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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 23rd May 2020, 1:20 pm   #1
vinrads
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Default Mains Transformer Question

I am checking a tx for the amp build it uses 4x EL506 the circuit shows a bridge rect , I have a tx with centre tap 300.0 300 v I can separate the winding's parallel them in phase to give me 300v double the current , or would using the tap to earth and two diodes still full wave , give the same current .Mick.
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Old 23rd May 2020, 9:23 pm   #2
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Default Re: Transformer Question

You can only parallel the secondaries if they are identical. This is not lkely to be the case and circulating currents may well cause problems.
I would suggest using a bridge rectifier for each winding and connect the DC in parallel. Diodes are cheap enough.
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Old 24th May 2020, 7:02 am   #3
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Default Re: Transformer Question

You won't get 300v you'll get about 420v bridge rectified.For 300v HT (which is a bit low) you'll need a tfmr with 210/220 secondaries.

Andy.
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Old 24th May 2020, 9:16 am   #4
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Default Re: Transformer Question

If this refers to the Brimar VA11 amplifier on your ECC807 thread, then you could simply use the two diodes and centre tap to 0 volts idea (that is a 'full wave' rectifier). The current available will be what ever the transformer is rated at. Yes you will get 400+ volts DC, but that is what is needed?

For information, if you did parallel the two 300 volt windings do not assume you would get 2 x the current. Unless the transformer is rated for use in bridge mode best to estimate 2/3 the 'total' current.
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Last edited by snowman_al; 24th May 2020 at 9:21 am. Reason: circuit added
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Old 24th May 2020, 9:53 am   #5
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Default Re: Transformer Question

Hi Two bridge rect's looks like the safest way to go , if I use this tx , it also has two 6.3 sec very useful as I will need 3.2 amps just for the output valves. I have asked for a quote from Primary Windings , Mick.

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Old 24th May 2020, 10:06 am   #6
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Default Re: Mains Transformer Question

Another question, can you wrap the laminations on a TX with aluminium? Why? I need to fit a cowl to convert it to a drop through type, the cowl I have is slightly too big. I had this one which was total rust, after much grinding and polishing, it's now looking good

Mick.
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Old 24th May 2020, 12:01 pm   #7
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Default Re: Mains Transformer Question

As long as you don't create any shorted turn by doing so, yes.

Essentially, don't do any conductive wrapping that passes through a winding window.

I once saw the results of an earth wire neatly routed through between two of the limbs of a 3-phase transformer by an inexperienced production technician! The TX won, whilst it hummed happily to itself.
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Old 24th May 2020, 1:41 pm   #8
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Default Re: Mains Transformer Question

So what you are saying is to leave a gap? If so, that's no problem.

Thanks, Chris.
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Old 24th May 2020, 3:36 pm   #9
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Default Re: Mains Transformer Question

Two bridge rectifiers will work out the same as two diodes in the classic centre zero tap full wave rectifier. Most of the diodes won't be doing anything.
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Old 24th May 2020, 4:45 pm   #10
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Default Re: Mains Transformer Question

Be aware that the mounting screws go through holes in the lams, so part of the transformer cuts them. these screws with the bell and the chassis can make a shorted turn on part of the flux. So insulate one end of each screw.

Had endless hours of discussion on this point because the safety people wanted to know that the bell on a transformer mounted on a rear panel was properly grounded because it was exposed to people.

David.
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Old 24th May 2020, 5:45 pm   #11
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Default Re: Mains Transformer Question

That's something else I didn't know , thank's David.
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Old 25th May 2020, 12:16 am   #12
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Default Re: Mains Transformer Question

Quote:
Originally Posted by merlinmaxwell View Post
Two bridge rectifiers will work out the same as two diodes in the classic centre zero tap full wave rectifier. Most of the diodes won't be doing anything.
No. The two windings will be separated and both of the windings will be passing current 100 times per second resulting in higher efficiency. The transformer power limit will now be due to heating in the primary.
An optimum transformer will have a similar amound of copper in the primary and secondary however the transformer which has windings which pass current only some of the time will need more secondary copper.
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Old 25th May 2020, 12:29 am   #13
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Default Re: Mains Transformer Question

That's at least three things I didn't know!
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