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Old 10th April 2013, 12:45 AM   #1
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Default adding series resistance to transformer primary

Hello all,
In order to get the B+ voltage I need, with a nice C-core transformer with a 1.9 ohm primary R and a 8.4 ohm secondary R, I can add some series resistance to change the ratio of series resistance to load resistance.
I'm using the charts in the Valve Wizard's "Power Supplies for Valve Amplifiers." And the equations too. I'm trying to drop the B+ coming off the secondary from 230 V (115 V primary) to 205 V. This is actually quite necessary for my design.

However I would need to add about 40 ohms to the secondary side, and that adds up to a lot of heat from power dissipation, heat sinks required, etc. But I see that every ohm added on the primary side is worth 4 on the secondary side ((Vprimary/Vsecondary)^2). So I could add 10 ohms to the primary side, possibly even splitting it between the two AC legs, and way less heat.

Is there any reason why this won't work, or problems to look for? It seems to make the whole design elegantly possible.

Thanks,
Ted
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Old 10th April 2013, 01:26 AM   #2
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You have a turns ratio of 1:2, therefore the primary current is double the secondary. With double the current the heating effect is 4X greater, precisely cancelling out the 4X smaller resistor value to give zero net change in heating effect.
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Old 10th April 2013, 01:35 AM   #3
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Better to add the resistance on the secondary side, because you can make it part of a CRC filter (just split your capacitance and put it in the middle) and get reduced ripple.
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Old 10th April 2013, 01:36 AM   #4
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by tapehead ted View Post
Hello all,
In order to get the B+ voltage I need, with a nice C-core transformer with a 1.9 ohm primary R and a 8.4 ohm secondary R, I can add some series resistance to change the ratio of series resistance to load resistance.
I'm using the charts in the Valve Wizard's "Power Supplies for Valve Amplifiers." And the equations too. I'm trying to drop the B+ coming off the secondary from 230 V (115 V primary) to 205 V. This is actually quite necessary for my design.

However I would need to add about 40 ohms to the secondary side, and that adds up to a lot of heat from power dissipation, heat sinks required, etc. But I see that every ohm added on the primary side is worth 4 on the secondary side ((Vprimary/Vsecondary)^2). So I could add 10 ohms to the primary side, possibly even splitting it between the two AC legs, and way less heat.

Is there any reason why this won't work, or problems to look for? It seems to make the whole design elegantly possible.

Thanks,
Ted
Dear Ted, I guess you didn't yet grasp how a transformer works .... or Ohm's Law for that matter.
You seem to be trying to transform 230V to 205V, so you seem to be chasing "a resistor value which will drop 15V" .
Fixed, I presume, as if it were an extra transformer tap.
Problem is, it does not work that way, current drop is variable, and depends directly on current consumption.
Which you have not stated and besides we have no indication that it will be constant anyway.
Please draw the schematic of what you are trying to achieve.

Last edited by JMFahey; 10th April 2013 at 01:38 AM.
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Old 12th April 2013, 12:54 AM   #5
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Thanks all-
(Incidently I have only positive associations with the name "Abraxalito", though I've never heard it before...)
I missed that about the current being transformed right along with everything else- whoops! There goes my free lunch.

The Valve Wizard is big on adding series resistance, not least to induce current-dependent (dare I say of course?) voltage sag, like what happens when a tube rectifier and it's resistance are added in series. So there's all these charts and equations and the like, and at least on the series side he writes that you can just add series resistance. I am actually aware that it is current dependent.

I'm certainly in danger of understanding this a lot better, and I thank you all for helping me out.

Now that my free lunch has disappeared, I see the advantages of using the series resistance as part of a smoothing filter. I'm also noting that on the Wizard's charts (which, hopefully, reflect reality) I can lose a fair amount of voltage when the ratio of Rseries to Rload is smaller than about .01, by using a big enough capacitor to drop the number 2pi(f)CRload to 1 or so. Off to shop for fatty capacitors...

One thing I need to get clear on is exactly what the DC load current (not including any heaters) consists of and when it changes. I think I'll start another thread on that if the answer doesn't pop up in the archives...

BTW If I was capable of posting pictures I would... at some point I may get that together.
Ted
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Old 12th April 2013, 07:59 AM   #6
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Historically I replaced a LOT of metal rectifiers with silicon ones (in record players. TV's etc.) - the technique was to mount a silicon diode and a wirewound resistor on a piece of tag strip. This duplicates the added voltage drop across a metal rectifier, or indeed a valve one.

However, and fairly obviously, it only does this under load - so the capacitors must be able to withstand the higher initial voltage. This isn't a problem with metal rectifiers, as they also exhibit a similar effect, with lower voltage drop until the valves warm up.
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Old 12th April 2013, 11:36 PM   #7
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This is, I believe, the first I've heard of metal rectifiers. I'll be looking into that.

I've seen schematics where the additional series resistance is switchable in and out. I could see where at a certain ratio of Rseries to Rload, a pot could be used for a variable sag effect, but also to compensate for variations in mains voltage.

Thanks!
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Old 13th April 2013, 01:35 AM   #8
Enzo is offline Enzo  United States
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Metal? As in maybe selenium rectifiers? They have metal fins. Or is this something else?
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Old 13th April 2013, 09:13 PM   #9
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There's a picture in _Valve_Amplifiers_ of some kind of Cthulu-looking metal rectifier Thing in a glass bottle... I don't suppose that's what you mean?

I found out I'm dropping some voltage across the OT, so that will get me right where I need to be.
I do note that Mouser has .8A, 2.5#, 0-240 variacs, so I could mount one on the safe side of the power transformer and scale away with minimal heat- I didn't actually get a figure for the heat a variac puts out, but it's got to be less than burning off the power with a resistor. And far more efficient. Probably not necessary, but imagine one as a "volume" pedal- really a "scale" pedal. Oh the possibilities of low high voltage.

Last edited by tapehead ted; 13th April 2013 at 09:14 PM. Reason: spelling
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Old 14th April 2013, 01:33 PM   #10
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Originally Posted by Enzo View Post
Metal? As in maybe selenium rectifiers? They have metal fins. Or is this something else?
Yes selenium ones - commonly known as 'metal' rectifiers in their day, presumably because of the metal fins of their construction?.

Smell horrible when they fail
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