[Amps] The power of an oversized electrolytic capacitor / how much PS filter C is enough?

Leigh Turner invertech at frontierisp.net.au
Thu May 10 04:19:51 PDT 2012

Hi Jim,

There are two aspects to consider here; the static and dynamic voltage
regulation, and the amount of ripple on the B+ plate supply.

In CW and SSB amplifier service the keyed and syllabic fluctuation of load
defines the requisite dynamic regulation capability of the HV power supply.
Achieving static and dynamic regulation under 5 percent would be good design
practice. As for rms ripple voltage, a target of less than 2 to 5 percent of
the DC supply voltage under full load current would be considered good
engineering practice.

A large energy storage capacitance on the HV rail makes sense from the point
of view the amplifier load current requirements have a large peak-to-average
ratio in SSB service. A stiff B+ voltage also helps with minimising IMD.

For common 3 kV @ 1A amplifier power supplies, i.e. a load resistance of
circa 3000 Ohms, a reservoir capacitance of about 60 uF would be sufficient
to yield the above respectable regulation and ripple objectives. One might
double the filter capacitance to circa 100 uF for good measure; but going
beyond that amount of capacitance would be a futile exercise in diminished

Now your 7 kV B+ supply corresponds to a PS load resistance of circa 7000
Ohms, so there is an associated relaxation in the capacitance required to
meet a given percentage ripple voltage objective.

The tolerable ripple level superimposed on the HV plate supply above which
incidental AM hum sidebands appear on the radiated signal is an interesting
question; the anecdotal experience of Rich's friend with only 2 uF of filter
capacitance suggests there's a large tolerance to ripple on the plate supply
with the tube and tank circuit exhibiting a good PSRR.

We can note the amplifier in question here uses a 4-1000 tetrode, and that
such tubes exhibit a plate current virtually independent of plate voltage,
i.e. tetrodes are a constant current device. Such a characteristic would
make it more immune to ripple and noise on the B+ plate supply.


-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Jim Thomson
Sent: Tuesday, 8 May 2012 5:19 PM
To: amps at contesting.com
Subject: [Amps] The power of an oversized electrolytic capacitor /

Date: Sat, 5 May 2012 20:59:29 +0930
From: "Leigh Turner" <invertech at frontierisp.net.au>
Subject: Re: [Amps] The power of an oversized electrolytic capacitor /

I agree Rob; a fraction of that massive HV PS capacitance would create an
unperceivable difference in the Tx performance and signal at a distant Rx
station :-)


### Leigh, how many uF in your opinion, do you suggest I use on a 7.7 kv No
load B+ supply. 
What is the bare minimum I could get away with. I’m being serious here.
Rich Measures has a friend with a 4-1000 amp...and he only uses 2 uf for a
filter cap...and No choke anywhere. Nobody hears any hum. 

On a similar note, a friend was on one night, toggling between his SB-220
and his Alpha 9500.  Nobody could tell the difference between the 1200w of
the SB-220.....and the 1500w of the Alpha 9500.   

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Rob Atkinson
Sent: Saturday, 5 May 2012 8:53 PM
To: Jim Thomson
Cc: amps at contesting.com
Subject: Re: [Amps] The power of an oversized electrolytic capacitor

832 uF at 7.7 KV.  No offense, but that's a waste of capacitors.


On Fri, May 4, 2012 at 9:28 AM, Jim Thomson <jim.thom at telus.net> wrote:

> ###  BTW, My latest creation uses 24 x 10,000 uf @ 450 vdc caps, all in 
> series... and charged up to 7700 vdc. That’s one bank.  I built a 2nd 
> identical bank of 24, and parallel the 2 x banks together... onto a common
> buss....

###  I got them for dirt cheap.  There is No way I would build something
like this by paying full price for new caps. I wouldn’t buy a brand new vac
cap from Jenning’s either, they are cost prohibitive.  But I have managed to
buy surplus, nib, sealed in the box, Jennings ceramic vac caps.   They come
encased in a metal foil bag which has been vac sealed.  You poke a pen knife
into it, and whoosh, the air rushes into the bag.  Once bag is removed, then
you unseal the box...and the new vac cap is inside a box, lined with foam
rubber on all 6 x sides.  

later... Jim  VE7RF

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