Big B+ voltage drop using diode rectifier. - Page 4 - diyAudio
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Old 15th November 2012, 02:40 AM   #31
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now everyone is talking about the transformer.

If it is to spec. as you stated 372-0-372. now what you need to do first when ever you using a transformer for the first time, you will need to measure the total secondary. This means from ct to voltage taps. note any differences. (this should be done with a true RMS DMM). remember to fuse the primary.

for example:

now take that measurement, say its 371 on one of them and 372 on the other. you will need to use the smaller no.

so take that and we are going to get the peak value. but we will use the smaller number: 371 x 1.414
If we live in a perfect world, this power supply would put out 524.6V so that means we need caps that can handle this no problem. so using the 80% safety margin factor, we need caps that are rated for 655.75 VDC.

so now we see our main cap ratings. now rectification. I like using the 6A10 diode because of the inrush current handling and low forward junction drop. one diode per voltage side and of course, ground the ct.

next, the caps:

as stated above, we take into account the margin. now I will select from my parts supplier some 330Uf @400V. around $6ea I get 4 of them.

next the schematic
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Old 15th November 2012, 03:25 AM   #32
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the schematic:

power supply | Flickr - Photo Sharing!
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Old 15th November 2012, 04:39 AM   #33
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now with your meter set it to ac measure the AC voltage at the output of the rectifier. if this is more than 5mV add more caps.

but in reality your voltage should sag slightly during operation.
its only bad when your power supply can not recover fast enough

as the output tubes conduct they form a resistance that is in series with the output trans. (pri) resistance and parallel to the power supply transformer's (sec) resistance. Any voltage measurement along the points would be measuring that point of the voltage divider network.
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Old 15th November 2012, 07:42 AM   #34
vingenz is offline vingenz  Italy
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Originally Posted by Enzo View Post
From your report, your AC voltage at the transformer was dropping considerably. .
No, was dropping only 10-12 volts. After the rectification DC dropping about 60v.
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Old 15th November 2012, 10:59 AM   #35
roline is offline roline  United States
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Location: East Tennessee
What is the output voltage at the first cap and second cap ;
without all tubes...
with just the input driver tubes...
with added one set of output tubes...
all tubes......

Is there any possibility of the amp being in oscillation??
Do you use grid stoppers on all the gates?
SO many tubes, SO little time!!!

Last edited by roline; 15th November 2012 at 11:01 AM.
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Old 15th November 2012, 05:24 PM   #36
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Location: Macedon NY
Originally Posted by vingenz View Post
No, was dropping only 10-12 volts. After the rectification DC dropping about 60v.
The AC output from the transformer will be "flat-topped" since the filter cap is charging only at the peak. Current waveform has perhaps 3:1 peak-to RMS "form factor" - NOT 1.4 like a sine wave! PEAK voltage sags much more under load than RMS. And PEAK voltage is what determines output voltage. Larger first cap will help a little (average output is increased by half the amount the P-P ripple decreases).
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