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Old 9th Dec 2021, 10:16 am   #21
kalee20
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Default Re: Curious issue with replacement coupling transformer

Bearing in mind the original transformer is failing short-circuit primary:secondary, that says it has (possibly) minimal insulation between the windings.

Close coupling between the windings is exactly what's needed for low leakage inductance and good HF response, though it comes at the price of not-so-good isolation. It's possible that the original is wound bifilar, with the start wires taken to anode and grid, and the finishes taken to HT+ and chassis (or GB-) which would give the best possible frequency response - but at increased risk of breakdown between the two windings insulated only by the wire enamel.
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Old 9th Dec 2021, 2:17 pm   #22
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Default Re: Curious issue with replacement coupling transformer

Note also that power valves of this vintage need quite large swings of grid voltage.

With the driver anode going to HT via the primary, then the anode end of the primary will be swinging quite a way above the HT voltage.

So you either get more HF roll-off, or you get lousy reliability in the transformer. It seems the original manufacturer chose the latter.

David
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Old 9th Dec 2021, 4:09 pm   #23
kalee20
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Default Re: Curious issue with replacement coupling transformer

Agree with Dave!

The high signal voltage drive requirement may have dictated transformer loading rather than RC coupling on the driver stage.

Though it may be possible to use highly insulated wire (Furukawa Tex-E for instance) to wind a reliable transformer bifilar fashion. If the windings are suitably connected, the HT voltage appears uniformly between the two wires throughout their lengths, irrespective of signal voltages, and this also gives the minimum capacitance. (It's only possible of course in a 1:1 transformer).

Incidentally, before anyone points out what I posted last night:

Quote:
Originally Posted by kalee20 View Post
Below is loss characteristics of 0.3mm silicon-iron (source: Telmag 'C' core data book). You can see that at 30Hz and 100mT, loss is 7mW/kg. At 10x the frequency (300Hz) the losses are 140mW/kg, not 700mW/kg. It's a 14x rise.
... that 7 to 140 is a 20x increase not a 14x increase, I'll have to own up it was a mental aberration! (The thrust of the argument still stands, that decrease in losses at HF due to flux density reduction, outweighs the increase due to the higher frequency).

A strip-down of the original failed transformer would be most instructive, especially as it's otherwise of no further use apparently!
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Old 10th Dec 2021, 4:23 pm   #24
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Default Re: Curious issue with replacement coupling transformer

Thanks to everyone who's posted thoughts and observations here.

Just to clarify, original replacement parts were not available from the manufacturer,, but the replacements fitted have been custom made to the exact specification of the originals, as provided by the manufacturer.
I am discussing this with the amplifier manufacturer and will update once I receive some feedback.

Andy
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Old 10th Dec 2021, 4:44 pm   #25
kalee20
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Default Re: Curious issue with replacement coupling transformer

Hmm!

It could well be that the replacements do indeed meet the original specs, but that the original transformers far exceeded the original specs. So you notice a difference.

What seems clear is that the replacement transformers are not the same as the original transformers, irrespective of what the minimum spec is.

Keep us posted, Andy, this is all good stuff!
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Old 10th Dec 2021, 5:51 pm   #26
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Default Re: Curious issue with replacement coupling transformer

They may well meet the published specifications, not all of them are published.
 
Old 15th Dec 2021, 11:03 am   #27
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Default Re: Curious issue with replacement coupling transformer

Well, the mystery is now solved although I’m not entirely sure I know why!
According to the manufacturer the performance of the transformer is tied to the orientation of the primary to the secondary windings – that is to say that the start and end of both windings have to be in the correct orientation. I had reversed start and end of the secondary winding due to slight differences in wiring colours.
Now this is corrected, all is well.

Interesting and a lesson learned !

Thanks for all the contributions to this thread.

Andy
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Old 15th Dec 2021, 12:23 pm   #28
kalee20
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Default Re: Curious issue with replacement coupling transformer

Thanks!

It could be a manifestation of what I was trying to say in post #21... that the ends of the windings which are in close proximity have to be at the same AF potential - either both 'cold' or both 'hot'.

Having one winding reversed adds capacitance to ground, which hurts HF response.

It's all fun!
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Old 15th Dec 2021, 12:25 pm   #29
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Default Re: Curious issue with replacement coupling transformer

Quote:
Originally Posted by Ted Kendall View Post
Quote:
Originally Posted by yestertech View Post
the makers recommended replacing both to preserve the channel matching etc.
Illogical, captain - it seems they are saying they can't guarantee the spec of their own transformers, never mind anybody else's. I don't think you have much to lose by stripping the faulty transformer and rewinding it in the same fashion. Are the manufacturer's replacements unobtainable or merely prohibitively expensive?
Further than that, such a statement throws doubt on their entire production of new amplifiers.

If two individual transformers produce a discernible difference in the sound, well, they can't both be right at once, can they? They could still both be wrong, though.

As there must be a steady flow of mixed (differently) wrong and possibly right transformers that get matched and paired-up for stereo amplifier production, some amplifiers therefore will be shipped with closely matched but wrong transformers. These will not sound as good as the ones which through luck might have got a pair of matched, good transformers. After all, the manufacturer says ther is a difference due to these parts.

You can see why audiophiles treat the application of logic to their pursuit as an attack on their very existence

David
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