First PSU Capacitor

mr2racer

Member
2009-07-09 12:53 am
Hey All,

I know that all tube rectifiers have a maximum first cap value in the power supply to protect the tube from inrush current at startup. My question is is there a minimum value? I mean that the first cap takes a real beating so a very high voltage cap is best to make it more survivable. But does capacitance also come into it? I have some .068uf 1000 volt caps could they be used with a 5U4G tube?

Thanks, Kevin
 
If the cap is omitted then you have a simple LC or RC filter, which yields much lower DC output voltage relative to CLC and CRC filters. There's a zone between zero and several uF where DC output voltage increases rapidly with increasing filter input side capacitance, which makes the capacitor's value a critical parameter if you need to hit a target voltage.
 
The larger the capacitance in the 1st position, the "stiffer" the supply. SS diodes have a distinct advantage in the turn on surge dept., that enables large caps. to be employed. A big cap. in the 1st position is not without a downside. Check the archives for my posts about "hash" filters.

For a 5U4, insert a CL-130 inrush current limiting thermistor between the rectifier and the PSU filter. You can slightly exceed the data sheet 1st cap. limit, in safety. An additional benefit of the thermistor is a small delay in B+ rise. Remember, directly heated vacuum rectifiers, like the 5U4, turn on almost as quickly as SS diodes do.
 
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mr2racer said:
My question is is there a minimum value?
It depends on what you mean by minimum. You can omit the cap altogether (if there is a choke) but then you get more ripple and less DC voltage; fine if this is what you need. A small value for the cap reduces ripple a bit, but can greatly increase PSU DC output resistance (droop); people suggest this to allow for voltage adjustment but in my view it is a bad idea. My advice is to omit the cap, or ensure it is large enough to act as a proper reservoir.

I mean that the first cap takes a real beating so a very high voltage cap is best to make it more survivable.
No. It is ripple current rating which matters - assuming the voltage rating is sufficient. Any film cap with sufficient voltage rating will be good enough, and most electrolytics will be good enough too as they are designed for this duty.

So there really wouldn't be much difference between 10uf and 40uf?
I don't know what 10uf is. 10uF and 40uF would both work, but you would get less ripple and less droop with 40uF.
 
I doubt that the thermistor will slow things down enough for indirectly heated types to start conducting fully. The thermistor will tame the turn on surge, which protects the rectifier. As it conducts, the thermistor heats and its resistance drops. That's a negative temperature coefficient (NTC). A voltage drop exists in the cold thermistor that gradually declines, with heating due to conduction.

While a NTC thermistor slows things down for only a few seconds, that's enough time for a SS rectified bias (C-) supply to rise and electrostatically protect power O/P tube cathodes.
 
Hey All,

I know that all tube rectifiers have a maximum first cap value in the power supply to protect the tube from inrush current at startup.

I ignore that. Instead, I figure the Isurge and make sure it doesn't bust that spec. Sometimes, that max suggested Ci is too large. That was the case with a project using the 5U4GB. 40uF would have exceeded the Isurge= 1.0A. 34uF (two 68uF/350V capacitors in series) got that down to ~800mA/plate.

My question is is there a minimum value?

Yes: 0uF (makes for an L-input ripple filter that reduces the avaulable DC, but handles high currents better than a C-input filter).

Thanks. So there really wouldn't be much difference between 10uf and 40uf?

It would make the difference in that a 10uF reservoir capacitor will make for lower DC, and higher ripple.
 
You can check out the basic on Power Supply Basics.
You can google "power supply capacitor filter calculator" which will give you a fair bit of info in calculating the capacitor value.
From the schematic I've seen, the 1st cap in high voltage PS capacitor are in the range of 100uF to 470uF depending on the amplifier (in general base on the required DC current).
If you are designing a new schematic, you can google around to choose similar value. Or you can base on other people's theory and do the actual calculation.
Otherwise, use the value suggested in the schematic.
 
cl1238 said:
From the schematic I've seen, the 1st cap in high voltage PS capacitor are in the range of 100uF to 470uF depending on the amplifier (in general base on the required DC current).
Hopefully not with a vacuum rectifier. They generally need smaller reservoir caps if the rectifier is to have a long and happy life.

If you are designing a new schematic, you can google around to choose similar value. Or you can base on other people's theory and do the actual calculation.
Otherwise, use the value suggested in the schematic.
Or when all else has failed, try reading the rectifier datasheet.