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Author Topic: Rectify 110v?  (Read 6235 times)

Fixr1984

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Rectify 110v?
« on: April 07, 2012, 01:24:54 PM »
I found a tube preamp that I was thinking of building and it calls for 150v for B+.  Can I rectify my 110v from the wall outlet to DC and use that?  Do I need to actually build a power supply using a transformer?

spud

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Re: Rectify 110v?
« Reply #1 on: April 07, 2012, 04:07:13 PM »
It's super dangerous to do it that way without a transformer.  What you want is a an isolation transformer that has seperate primary and secondary windings.  Believe it or not this was actually done on some old tube amps - I believe even models that Sears sold (Silvertone, for example).  They were just being cheap and pretty reckless.  Don't be cheap and foolish - get a transformer and do it right. 

For a cheap solution, get 2 wall warts that put out about 12v AC or so (more or less is ok so long as they are the same).  Also whatever kind you choose and make sure you get some with enough current output to handle your complement of tubes and the rest of the circuit.  Then wire them back to back - like this:

(input) 120VAC -> WW1 -> 12VAC - 12VAC -> WW2 -> 120VAC (output)

A tricky way to do this would be to have a jack on the outside of pre-amp that accepts the 12v output of the first WW (WW1) then wire that receptacle to the other WW (WW2) that's inside your pre-amp housing. 

This will provide isolation since each of the wall warts is isolating the mains from the output (so actually double isolation).  One thing you will need is 6.3V AC for the heaters.  You could tap into the 12V from the output of WW1 and with dropping resistors or even with a pair of ZENER Diodes (use 1 watt 5.6V Zeners) to get it down to 6.3v -or if you want you can rectify that too (plus filter) and have DC heaters - do the rect before the Zeners and you will only need 1 to clamp the voltage to 6.3v - but you'd still use 5.6v zeners though.   

Jim

Fixr1984

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Re: Rectify 110v?
« Reply #2 on: April 07, 2012, 04:35:40 PM »
So I would basically run the second one backwards and the I would rectify the 120VAC when I am done

J M Fahey

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Re: Rectify 110v?
« Reply #3 on: April 07, 2012, 11:54:07 PM »
The back to back transformer trick is fine, but what was said about the filaments is not so.
You are mixing (incorrectly) AC and DC circuits.
If you have 12A*7 tubes in the preamp, you can wire them for straight 12VAC filaments.
The first (220/12) transformer must be rated at least 2X the filament current consumption.

spud

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Re: Rectify 110v?
« Reply #4 on: April 09, 2012, 10:58:12 AM »
Hi Juan, 

Good points.  I was thinking that the transformers would have to be rated high enough but didn't realize that the first would have to be so high (double) in terms of current.  But I suppose it has to come from somewhere!  And that would make sense if you are "stealing" the filament source from that one, it would have to be high enough to still provide enough current for the rest of what you're doing with it.  Not trying to argue with you but, I'm trying to understand your statement that it would be "incorrectly mixing AC and DC circuits".  In the tube amps I've built (2 completed and working on #3) the filaments were all AC but the rest of amp was all DC.  This was at 6.3 VAC and the heaters were wired in parallel.  I was thinking they could also have been DC at 6.3 in parallel since he's got 12v AC to work with is this ok?  So I guess my thinking is wrong here, can you explain since I'm not understanding why.  And yes, your right, he could do 12VAC heaters as well with probably less work just wired in series, but I'm just used to thinking about heaters at 6.3v! 

Hey, maybe you were thinking that I meant that the second wall wart was DC?  If so, I apologize for not being clear, BOTH have to have AC secondaries, or this won't work. 

Thanks - and as always, appreciate your insights,

Jim

J M Fahey

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Re: Rectify 110v?
« Reply #5 on: April 10, 2012, 11:32:41 PM »
VERY short answer: you can turn 12.6VDC into 6.3 VDC with 6.3V(obviously DC) Zeners, because you are substracting a DC value (the Zener rating) from another DC value .
It would also be very inefficient, enough to render the project useless, but we are not analyzing that now.
But you can *NOT* use the same 6.3V Zeners with a 12VAC source, you can't simply substract them.
Rather than me explaining AC circuits 101, take 2 sheets of paper.
Call one DC
Draw a graph showing 12.6V DC along time (it will be a horizontal line) and substract from it 6.3V , you'll get another horizontal line, now at 6.3V
Fine.
Call the other sheet AC.
Draw a 12.6V RMS AC signal, you'll have a beautiful sinewave.
Now substract from it, at every point the amount of 6.3V (fixed, that's what a Zener does).
Look at the new waveform you found.
Do you like it?
What's its RMS value?
Will the filament like it?
Enjoy your homework.

spud

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Re: Rectify 110v?
« Reply #6 on: April 11, 2012, 12:11:05 AM »
But wouldn't this do the job -

http://www.electronics-tutorials.ws/diode/diode_7.html

About 3/4 way down, Zener Diode Clipping Circuit.  I used it to regulate 6.3v AC heaters since I was reading over 7v due to the way the PT was wound - 115v pri where we get 124v normally.  With 2 Zeners configured Cathode to Cathode and the Anodes attached to the 2 sides of the heater filaments (I put them at the start of the circuit just after the PT), I could get 6.3v pretty steady.  I used 1 watt 5.6v Zeners since .7v is forward bias voltage giving a total of 6.3v (still I wonder if someone actually made the 5.6v Zeners just for this purpose since they are perfect for it). 

Anyway, this is what I thinking about when I mentioned Zeners - I guess I wasn't clear. 

Jim


J M Fahey

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Re: Rectify 110v?
« Reply #7 on: April 11, 2012, 01:22:27 AM »
You found a nice way to burn transformer windings, zeners or both. ;)

spud

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Re: Rectify 110v?
« Reply #8 on: April 11, 2012, 09:23:50 AM »
So based on your answer, the Zeners won't take it and if they don't fail it could cause the transformer to fail?  So in reality, the example I showed is pretty flawed.  Or is it only usable when say the voltage being clamped/clipped is not significant or is it the current that would cause the failure?  Would larger zener's work - say 5 watt?  It seems to work fine for dropping the less than 1 v in my usage but in this usage (dropping 6vac) it would basically be a failure waiting to happen, if I'm understanding you.  The 6 vac at 1 amp would be 6 w - much more than the 1 watt resistor or 2 in series.  I see what your saying.  In my usage if the voltage drop is .7 (or so) and I have 2 x 1 watt 5.6v Zeners, they must be seeing about 2.1 watts or more as that circuit is a 6.3vac 3 amp filament - does the Zener get hit with the current if it's not actively clamping the voltage?  Maybe I should swap these for 5 watt Zeners.

Thanks for the insights -

Jim

spud

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Re: Rectify 110v?
« Reply #9 on: April 11, 2012, 10:28:36 AM »
Just checked, they are 5 watt Zeners (1N5339B) - so they should be ok for clamping that small amount (.7v).  The Zeners for clamping the 12v down to 6.3 was a bad suggestion - don't do it.  Run the heaters in series and use the 12vac and that will be simpler and work without issues. 

Jim

phatt

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Re: Rectify 110v?
« Reply #10 on: April 11, 2012, 11:04:50 AM »
You can't clamp AC power like that.
Zener regulation is referring to DC Voltage.

The circuit you linked to is describing Low current AC *SIGNAL* clamping/limiting which is very different application to AC heater circuit where current is very high.

I've seen a full wave bridge used to drop half a volt in AC Heaters that may work but 7 volts is on the edge of the 10% tolerance for filaments so it's not extreme over voltage.
(Can't remember off the top but I'm sure I read +/-10% for filaments,, others will know)

Here is one way to do psu tricks from 12VAC.
note the filament supply is wrongly labeled it should be neg.
Phil.

J M Fahey

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Re: Rectify 110v?
« Reply #11 on: April 11, 2012, 12:48:29 PM »
If you have 7VAC and want to "clamp" them to 6.3V using 6.3V Zeners, YOU ARE NOT CLAMPING 0.7V BUT 3.6V
Furthermore Zeners behave as SHORTS above their Zener voltage.
So your transformer behaves as if it has a shorted 3.6V secondary and will deliver into it all the current allowed by its internal resistance.
That (huge) current will quickly kill the Zeners and if the transformer is small enough as not be able to do it, it will die itself trying.

spud

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Re: Rectify 110v?
« Reply #12 on: April 11, 2012, 02:14:42 PM »
Ah, I see - thanks Phil and JM.  So just by sheer luck, my clamp is "working" but puts the Zeners and my PT at risk.  I'll take it out and I'll have live with the over voltage.  Or how about some small value resistors? 

When I do the math for this I'm getting weird results: 

R=V/I so

R=.7v/3A
R=.233 ohm????

Can't be right - never heard of a .233 ohm resistor???  I must be doing something wrong here...any help is appreciated.

Or should I just insert a voltage regulator of some kind - recommendations?

Jim

J M Fahey

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Re: Rectify 110v?
« Reply #13 on: April 14, 2012, 10:38:31 PM »
TONS of amplifiers use 0.22 ohms resistors, typically in 2W and 4W sizes, so any parts distributor carries them.
Search their online catalogs for 0.22 ohm 2/4/5/7W ceramic resistors.

spud

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Re: Rectify 110v?
« Reply #14 on: April 16, 2012, 09:10:04 PM »
Yes, indeed - found some at Mouser.  Not bad prices. 

Thanks for the help,

Jim