Help with designing a power supply for a bunch of tube pedals (UK voltages)

[yeeshkul]

Ah, that is a lot. You may like to start with something smaller and cheaper, just to find about how it really works a measure it, before you build a power station 
I am not being a dick, i mean it well  - when i imagine my first power project being a 48W power supply i would be scared to measure the parameters - which is what you really should do - at least to find out the right fuses.

[frequencycentral]

........before you build a power station 

On the positive side - we tend to turn the heating right down when I power up my pedalboard. 

[yeeshkul]

 

[gez]

Penfold wrote an excellent introductory book on designing power supplies.  Worth a read before you embark on anything.

You can buy 12V PSUs that kick out 1.2 A for less than a tenner.  They're a little brick-like, but an alternative is to use smaller, cheap AC transformers and do the rectification and regulation on-board.  You could probably pick up a cheap bench supply on eBay.  New, they might not be too much.  All alternatives.

[Ripthorn]

The smaller module looks good. What total current draw you expect you may need? I am not experienced in tubes at all 

I have three tube pedals that draw 300ma each. I have four more that draw up to 150ma each. My total draw with what I have right now is 1.5 amps. I'm planning more tube pedals such as phaser, envelope filter, wah, and maybe a sample rate reducer. The phaser and filter could both end up being four tubes each and drawing 600ma each. I guess I need to build one with enough ma not to run out of steam in the near future!

The more VA out of a transformer, the more expensive it will be.  You may want to consider wiring up a couple 2A transformers in parallel, basically have one enclosure that has two (or three) independent power supplies inside.  Yes, that could be bulky, but it could well be cheaper than trying to find a 12V, 6A transformer.

[mth5044]

I would love to see this come to fruition with a PDF file   

[Ice-9]

I would like to mention an obvious point but often overlooked, don't forget to include a fuse in the HT side.

[Ice-9]

I would like to mention an obvious point but often overlooked, don't forget to include a fuse in the HT side.

Sorry that should read HV , high voltasge side (240 mains) not HT

[yeeshkul]

... not saying that the careful picking of the primary fuse was described step by step in this thread 

[Ice-9]

... not saying that the careful picking of the primary fuse was described step by step in this thread 

Yeah ! sorry , and i know you lot know ya syuf , but when it comes to safety theres no harm in mention it over.

[yeeshkul]

when it comes to safety theres no harm in mention it over.

absolutely, you are right 

[punkin]

splendid thread...really!

[frequencycentral]

OK Jan, just digesting all this information, and trying to understand it all.

I completely agree with everything your bro said but the output voltage: 17V - 1.1(to 1.4 - see the catalogue for the particular bridge) -3 = 12.9 ... i would call it a bit risky but i don't want to put you off mate, just give it a try. It is sure big enough to feed 12 outputs, the only trouble can be caused by the non-linear behaviour of 7812 when it has not enough voltage across the legs. Also mind the 10% mains drift. After all the best way to learn is to put your foot into it, no pussyfooting and learn from your own possible mistakes. If you find out the strange things are going on with the output voltage when you draw something close to 300mA - you can always make a non-stabilised power supply out of it.
I think that stabilisation is a bit exaggerated feature anyway.

What you are saying here is that the output voltage of the transformer after rectification is 12.9v, and the 7812's really need to see 15v for linear behaviour. Have I understood correctly? So really I should be looking at the next transformer up?

So instead of the 6V+6V 50VA TOROIDAL TRAN (12020) (RC) 88-2614 I should choose the 9V+9V 50VA TOROIDAL TRAN (12021) (RC) 88-2616 ?

Or can you recommend the right one from this range:

http://www.rapidonline.com/Electronic-Components/Transformers/Toroidal/Toroidal-transformers/35506/kw/Toroidal%20transformers#techspec

[yeeshkul]

Rick i may confused you because i subtracted those 3V as well.
What i say is: (12*1.414)-1.4 = 15.5V

I consider 1.4 drop on the bridge which is maximum that can drop there (the worst case, it will be probably around 1.1 - see the catalogue for a particular piece).
So now you have 15.5 V at the input of 7812. That can be enough because it makes 3.5V across the 7812, but keep on mind that the mains voltage can also swing a bit (10%) around the 230V. The worst case is let's say 210V instead of 230. And that may cause your power supply go wild in the middle of a gig. I would go for 15V trafo just to be sure it won't fail me.

Also, the secondary voltage of a transformer depends on the current you draw. A catalogue simply states a nominal voltage, which is a voltage related to the maximum current draw recommended for the particular transformer - by the other words - it states the maximum current draw that will be supported by the nominal voltage. When you measure voltage on an open end trafo, you will get more (for example you can get  about 18-20V on 15V trafo this way but as far as you connect a load to the secondary, the voltage goes down, reaching its minimum at the maximum current draw). That means, that you may want to skip the whole stabilisation thing and go for a simple rectified power supply without 7812 and then you will get away with the 12V(nominal) trafo on condition that it provides the necessary current .

example: a german transformer HAHN 230V/1x15VAC/667mA says:

Primary voltage:   230 Vac ±10 % 50–60 Hz
Max primary current:   75 mA at full load
Secondary voltage, open circuit:   18VAC (there is a table attached with voltages for various output currents)
Deviation from nominal secondary voltage:   ±5 %
...
Height:   34.6 mm
Fastening holes:   Æ4.2 mm
Weight:   335 g

VAC - before rectifier
DC - after
Which means you will get 18VAC when there is no load - but 15V(+- 5%) when you connect a load that draws 667mA.
It says max primary current is 75mA, so that your primary fuse should be a bit bigger - here i very much recommend using your own DMM te get a proper value fuse.