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Old 6th February 2014, 03:45 PM   #1
emosms is offline emosms  Bulgaria
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Default RC chains capacitor swap

Hi,
A very lame question:
Click the image to open in full size.
This is part of ps for a tube project.
What if a swap the capacitors c1 and c2 - 22uf first, 100uf second.
I guess there would not be any difference in ripple smoothing.
---
Reason - a smaller 100uF cap/400V for a smaller ps unit.
Anyway the voltage will be less than 400v
---
Unfortunatelly, I am going to use the resistor only to drop the voltage to the value needed for the tubes, not really calculating the rc chains.
Voltage taken on the second cap.
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Old 6th February 2014, 04:41 PM   #2
DF96 is offline DF96  England
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Both caps need to be able to take the full voltage.

Putting the small cap first will reduce charging pulses and make good grounding slightly less important. It will increase voltage droop under load. Ripple at the output will be largely unchanged.
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Old 6th February 2014, 05:36 PM   #3
emosms is offline emosms  Bulgaria
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Thx,
What do you mean "make good grounding slightly less important"?
Some more details pls, about how it affects grounding, e.t.c... Where can I do some readings about it?
---
No load voltage is going to be app. 360, maybe more. (going to test a custom made transf. these days)
I wanted to put 22(47)uF/450v first and 100uF/400v second.
.. But, the supply will be detachable, so with no load there would be almost no drop at C2.
or at least I think so.
---
p.s.
Got this software, but cant really use it yet
PSUD2

Last edited by emosms; 6th February 2014 at 05:39 PM.
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Old 6th February 2014, 05:55 PM   #4
DF96 is offline DF96  England
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There is plenty about grounding both here and elsewhere on the net.

If the charging pulses are slightly smoother, due to a smaller reservoir cap, then they will create slightly smaller problems due to ground currents or magnetic induction.
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Old 6th February 2014, 06:08 PM   #5
AndrewT is online now AndrewT  Scotland
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I prefer the small cap before the big cap.
This is because the big cap supplies virtually all the near instantaneous current that the load demands.

But the first cap of a rCRC string takes a lot of punishment. It must have an adequate ripple capacity.

4k7 and 100uF is an enormous RC time constant.
That makes me ask: is the 4k7 also doing a double duty? Is it dropping the high B+ voltage to a lower voltage to suit another circuit?
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Last edited by AndrewT; 6th February 2014 at 06:10 PM.
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Old 6th February 2014, 06:33 PM   #6
emosms is offline emosms  Bulgaria
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I am following a 'verbal rule' that a drop of f.ex. 30 volts is good to achieve ripple smoothing.
100uf - the bigger the better 'verbal rule' .
---
Ok, I will do some more reading about valve ps, but the primary purpose of 4k7, or whatever it comes, is to set my V1 voltage to 300v under load.
Then there will be anotherRC chains going parallel, to supply other valves:

Click the image to open in full size.

Considering heater elevation as well - I am going to test a topology with a tube cathode follower.
Testing anyway with a heater elevation for hum reduction (non tube CF version)
---
Calculated the reservoir capacitor according to
http://www.valvewizard.co.uk/smoothing.html
It comes about 15uF. Suplying preamp with a current draw of 5mA

Last edited by emosms; 6th February 2014 at 06:51 PM.
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Old 7th February 2014, 10:27 AM   #7
DF96 is offline DF96  England
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Be aware that a highish LF/subsonic supply impedance (4.7k) may help create the conditions for motorboating.
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Old 7th February 2014, 02:42 PM   #8
emosms is offline emosms  Bulgaria
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Is that a matter of concern, as the PS will supply only a preamp of 2 tubes?:
Quote:
One common cause is feedback through the plate power supply circuit.[2][4] The power supply provides DC current to each tube's plate circuit, so the power supply wiring (power busses) can be an inadvertent feedback path between stages. The increasing impedance of the filter capacitors at low frequencies can mean that low frequency swings in the current drawn by output stages can cause voltage swings in the power supply voltage which feed back to earlier stages,[2][4] making the system a subaudio oscillator. This is caused by inadequate power supply filtering or decoupling.
Otherwise I think get it (more or less )
Quote:
Low frequency oscillations like motorboating indicate that some device or circuit with a large time constant is involved, such as the interstage coupling capacitors or transformers, or the filter capacitors and supply transformer winding.
Do I still have large time constant, if I keep 100uF as the 'reservoir' cap, or better just to lower the value (22uF + 47uF or 47uF + 47uF)
p.s.
got my custom transf. to test . Much better than coupling а bunch of small transformers
http://s10.postimg.org/xv293p8xl/P1010240.jpg
http://s9.postimg.org/4rsln57tb/P1010239.jpg

Last edited by emosms; 7th February 2014 at 02:49 PM.
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Old 7th February 2014, 02:49 PM   #9
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by DF96 View Post
Be aware that a highish LF/subsonic supply impedance (4.7k) may help create the conditions for motorboating.
does swapping the 22uF and 100uF affect this instability?
Does one order (of big/small) differ from the other order (small/big) as far as instability is concerned?

Would splitting the 4k7 into an RCRC of 2k2 100uF 2k2 22uF change the stability?
At the moment he shows rCRC
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Old 7th February 2014, 04:04 PM   #10
emosms is offline emosms  Bulgaria
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Great option
Splitting 4k7 (whatever), 1 extra resistor before C1 to set the voltage and tweak it all.
100uF as C2.
Maybe adding a third RC chain for the V1 plate (top right corner).
//V1 is going to be a penthode.
Lowering all the secondary RC caps to 10uF/400v (out of space into the device's box..)
Components up to C2 (plus the heating stuff) are in the PSU box
---
No calcs and full understanding, space/design considerations before the math .

Last edited by emosms; 7th February 2014 at 04:09 PM.
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