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Old 12th December 2012, 10:32 PM   #11
FoMoCo is offline FoMoCo  United States
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Quote:
Originally Posted by woodrough View Post
Cool, screw diodes anyway. That's why I chose the rectifire in the first place. So I measured the resistance of the center tap and read 90ohms averaged from both legs. And read from the primary side 8ohms per 120v section. Being that I'm using 120v, I will be running the primary in paralell making it 4 ohms.

So 90 + 4 = 94 ohms! I think that's a healthy enough resistance for the rectifier ya? Does this sound right? Seems like it to me.
No. The primary resistance is reflected to the secondary. You need to multiply it by the square of the turns ratio.

90 + ((210/120)^2)*4
90 + (1.75^2)*4
90 + (3.0625)*4
90 + 12.25
102.25

In my opinion you need neither the resistor or the diode.

Last edited by FoMoCo; 12th December 2012 at 11:02 PM. Reason: Left out a 2 in the first equation.
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Old 13th December 2012, 09:28 AM   #12
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Hey there,

FoMoCo was faster than me, regarding the correction of the reflected primary impedance

I am not sure if I understood you right, but if the secondary impedance is 90 ohms in each leg (from center tap to *each* end of secondary winding), then you're safe. If 90 ohms is the total resistance of the secondary (end to end), then it might be a bit low and need resistors.

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Andreas
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Old 13th December 2012, 11:45 AM   #13
FoMoCo is offline FoMoCo  United States
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Originally Posted by Rundmaus View Post
Hey there,

FoMoCo was faster than me, regarding the correction of the reflected primary impedance

I am not sure if I understood you right, but if the secondary impedance is 90 ohms in each leg (from center tap to *each* end of secondary winding), then you're safe. If 90 ohms is the total resistance of the secondary (end to end), then it might be a bit low and need resistors.

Greetings,
Andreas
I took it as he measured from center-tap to each leg and then averaged the two readings. If, indeed, the 90 ohms was the entire secondary, then I have to second Rundmaus'. Some resistance may be required.
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Old 13th December 2012, 12:47 PM   #14
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Tube rectifiers has been working fine several decades without diodes nor resistors. Thousands of radios, TV and amplifiers are the evidence of it.

The only rectifier that almost always uses a resistor of about 50R was in the half wave rectifiers like 35Z5 or 35W4 who operates directly from power cord.

I only suggest to include a fuse in the center leg of the transformer to the chassis in order to prevent any short circuit.
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Last edited by Osvaldo de Banfield; 13th December 2012 at 12:50 PM.
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Old 13th December 2012, 02:22 PM   #15
DF96 is offline DF96  England
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Most radios had transformers with much higher secondary resistance.
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Old 13th December 2012, 03:12 PM   #16
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Of course, and also more resistance reflected from primary, given the step up voltage transformer ratio, and also lowest winding (Copper) and core (Iron) quality (And higher looses), all of them can also be reflected as resistance in secondary or primary side, although is more common to reflect them into primary side.
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Old 13th December 2012, 03:45 PM   #17
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Oh! This is totally news to me. Thank god I asked you guys this.
The transformer resistance formula is a good thing to know (Never saw it before). I measured the ct to one leg at a time and averaged their two ohm values so all looks right.


So it looks like I don't need anything on the legs! nice!

I was gonna ask about the fuse eventually. Is it smarter to put it on the primary side or the CT of the secondary? Origionaly I was gonna put it after the switch on one lead.
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Old 13th December 2012, 03:53 PM   #18
FoMoCo is offline FoMoCo  United States
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I'd put it on the primary side. Otherwise the fuse doesn't protect the transformer.

Putting it on the secondary subjects it to higher voltage. You'd also need two fuses, one before each rectifier. The center tap is particularly a bad idea. If one of your rectifiers shorts, it doesn't protect at all against that.

No offense. But, it seems that maybe you shouldn't be playing around with high voltage at this stage of the game.
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Old 13th December 2012, 03:58 PM   #19
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Im following these schematics pretty closely but also at the same time, Im getting a lot of unique tips that defy the schematics. Such as LimR values, transformer resistance & fuse placement (as mentioned above). I understand the danger of high voltage but thats why I am raising these questions in the first place.
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Old 13th December 2012, 04:16 PM   #20
kevinkr is offline kevinkr  United States
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Place the fuse on the hot side and before the switch, that way if the switch develops a short to chassis (I have seen it happen) the local fuse can blow rather than relying on a distant circuit breaker to protect the intervening wiring. Make sure that the chassis is grounded.

I use the series diodes with all modern 5AR4 as it greatly reduces their propensity to fail due to arcing during warm up and during a hot start event. (Supply interruption)
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