Nothing In Particular

[izzy wizzy]

Our mice are few and far between and only in the garden, so far. Hedgehogs (2) and bastard squirrels prolific.

Quandary.

Hedgehogs were something we encouraged in our UK garden. Here they are a pest that we trap for. Hard to make the transition. They're bloody huge as well.

[andrew Ivimey]

This is a hedgehog...

[Dave the bass]

I'm reading Geddy Lee's book, 'My effin' life'.

The word 'shoddy' comes up, his Dad worked for a while in a shoddy mill he says.

I know we've covered this before, he gives a description of the word and says "Shoddy was inferior material spun from waste woolen cloth and used for stuffing inexpensive furniture ".

But still...

Pfffttttt, shoddy!

[simon]

I'm having a moment and can't convince myself I'm correct. With a traditional centre tapped mains transformer intended for full wave rectification, if the whole of the secondary is used with bridge rectification the current capacity of the secondary should be halved (compared to full wave CT) because all the secondary is being used all the time. Right?

[Max N]

Does this help?
https://www.sowter.co.uk/rectifier-tran ... lation.php

[simon]

Not really, but thanks Max. It's permissible Iac of the secondary that's eluding me - I think it's half the rating because the full winding is in use all the time with bridge rectification, whereas each half winding is on then off then on again etc. so full current rating for full wave rectification. Though it's probably not quite that simple.

[Nick]

So from the link Max gave

FULL WAVE: Iac = Idc

BRIDGE: Iac = Idc x 1.61

So from that I would say that the rating of a transformer rated for full wave should be de-rated by 62% (1 / 1.61 = 0.62 )

[simon]

Yep, I get that. Maybe I'm just making this too complicated, in my own mind at least.

But yes I suppose if a CT trafo is rated at say 200mA, the two secondaries in series would have a capacity of 200/1.61 = 124.2mA. Better than half anyway.

[Paul Barker]

I'm having a moment and can't convince myself I'm correct. With a traditional centre tapped mains transformer intended for full wave rectification, if the whole of the secondary is used with bridge rectification the current capacity of the secondary should be halved (compared to full wave CT) because all the secondary is being used all the time. Right?

Correct

[Paul Barker]

I dont agree with Sowter when you wind a power transformer you choose the core size by the power only half the centre tapped winding is working at a time. So the current through half the winding is taken into account. The core and A which is the core suface area is measured for that duty. Which is 50%. The bridge rectifier is working all the time so it requires double the size of A. Primary winding gauge is also different requirement.

[Nick]

I dont agree with Sowter when you wind a power transformer you choose the core size by the power only half the centre tapped winding is working at a time. So the current through half the winding is taken into account. The core and A which is the core suface area is measured for that duty. Which is 50%. The bridge rectifier is working all the time so it requires double the size of A.

All that is true, but Simon is asking about Idc, so the current that is available after the rectification and smoothing. Or at least that's what I think he is after. The power that the secondary can provide (as you describe) would be the only thing you are interested in if you were using the secondary to power a resistive load with AC.

[Morgan Jones]

I'm having a moment and can't convince myself I'm correct. With a traditional centre tapped mains transformer intended for full wave rectification, if the whole of the secondary is used with bridge rectification the current capacity of the secondary should be halved (compared to full wave CT) because all the secondary is being used all the time. Right?

Yes. When you connect a bridge rectifier across all of that secondary you are loading the transformer more heavily because you obtain double the voltage (at the same current).

It's worth noting that because the centre-tapped winding only uses half at a time, it effectively wastes winding window. Winding window that could have been used for thicker wire to feed a bridge rectifier. If you want a valve rectifier but are having a transformer made, use a bridge rectifier (half using the rectifier valve, and half with 1N4007), allowing a single (thicker) HT winding that has lower losses and is cheaper to make.

[simon]

Thanks all. I'm relieved I'm not going completely mad (just a little). I'm scratching my head with the mains txs I have, trying to work out options.

Why are they so expensive to buy now? (Rhetorical question.)

[Paul Barker]

Lots of stuff in our pursuit’s are costly.

[Paul Barker]

This is probably a good time to say. I’ve come to realise I cant make any amps or power supplies into boxes. My left hand has a long way to go. Though I need to use it its baby steps.

So what Ive decided to do is buy more octal relay bases and use only octal valves, but also the GM70’s. Might aswell just build it to do 20 watts class A1 only on the tributes 10k into 4 ohm, such as for my Celestion 4 ohm A2’s. And nobody knows the correct impedance for the esl’s so 4 ohm is best endeavour with those also.

So signal section on a bamboo board from pound type of shop. Everything will fit.

The power section will not be monoblock same board again.

All I’ll want to do is use screw connections like the relay bases and connection blocks screwed to the boards.

The input stage changed to 6SL7 srpp. The driver is still the EL38 trioded srpp cL coupled to GM70. Up to the gm70 everything is octal.

Now Ive made it achievable.

Its a good job I had already built my EL38 amp. Its the last amp with a chassis for a long time. That doesn't matter, against the stress factor of the challenge could raise my bp and …….