Selecting audio tranny core size

[ashok]

How does one select the core size of a transformer meant for audio use. For a power transformer we use the power line frequency and the maximum operational flux density in the core.
For audio do we use say 20Hz as the frequency and the same flux density as for the line frequency? The core will be CRGO silicon steel core.
Thanks,
Cheers.

[corbato]

Hi Ashok
Thanks for starting this thread, which will specifically help DIY’er in countries such as India. While NOS and OUS Tube are available quite easily the biggest problem in India is the output trannies.

But it is easy to get a good quality trannies custom made in India from specialist or small shops. While these folks usually lack the theory they are quite skilled and if given exact specs then good quality trannies can be expected rather cheaply.

So I take the liberty to add to your query and wonder if some can help with the following:

EL84 SE Transformer capable of max 10 watts at 20hz to 20khz, 50mA.

What core size?
What Wire gauge for primary and secondary
What gap?

Same specs for a 6L6 SE also.

Any help will be greatly appreciated.

[ashok]

Hi Corbato,
There are many transformers winders here who claim that they can wind audio transformers. Most are younger than me and so they would not have gone through the 'tube era'. That does not mean that they cannot do it but that they have not spent enough time with these things.
I still have people come in and see the tubes and ask me what they are. In the electronic market in Chennai, Mumbai and Delhi shop keepers have smiled and said "why do you want those things. Transistors are far better. Tubes are ancient things, no one uses them nowadays".

So I would prefer to wind my own audio transformers. As I understand the CRGO cores should be good enough. I would like to know what winding schemes are the best and what core sizes to use.

I am sure there are are many DIY'ers or experts who would not mind sharing their expertise. I used to wind my own power transformers several years ago.
Cheers.

[corbato]

Hi Ashok

You right in implying that experience in vital. The sound of trannies I had “reversed engineered’ are nowhere near the original that were probably made by Philips India in the ‘60s.

Tubes are ancient. No body wants them. Perhaps that is the reason (thankfully) that they are cheap here. You can find an ok collection of trannies for tubes such as ECL82/86 (radio use), but I guess high quality amps were not made that abundantly during the tube era, so finding matching trannies is a tall order. Few months ago I bought a pair of DELTA Push Pull items (4K primary) that I intend to put to use in a 4 x EL84 amp someday. These are I guess still made to order by them. But not UL taps and certainly no full coverage of the audio band. Pic is attached.

I have more than 50 NOS and OUS output tubes with me. Sadly no output trannies to match.

Cheers

[ashok]

What is the normal air gap for SE transformers. I guess this must be related to the transformer size ( wattage) an dthe DC current flow through it. Are there any standard gaps used in practice. I see 0.5mm mentioned at many places. Are ther are graphs on the web showing the effects of air gap length?
Thanks.

[Jimmy154]

I have the same question as the guy who posted this thread. I'm winding a 12 mH tranformer type inductor out of 12 gauge wire. I'm not sure on the core size or air gap. Also I'm not sure what the differnence between CRGO silicon steel and CRGO steel is. I think they're the same thing.

But none of that is too important. I would just like some one to tell me where I can get CRGO steel or even better CRGO laminated steel or still even better CRGO from dismantled tranformers. I've only seen suppliers in India, but I'm in America.

[Bandersnatch]

Hey-Hey!!!,
For a general place to start with, take the power transformer core and double it for a 30 Hz function and double again for 15 Hz. Since you probably won't be going to 15 Hz, the flux density won't be at power tx core levels( good thing too ). So for an audio OPT, 4x the size of a PT is a reasonable place to start.
regards,
Douglas

[ashok]

Looks like this fairly neglected thread has come back from cyber space.
Lets put up some serious information here and hope guys will try out their own transformers and report back.
I will put up the equations that I have for selecting cores. I rediscovered them in my old files. All that in the next post. Till then hope we have others who can give their two cents worth.
Cheers.

[Tony]

For a general place to start with, take the power transformer core and double it for a 30 Hz function and double again for 15 Hz. Since you probably won't be going to 15 Hz, the flux density won't be at power tx core levels( good thing too ). So for an audio OPT, 4x the size of a PT is a reasonable place to start.

just as i thought. i would go to 15hz though, so now the fun part, how do we partition the coils to minimize high frequency losses due to to inter-winding capacitance, or stray capacitance, any suggestions would be welcome....

[fdegrove]

Hi,

SOME THEORY.

SOME MORE

Cheers,