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Old 20th Oct 2016, 3:40 pm   #1
bikerhifinut
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Default "Bucking" Transformer.

The idea of one of these is appealing to me to get my 253v mains down to 230Vfor a pair of transformers.
I saw this link and I figured I could build a couple of these in individual transformer boxes with IEC output leads to my valve amps.
I was thinking of using 2amp transformers as I use 1.5 amp fuses. that roughly equates at 50VA for a 24 volt transformer?
And I assume toroids will work in this function?
Thanks to Ed Dinning for suggesting this a while back, its taken a year to take fruit............

Can anyone see why I shouldn't try it?

http://dms-audio.com/node/12

Andy
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Old 20th Oct 2016, 4:45 pm   #2
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Default Re: "bucking" transformer.

I have got one to do in the next few months. I am going to try to hide the bucking transformer inside the cabinet.
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Old 20th Oct 2016, 5:55 pm   #3
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Default Re: "bucking" transformer.

I'd like to do that but space under the chassis is too cramped.
I also figured doing it this way, I can utilise a couple other Japanese transformers that I have that run at 220V nominal mains.

I did wonder if I used a 100VA transformer could I just build one Bucking unit and run both monobloc power amps from it in parallel so to speak or are there interaction issues? I wouldn't have thought so as its not really any different to running 2 transformer primaries off side by side mains sockets or having 2 or more mains transformers inside a piece of kit ?

Just one neat solution.............. I hope.
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Old 20th Oct 2016, 6:19 pm   #4
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Default Re: "bucking" transformer.

It is basically down to the total loading on the bucking transformer. If it is big enough it should be fine with both amps connected to it.
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Old 20th Oct 2016, 7:22 pm   #5
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Default Re: "Bucking" Transformer.

Yes, toroids will be fine.

As you say, just work out the current drawn by your load (2A), multiply it by the voltage you are raising or lowering the voltage by (23V), and that's your transformer rating.

Bigger transformers usually have better regulation than small ones. Small laminated-core transformers are really bad, largely because a disproportionate amount of space is taken up meeting insulation clearance requirements (largely unnecessary in this application). So if you don't want your 230V to sag excessively, spend a few more quid on beefier transformers.
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Old 20th Oct 2016, 9:25 pm   #6
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Default Re: "Bucking" Transformer.

grand kalee!

I was looking at getting a beefy toroid say up to 100VA. Would I be OK feeding 2 mains transformers off it rather than build 2 separate bucking units?
I had a notion to Build one up in a nice project box with a couple of IEC outlets to use with a couple of IEC plug to socket leads straight into the IEC sockets of my amps?
A.
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Old 20th Oct 2016, 9:32 pm   #7
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Default Re: "Bucking" Transformer.

how about one of these?
Wire the mains to the 240V and take the live to the amps from the 220V outlet?

http://www.maplin.co.uk/p/vigortroni...-186-100-a32qk

A.
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Old 20th Oct 2016, 10:54 pm   #8
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Default Re: "Bucking" Transformer.

Probably not beefy enough. An auto transformer will need to be rated for the total load taken whereas the bucking arrangement only needs to be rated for a little over 10% of the total.
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Old 20th Oct 2016, 11:36 pm   #9
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Default Re: "Bucking" Transformer.

Thanks.

I've ordered up a 160VA 2x12V toroid and some IEC chassis plug/sockets to put in a project case I have here. I'll keep you posted with pics if it works.
I can see this device being useful as I have some 220V transformers that will now be useable in Valve amp projects.

I am assuming it's ok to run both power amps off one bucking circuit? the toroid is well specced to cope.

A.
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Old 22nd Oct 2016, 11:19 am   #10
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Default Re: "Bucking" Transformer.

I power a pair of quad ii's off a single bucking transformer without any problems. I am using a toroidal transformer made by rotork, salvaged from a skip at work.
Glyn
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Old 22nd Oct 2016, 12:18 pm   #11
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Default Re: "Bucking" Transformer.

Thanks glyn.
I'm just waiting for my parts to arrive to make the neat job.
In the meantime I have found a big 24V transformer salvaged from a CB radio transmitter and am tempted to "breadboard" a bucking unit.
Even if it doesn't solve one of the issues, I will have got the electronic issues sorted by dint of the PSU running at the correct lower voltages. I realise I must have been putting the circuit and components under considerable stress.

A.
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Old 22nd Oct 2016, 1:14 pm   #12
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Default Re: "Bucking" Transformer.

You could apply a 750 VA bucking transformer to an entire 32 amp ring main, which would probably be kinder to your other appliances as well .....
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Old 22nd Oct 2016, 6:46 pm   #13
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Default Re: "Bucking" Transformer.

Well its in. Just a lah up. It turns out that my transformer is 20-0-20V so I only used one half of the secondary todropmy mains from a measured 253V to 233V. There wasn't any sag so with both monos in the circuit I was getting spot on give or take a couple of volts as it drifted up and down with time.
Then I did some first aid. Replaced the anode resistors on GZ34 with 100R at 7W as that's what I had in the box. The 220R in the circuit, one was O/C and the rest had signs of extreme thermal distress, not surprising considering how much over voltage I had to drop.
Switched on and first thing I noticed was that the transformers are noticeably quieter. Not silent but a definite improvement.
And to measurements.
EL34 heaters: 6.25V
adjusted DC heaters down to 6.3V.
Secondary Volts off load at 418V, not far off the 410 spec and it is off load.
On load not much different after fitting G34. Around 415 to 420 depending on mains in.
Volts on anodes after 100R : 409 so dropping 9 to 10V
HT on CT of OutpUt transformer : 450 to 455 so that's bang on the design parameters for the leak circuit. EL34 cathodes at 36V, a bit on the high side but acceptable. so pulling around 160 mA per amp taking into account 5 mA for preamp and phase splitter valves.
Transformers running noticeably cooler now they are at design voltage.
They still buzz a bit from the windings and nothing can be done there, but overall it's a result and I am glad the voltages are correct and components aren't stressed like they were.

When I get the toroid delivered I will utilise that and the IEC sockets in the ABS project box as it should run silent. Then it will look and be right and nice and safe electrically.

it's been well worthwhile and I now have a means of powering 220V AC equipment that I own without fear.

A.
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Old 22nd Oct 2016, 6:53 pm   #14
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Default Re: "Bucking" Transformer.

Quote:
Originally Posted by julie_m View Post
You could apply a 750 VA bucking transformer to an entire 32 amp ring main, which would probably be kinder to your other appliances as well .....
I wonder what my sparky would say if I asked him to install one?
It's an idea for the room with the stereo as I use a lot of valve kit. Fortunately my preamps and phono stage use fully regulated HT and LT supplies.
A.
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Old 22nd Oct 2016, 6:59 pm   #15
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Default Re: "Bucking" Transformer.

Quote:
Originally Posted by bikerhifinut View Post
I wonder what my sparky would say if I asked him to install [a whole-house bucking transformer]?
Probably not much. But check after he's gone, that he wired the secondary the right way around
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Old 22nd Oct 2016, 7:31 pm   #16
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Default Re: "Bucking" Transformer.

You can get 'whole house' voltage reducers which are supposed to reduce power consumption.
 
Old 22nd Oct 2016, 7:50 pm   #17
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Default Re: "Bucking" Transformer.

But they don't, really ..... Switched mode power supplies and some kinds of motors draw more current at lower voltages. Thermostatically-controlled heaters and fridges stay on for longer (same joules = fewer watts * more seconds). Tungsten filament light bulbs have a positive temperature coefficient, therefore a lower resistance at a lower temperature, so will admit more milliamps at a reduced voltage; resulting in less economy than expected (and the reduction in light output depends on the reduction in power squared).

You don't get anything for nothing in this life. Auntie Thermodynamics has spoken!
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Old 22nd Oct 2016, 9:09 pm   #18
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Default Re: "Bucking" Transformer.

Hence the italic supposed!
 
Old 22nd Oct 2016, 10:14 pm   #19
bikerhifinut
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Default Re: "Bucking" Transformer.

Just to steer this back folks.
it looks like bucking transformers are a neat solution to high voltage mains with valve equipment.
Now I have had a bit of thought. How about a switched Bucking unit, i'e. get a transformer with tappings at 10V 20V and 30V.
fit a 3way switch and you have an option to adjust for any voltage down from 250V by switching in the incremental windings to get down as far as 220V if needed?
I probably just described an autotransformer.
A.
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Old 23rd Oct 2016, 2:34 pm   #20
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Default Re: "Bucking" Transformer.

Not an autotransformer (except insofar as no isolation) but you will need to rate it for the largest volt drop secondary VA- 30V * maximum line current drawn.
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