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diyAudio Member

Join Date: Sep 2010
5AR4 PSU

hello guys,

I am designing a power supply for an amplifier, but I have some doubts about the calculation for Minimum Limiting Resistance (RLIM (min))

Rt= Rs + (n^2)* Rp
Where:
Rs = DC resistance of one half of the transformer secondary winding.)
Rp = DC resistance of the transformer primary winding.
n = Secondary to primary turns ratio (equal to the secondary voltage divided by the primary supply voltage).

Can someone please confirm, if my calculations are correct, in this configuration I have 250V per plate right?

The limiting resistor per plate at 300V is 75 ohms, by my calculation I have 133 ohms therefore no need to place a resistor. Assuming my calculation is correct.

Thank you guys for the help

MFF
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 1st January 2014, 06:22 PM #2 diyAudio Member   Join Date: Sep 2004 Location: Budapest "Rs = DC resistance of one half of the transformer secondary winding." Why "one half"? HV secondary is 250V+250V= 500V AC, with 42.43R (correct?) DCR. Without SS diodes this is the half wave rectifier.
 1st January 2014, 06:24 PM #3 diyAudio Member     Join Date: May 2005 Location: Dallas (but I am not a Texan!) The secondary is a little confusing. Is it 2x 250V (500V total)? If so... I think "N" is also per plate, not total, so should be 250/240... Pete
diyAudio Member

Join Date: Sep 2010
Quote:
 Originally Posted by pmillett The secondary is a little confusing. Is it 2x 250V (500V total)? If so... I think "N" is also per plate, not total, so should be 250/240... Pete
Hi Pete
Yes is a 2X 250V total of 500V
you're right is 250/240 so after recalculating we have TR= 21.2 + 1,08 x 5,2
RT= 26,8.

As we have 250V per plate and by the datasheet of the 5AR4 the minimum for 300V per plate is 75ohms so I need 75 - 26,8 = 48,2 ohms resistor I am, right?

Thanks for the help

MFF

diyAudio Member

Join Date: Sep 2010
Quote:
 Originally Posted by euro21 "Rs = DC resistance of one half of the transformer secondary winding." Why "one half"? HV secondary is 250V+250V= 500V AC, with 42.43R (correct?) DCR. Without SS diodes this is the half wave rectifier.
The Valve Wizard

diyAudio Member

Join Date: May 2005
Location: Dallas (but I am not a Texan!)
Quote:
 Originally Posted by MFF Hi Pete Yes is a 2X 250V total of 500V you're right is 250/240 so after recalculating we have TR= 21.2 + 1,08 x 5,2 RT= 26,8. As we have 250V per plate and by the datasheet of the 5AR4 the minimum for 300V per plate is 75ohms so I need 75 - 26,8 = 48,2 ohms resistor I am, right? Thanks for the help MFF
That seems about right. It will drop your B+ voltage a bit...

 1st January 2014, 08:09 PM #7 diyAudio Member   Join Date: Sep 2010 will be around 14V. I am taking the high voltage B+ from the CT of the 5AR4 filaments winding, this way we have less noise from the 5AR4 filaments into the B+ . this is correct? Thanks MFF

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