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Old 3rd June 2004, 06:17 PM   #1
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Default transformers

hello,

i'm rather new to tubes, have red a lot about it, but never build something.

I know that transformers are very important for the sound.
But why do we use very expensive power transformers ?
the output-transformers: ok ; that has to have great bandwitch low resistance, capacity.....

But the power transformers: can't you use normal transformers: 1 for the B+ and another transformer for the heating ?

has somebody done this already (probably yes: and what did you think about it)

thanx a lot
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Old 3rd June 2004, 07:55 PM   #2
benny is offline benny  Australia
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firstly, 'normal' transformers, like what you'd normally get from an electronics shop are generally what's considered low voltage... these are cheaper to make as they require much less insulation and crap to make them safe. valves generally use high voltage trannys... like a couple of hundred volts at least, therefore, you need a higher voltage tranny. second, when you look at valve power tranny's, they generally have a few secondaries on them, which is just conviniant because it has all your filament suplys, and a HV winding for your plate suply, sometimes a 50V tap for bias suplies on this winding... and then they're made to look good... which i guess is important to a lot of people when they are pretty much as much on display as the tubes themselves in an amp... much nicer looking than those open coil designs... which by the way, aren't safe at high voltages... so don't go doing anything tricky like wiring a LV tranny backwards for HV...

but as you ask, no, you don't need to use these specail trannys. i have used alternatives myself, and so have plenty of other people... whatever you do, always use a tranny for isolation... even if you only need AC at mains voltage, you still need isolation. that's the most important thing to remember... the rest is up to you... so use whatever tranny suits your purpose and is safe to use in this purpose.

cheers
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Old 3rd June 2004, 10:27 PM   #3
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Hi,

One cheap simple method that I have used for preamplifier experiments is to use 2 standard 6V or 12V transformers connected back to back.

The first transformer transforms mains voltage to 6V or 12V for heaters and the 2nd transformer 6V or 12V side is connected in parallell of the first transformer output side thereby giving high voltage on what is normally the primary side that can be used for anode voltage supply.

This works very well with 230V primaries and also if you can find transformers with double 110V primaries. The first transformer need to be quite large as it has to carry all the power.

This may sound expensive but actually it is not as often standard 6V or 12V transformers are very cheap, so for preamps and other circuits that doesn't that much anode voltage it is an alternative, there are of course the possibility to use a voltage doubler on the 2nd transfomer to get even higher anode voltage.

This method can also be used for more complicated schemes, I used 3 cheap standard transformers when I built the prototype of my new DC coupled preamp where I needed 30VAC for heaters, 110V and 300VAC for B+ and B-.

Regards Hans
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Old 3rd June 2004, 11:10 PM   #4
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Quote:
But why do we use very expensive power transformers
Which very expensive power transformers do you mean?

I use PT's with electrostatic screen that cost around 50.

Regards,
Bas
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