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Vacuum tube circuit output transformer substitution

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hsieh

Mon Oct 25 2010, 08:31AM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

I want to built a vacuum tube radio(just for fun).Bui I can't find a suitble output transformer(7Kohm:8ohm).Is there any substitution?(Some websites sell it,but it's too expensive for me)
the circuit

I ask a teacher at my school.He told me to wind 150 turns primary and 5 turns secoundary on a ferrite core.will this work?
I found another way to make this type of trnasformer on the Internet:
europa.spaceports.com/~fishbake/ot1/ ot.htm
What is the diffrence?

Thanks

Proud Mary

Mon Oct 25 2010, 09:37AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

è¬æ˜•å“² wrote ...

I want to built a vacuum tube radio(just for fun).Bui I can't find a suitble output transformer(7Kohm:8ohm).Is there any substitution?(Some websites sell it,but it's too expensive for me)
the circuit

I ask a teacher at my school.He told me to wind 150 turns primary and 5 turns secondary on a ferrite core.will this work?
I found another way to make this type of trnasformer on the Internet:
europa.spaceports.com/~fishbake/ot1/ ot.htm
What is the diffrence?
Thanks

If we assume a notional 10% loss, such that the effective secondary impedance is 7R2, then the impedance ratio is

7k/7R2 = 972

and the turns ratio is

sqrt972 = 31:1.

which is roughly the same turns ratio of 150:5 suggested by your teacher.

But there is a lot more to valve output transformers than this, including, but not limited to:

The transformer must be able to handle the constant DC current flowing through the primary, plus the AF component, without saturating.

The transformer must have the flattest possible frequency response.

It is possible to use mains transformers as valve output transformers, if the core laminations are removed and then re-inserted with an anti-saturation gap - in your case, a 240:8V transformer for example - but you must not expect hi-fi results from this type of quick fix.

Output transformer frequency response excursions can often be improved upon by means of negative feedback, but this will not cure a rotten transformer of its bad habits.

Good Luck!

Sulaiman

Mon Oct 25 2010, 12:06PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

A lot will depend on whether your amplifier is 'single-ended class-A' or 'push-pull class-AB'
If your amp is push-pull then the reference that you gave should be ok.
IF your amp is class-A (one output valve) then
the transformer primary also acts as an energy storage inductor (as implied above)
and a significant airgap will be required, which means quite a large transformer.
So, is your amp single-ended or push-pull ?

hsieh

Mon Oct 25 2010, 03:38PM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

Sulaiman wrote ...

A lot will depend on whether your amplifier is 'single-ended class-A' or 'push-pull class-AB'
If your amp is push-pull then the reference that you gave should be ok.
IF your amp is class-A (one output valve) then
the transformer primary also acts as an energy storage inductor (as implied above)
and a significant airgap will be required, which means quite a large transformer.
So, is your amp single-ended or push-pull ?

I'm building a radio, not an AMP.(I know the audio amplify stage of a radio might be similar to a small AMP)

The circuit is in my first post.

Proud Mary

Mon Oct 25 2010, 07:56PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

è¬æ˜•å“² wrote ...

I'm building a radio,

And with regeneration too, by the looks of the RF circuit. Oh, I do like that!

George Schmermund

Mon Oct 25 2010, 08:36PM

Registered Member #3304 Joined: Mon Oct 11 2010, 04:18AM
Location:
Posts: 9

Why not start with something simple like a one tube set, then work your way up?

If you get really ambitious, you can also build your own vacuum tubes!

hsieh

Tue Oct 26 2010, 12:49AM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

George Schmermund wrote ...

Why not start with something simple like a one tube set, then work your way up?

I want to try simple 1 tube circuits too,but these vacuum tubes may be not easily avaliable here in Taiwan.

In some circuits,I don't Know hom to find or make RF coils and transformers.

George Schmermund

Tue Oct 26 2010, 01:17AM

Registered Member #3304 Joined: Mon Oct 11 2010, 04:18AM
Location:
Posts: 9

Maybe you should crank it back a another notch. How about starting with a crystal radio?

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