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Old 31st May 2016, 12:27 pm   #1
ukcol
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Default A Question for the transformer experts.

Some time ago I acquired a circuit sheet for the Decca 1125 Intercontinental. I loaned it to Paul Stenning along with some other stuff when he was compiling the service data disc - so some time ago. The set is quite advanced on the receiver side for a domestic radio and I fancied getting one. Relatively recently I managed to win one with a very poor cabinet on eBay and apart from the usual AF11x problems the chassis was a good 'n'.

Anyway to cut from the waffle to the point of this post.

I noticed that the driver transformer is considerably larger than the the output transformer, which to me (in my ignorance) is counter intuitive. I realize that factors that have implications for size include impedance ratios, inductance required and expected winding currents. I also realize that push-pull transformers can be made smaller than single ended types of similar power because the DC currents cancel reducing magnetisation of the core, but I am surprised at the size of the driver transformer.

Given the requirements for this application would the output transformer be expected to be physically smaller?

The circuit, which is conventional, is shown below along with a picture of the transformers.
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Last edited by ukcol; 31st May 2016 at 12:29 pm. Reason: puntuation
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Old 31st May 2016, 1:40 pm   #2
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Default Re: A Question for the transformer experts.

You would expect the output transformer to be larger, yes. But you have hit it with the fact that the driver has to pass DC. Maintaining inductance, to give the sort of quality frequency response that this radio obviously aspires to, is not an easy task...
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Old 31st May 2016, 2:25 pm   #3
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Default Re: A Question for the transformer experts.

Thanks kalee20. The emitter resistor for the driver transistor (not shown in the snip) is 680 ohms with 0.8volts across it. The DC current through T1 primary is therefor about 1.2mA, can this be significant in terms of magnetisation of the core of T1?
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Old 31st May 2016, 5:41 pm   #4
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Default Re: A Question for the transformer experts.

Yes it would. The primary will have a few thousand turns on it, thus there will be a few ampere-turns of steady magnetising field produced. This is more than enough to send an ungapped core of this sort of size well towards saturation, so there has to be a gap introduced... Which then needs more turns to achieve the inductance desired. Luckily, inductance increases the square of the turns, so a solution is possible. But more turns increases self-capacitance, and leakage inductance, which both hurt high-frequency response. The answer? A bigger core.
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Old 31st May 2016, 5:56 pm   #5
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Default Re: A Question for the transformer experts.

Thanks again, that is a comprehensive answer and answers my question completely.
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Old 31st May 2016, 6:04 pm   #6
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Default Re: A Question for the transformer experts.

The driver transformer is working in a higher impedance environment than the output transformer, so it needs more inductance to give the same low frequency performance. It needs to be a gapped core to handle saturation, and that reduces the inductance per turn, so the number of turns goes up significantly, and that applies more ampere turns of magneto-motive force, so the fight against saturation is a bit of a stern-chase and the whole job is a bit harder than intuition would.suggest.

The huge numbers of turns forced fine wire ans interstage transformers are very susceptible to green-spot corrosion.

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Old 31st May 2016, 8:28 pm   #7
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Default Re: A Question for the transformer experts.

Thank you too David for your input.

For most of my working life I was a service engineer in the domestic Radio and Television trade and although I did a proper apprenticeship with day release college where we were taught electronics theory, what was taught was targeted at giving us the knowledge to diagnose problems. The design engineer by contrast would have been on a degree course in order to understand the subject at a level appropriate to his job. That hasn't stopped me over the years having an appetite for more knowledge of this fascinating subject though. I find the more the subject is understood the more enjoyable it is.
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Old 31st May 2016, 11:13 pm   #8
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Default Re: A Question for the transformer experts.

My design perspective allows me to sit down and work out things from first principle and so design things which have never been done before or to analyse faults which no-one has seen before (EG debugging first prototypes) But the radio and TV repair folk can run rings around me with their extensive knowledge of faults they've experienced. They are so much faster than me on known issues, and for vintage gear, most issues are already known. Relatively speaking I'm a plodder, but I regularly go off-piste.

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Old 1st Jun 2016, 12:06 pm   #9
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Arrow Re: A Question for the transformer experts.

Quote:
Originally Posted by Radio Wrangler View Post
My design perspective allows me to sit down and work out things from first principles.
The 'back to first principles' concept works for me too when I have an unknown piece of gear in front of me with an obscure fault to clear. However, when a 'I recognise this' item arrives with a 'known fault', diving in and at aiming at that 'known fault' can sometimes be very unproductive and time-wasting - it is too easy to make assumptions and jump to unproven conclusions in such a scenario. Employing the 'back to first principles' concept first in such a situation requires a good deal of self-discipline.

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Old 2nd Jun 2016, 7:13 pm   #10
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Default Re: A Question for the transformer experts.

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Originally Posted by Skywave View Post
........ it is too easy to make assumptions and jump to unproven conclusions.......
Although I had some field experience I spent the most of my working life as a workshop engineer. The people I worked for (in common with many) moved heaven and earth to try and prevent sets going in to the workshop for reasons of cost. Field engineers would be supported with access to a data base of common faults and help by telephone from workshop engineers. As a consequence pretty well every job that arrived at the workshop was a significantly "difficult" fault. The only exception was the CRT change that was always done in the workshop for safety reasons.


On the odd occasion however a set would get through the filter system and arrive at the workshop with a simple, common or straightforward fault and I for one would sometimes get caught out and not go straight to the simple solution.
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