powerampfreak said:
Hi,
Is it technically possible to use a "standard" 230Vac to 24Vac (for
example) as an output transformer in a tube circuit?
Regards
It is possible; but the quality will never be anywhere near as good as
even the most basic purpose-designed item. When designing a valve
output transformer you need to be able to optimise:
1) The turns ratio
2) The core losses and copper losses
3) The coupling between primary and secondary over a wide range of
frequencies.
4) The self-capacitance of the primary.
With an off-the-shelf mains transformer,
1) Will be fixed and if you are lucky it may be somewhere near what you
want (but it more likely will not). If it is wrong, you will prevent
the output stage from delivering its full power into the load. You may
also get flashover inside the windings or across the pins of the
valveholder.
2) Will be optimised for full load 50 c/s operation and will be nothing
like what you want. The core may run into saturation if you try to use
it at full power below 50 c/s; and this could damage the valves.
3) Is determined by the quality of the core material at medium
frequencies and by the configuration of the windings at high
frequencies. You can get away with power-grade laminations for
moderately demanding audio purposes (I have done so many times); but
audio grade will be better if you want low losses.
The windings need to be split into sections and interleaved, this is a
complex process which was well-understood by the 1930s, so you need to
read a designers' handbook from that era if you want to do it correctly.
The limitation placed on your frequency response can be partially
covered-up by negative feedback, but the losses due to poor coupling
cannot be overcome in this way.
4) Will set a limit to how much feedback you can use before it becomes
unstable. Phase shifts are caused by the self-capacitance of the
windings interacting with their inductance. Because of the large number
of turns of fine wire on the primary, valve designs become unstable at
lower frequencies and with far less feedback than transistorised
transformer designs (which need fewer turns). Even medium-quality
purpose-built valve output transformers are quite limited in the amount
of feedback they will allow, and mains transformers will be very limited
indeed.
Try it, have fun ...but don't expect miracles.
P.S. Always keep one hand in your back pocket when you work with live
valve equipment, it prevents an electric shock from travelling straight
across your chest and stopping your heart.