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chd_diy 30th December 2010 01:46 PM

5U4 Rectifier. Maximum filter cap value?
Hello all.
I have yet another potentially foolish question that I'm hoping someone will have the answer to.
I'm putting together a tube rectified power supply (for a power amp) which uses a 5U4G rectifier. The GE 5U4 data sheet lists (under typical operating characteristics) an input filter capacitor of 40 mfd.
They don't list a maximum value.
Mainly my question is, can a larger value cap be used in this location? If so, what is the maximum value you'd reccommend?
Also, is there any benefit (or danger) to using a bigger cap in this location.
Perhaps I should Just keep a 40 mfd at the input and feed it through a filter choke of around 10 H and pile on the caps after the choke.
What's the difference between adding more capacitance before or after the choke?
Should I start out with a choke, then go out to a large capacitor bank?
The GE data lists an input filter choke of 10 H.
I look forward to hearing your thoughts on all this. I'd rather check twice, solder once.
Thank you for your help.

kevinkr 30th December 2010 02:18 PM

Usually the capacitance value recommended in a tube data sheet is the maximum that should be used with a given type for good reliability. I would not go over 50uF under any circumstances and would recommend 30 - 40uF as reasonable values for the input capacitor based on my experience with this type.

Larger values may cause arcing during warm up and will also exceed the peak current rating of the rectifier. You can use more capacitance after the choke if you wish.

I recommend you download Duncans Amps PSUDII here:
PSUD will allow you to analyze the performance of your proposed supply design.

boywonder 30th December 2010 03:43 PM


Originally Posted by chd_diy (

What's the difference between adding more capacitance before or after the choke?
Should I start out with a choke, then go out to a large capacitor bank?
The GE data lists an input filter choke of 10 H.


Adding capacitance (more than recommended) before the choke can lead to arc over of the rectifier and other problems as Kevin mentioned. Also note that the data sheet specifies the minimum impedance of the transformer; if this is too low it can also cause arc over.

The formula for the effective plate supply resistance to the rectifier is Rsecondary +Nsqrd*Rprimary+Ra where Ra is the resistance that you may need to add in each leg of the transformer secondary before the rectifier.

If you start out with the choke first, you'll have an LC filter which gives about 90% of the transformer secondary voltage as a max B+; with a CLC design, you can get up to the 1.41 (sqrt2) times the transformer secondary voltage.

More henries=less ripple voltage. As mentioned check out Duncan Amps PSUDII, it will help you tremendously in this exercise.


SGregory 30th December 2010 03:54 PM

I only have to top sheet of the spec for the 5U4 but 40uF is max. As boywonder says you will need to make sure that there is enough resistance in the plate supply for that. If your PT is low impedance then you will have to add some resistance to the rectifier plates.

Vinylsavor 30th December 2010 04:00 PM


If you want your rectifiers to last long, stay clear of the max value.
Especially cheap chinese rectifiers are known for not beeing capable to withstand the max values of their original counterparts

If you want to have the least possible interference of the rectifier with the amp (cap charging current spikes) use low values for the first cap, or go for choke input supply

Best regards


pieter t 30th December 2010 07:19 PM

Not all power supply chokes are "good enough" for a choke input supply.
This choke must be able to handle the spikes and ripple coming from the rectifier and should be very well impregnated, preferably layer wound, or very audible resonance might occur.
However, when done well, choke input power supplies are the most quiet / silent, also because less strain is put on the power supply transformer.
With CLC suppplies often the current spikes in the first cap cause resonances in the power supply transformer (mechanical hum) which are almost impossible to cure.

Vinylsavor 30th December 2010 07:52 PM

Hi Pieter,

yes, absolutely right. You might get away with a standard choke if the current is derated by 50% or so and a small input cap (about 100nF).
But best is to use a suitable choke. Lundahl's chokes are made for choke inpout duty and he specifies the max AC voltage allowed across the choke. I'm sure Pieter also has the right stuff in his product selection ;)


chd_diy 2nd January 2011 01:40 PM

Thank you for your help
I wanted to thank everyone who responded with such indepth info. I really apreciate you sharing your knowledge and experinece with a beginner like myself.
The power supply design software you provided a link for is great. I've not had time to fully check it out, but from what little i've seen it looks like an invaluable tool to anyone building a power supply.

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