Using standard 50/60hz transformer as output transformer

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

On a sunny day (1 Apr 2007 13:31:45 -0700) it happened "powerampfreak" wrote in :

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

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.

Notice the toroidial type and low impedance output stage.

Tim

-- "Librarians are hiding something." - Steven Colbert Website @

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

** 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

** 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

losses).

amplifier.

millions in

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.

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.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

"Adrian Tuddenham Pommy Jerk"

** Shame about toroidal mains transformers. ** Shame about the wide range of toroidal mains transformers available. ** Shame about toroidal mains transformers being close to ideal. ** Same goes for any output transformer - d*****ad. ** Shame about toroidal mains transformers all using high grade GOSS. ** Shame about toroidal transformers not being mentioned in such ancient texts. ** Shame about toroidal mains transformers.

** Shame about toroidal mains transformers being BETTER than most convention E-Core output transformers. ** No miracles involved with a toroidal mains transformers. ** Nice tip.

Not very practical though.

....... Phil

On a sunny day (1 Apr 2007 21:22:29 -0700) it happened "powerampfreak" wrote in :

Warfare?

Jump from a plane? Forget about tube amps.

On a sunny day (Mon, 2 Apr 2007 21:51:18 +1000) it happened "Phil Allison" wrote in :

I do it all the time, and stay clear of grounded objects. As a TV service person (I did many years) how about in somebodies living room next to a central heating radiator with a tek scope on a live chasis?

Anyways I have survived 240V AC several times, even through the heart. I found I could no longer move my arms... but I could move my legs, so I jumped, that broke the contact. Keep head cool, only few seconds to think and find solution before you pass out. Do not try this at home.

Now _there_ is good advice.

out.

Does anyone know what made Phil Allison so pissed off ... hmm.. something I said?

Don't worry about, you didn't do anything wrong. He's like that all the time. Too bad too, cuz sometimes he has useful things to say. Most of the time though......well.......you'll see soon enough if you hang around.