 2 outputs of 12V so in order to make this work can i put 120V in the first input of my transformer then take 12V from the first output, route it to pads A and C and then conect the same output in parallel to the other 12V out of my transformer in order to take from the other 120V insert the voltage i need for pads B and D ??

I hope i did'nt confuse you.

[R.G.]

I think what you asked is whether you can just use the two 120V primaries, only use one for the AC power input, but the second one for the isolated 120V.

If so, the answer is yes, that will work. But you might not want to do it for safety reasons. When AC power transformers are made, there is special extra insulation between the AC power windings and the secondary windings. This is because there are all kinds of high voltage transients on the AC power line. The extra insulation, beyond what is needed just to work right, is to prevent these transients from arcing across to the secondary side and causing a shock hazard.

The two-transformers setup is designed to sidestep this issue by using a second transformer so that all the windings that are "secondary" are behind at least one of the extra insulation layers.

I recommend you use two transformers and stay safe.

[maroonedwizard]

Thank you R.G. for your reply.

[mojo_hand]

Here's a thought...

The two-transformer arrangement seems to have been the result of the author's wish to run the pedal off a wall wart.  If you're using a toroidal transformer, it sounds like you are using a different approach, and are plugging into a wall outlet.

If so, there's really no compelling reason to use a second transformer at all; absent the wall wart advantage, it becomes an isolation transformer, which tubes might benefit from a little, but definitely do not need.  You *can* get 120->12 off the one, then rectify and filter the 120 VAC coming out of the wall for the +140v.  This won't give you the line regulation that a lightly loaded transformer will, but when you look at the brute force RC filter sections that go into tube equipment, the net difference isn't likely to be significant under any normal operating conditions.  If you're worried about dangerously high voltages from your local AC provider (tubes shrug those off with relative ease, BTW), a few light-duty MOVs and a high voltage regulator IC would afford far more protection than a transformer, and need not be significantly more expensive.

As always when connecting something to a wall outlet, do make sure that you have a rock-solid connection to the ground prong, and don't forget the fuses!

[R.G.]

The tubes don't need the isolation, all right.

YOU DO. Many guitar amps, even more TVs were built that way, with no isolation transformer and rectified AC line providing the high voltage.

It is so dangerous that it is like a death wish to do that. What happens if the electrician wired the wall outlet backwards? Or if you're tired and it breaks and you just open it up and tear in, forgetting.

You're not protecting the tubes with the transformer. You're protecting YOU.