Using standard 50/60hz transformer as output transformer

Hi,
Is it technically possible to use a "standard" 230Vac to 24Vac (for
example) as an output transformer in a tube circuit?
Regards

 

Only if you don't much care about the audio.

 

It will probably work at 50/60 cycles, but take the feedback from the
secondary for audiophile performance.


martin

 

I have actually tried this in my school days or there about.
As those transformers have no air gap, you need an old tube type
one with 2 HV windings, say 220V to 300 + 300 + 6.3, and
then use a _balanced_ circuit (as used for dual anode rectifiers).
It worked, but not very good, but my impedance matching was likely not ideal.
Probably it is not really the sort of iron for HiFi too (high frequency losses).
Perhaps you could use a 220V transformer with a 110V tap....

Better is to forget tubes and output transformers, build a transistor amplifier.

Or you could wind your own transformer.
Actually, on the subject of amplifiers and transformers, I bought a McCrypt
PA3000 power amplifier from www.conrad.nl, for _69,95_ Euro (say 100$).
It has in it a 200W ring core transformer, 2 SEPP transistor output stages
with 16A transistors, delivers easily 100W sine per channel, has XLR and phone
input, has a temp controlled fan, has a 19inch housing with grips, has separate
volume controls for left and right, real mains switch....
If you add up the component prices it is impossible.
So I have given up now on audio amp design.
It says designed in Germany, but I suspect it is build in China.

It is so powerful that I have not run this one at 100% yet (oh yes it has
warning LEDS for clipping), have blown enough speakers the last few years.

BTW this is only for the REAR channels in 5.1
LOL
Can you do it for 100$?

I would not have written this if I had not fallen for the April 1 joke 'made millions in
electronics ;-( ).

 

It will certainly work, but you'll get some high-end rolloff from eddy
current effects. And don't go single-ended class A, or it could
saturate; these transformers aren't designed for DC in the windings.

Hey, lash up a breadboard and try it! Tell us how it sounds.

John

 

Sure.
http://geek.scorpiorising.ca/GeeK_ZonE/index.php?topic=2779.0

Notice the toroidial type and low impedance output stage.

Tim

 

I'm sure it'll be fine at 50/60Hz !

Graham

 

I made an output stage with a pair of triodes and a small EI
power transformer with dual primaries, and it had pretty
fair fidelity up to about 3 kHz, so AM radio would sound
fine through it. I am sure a toroidal power transformer
would work even better.

 

.... especially if you want to take the audio all the way
down to 20 Hz. You might take two of them and wire one of
the primaries from each in series but wire the secondaries
in parallel. Toroids are easy to stack, too.

 

Oh, another thing, This trick gives you something like
screen taps, too.

 

"powerampfreak"

** Go search the archives " rec.audio.tubes "

Toroidals with twin primaries of 120/120 are so usable.



........ Phil

 

"John Popelish"

** Mains toroidals often have a flat response to 100 kHz and beyond.

Virtually no magnetising current to cause distortion.

Only issue is that 120 volts rms per side at 50Hz may not be enough.



........ Phil

 

"John Popelish"

** That is a silly myth - you never need full power output at 20Hz or
anything near it.

A toroidal tranny that begins to saturate at say 130 volts at 50Hz will be
able to accept 65 volts at 25 Hz.

** Nice idea.


......... Phil

 

I suppose you've never heard any drum 'n bass style music then ?

Graham

 

"Eeysore =ASD fucked pommy IDIOT "

** **** off - you asinine tenth wit cretin.



....... Phil

 

For your information, I'm working at a audio power amp manufacturer,
only transistor designs though, but our biggest unit produce over 12kW
as a stereo amp.
My interest around tube designs has developed lately since I like to
build a tube amp myself... Try something I never did before.

 

We're not talking about Hi-Fi here, just for guitar amp purposes, then
you won't need a very broad audio range, maybe 50hz up to 5khz or
something.

 

"powerampfreak"
"Phil Allison"

** We were not talking to you - FUCKWIT !!!




......... Phil

 

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.

 

We?

How many do you speak for?

How do I make sure I'm not one of them?