There is a reason why plate modulated amplifiers operate in Class C or E modes.
That is because in linear modes do not produce modulate linearly. That is to
say
if you double the plate voltage the RF voltage peaks are not doubled and the
PEP power is not
X4. This is one reason why some amplifiers like the Heathkit Compact KW can
work well
in SSB or CW (running AB2) with one side of series wired filaments grounded and
12.6 volts
applied to the other side of the filaments/cathodes.
The end result is that 6 percent ripple on the plate if you are operating in
class AB2 not C does not
turn out to be 6% in the modulated carrier. It is somewhat less.
The other thing is that most of the time the average current on the power
supply in SSB will be
much less than that in CW so the ripple will be less there as well.
The ripple will, in SSB, appear to be a modulation on the modulation and not
a hum. This is not
so noticable, this was observed in the very early days when the filter chokes
for AM radios was also
the Magnet for the Speaker. All the ripple current flowed thru the filter
choke. It it did modulate the
audio somewhat but was not so noticable.
You will probably find many operating SSB have more full load CW ripple that
6%. But no one really notices
unless you apply a pure tone and there is a good S/R ratio on the receving end.
In the case of the cathode 60Hz modulaton in the Heathkit amplifier the
degree of modulation is actually
realated to the IMD, which is 30db or more down to start with and that is with
equal amplitude signals.
Whe one is 1/10 less than the other it is much further down.
73
Bill wa4lav
________________________________________
From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf Of
Vic K2VCO [vic@rakefet.com]
Sent: Sunday, June 19, 2011 1:34 PM
To: Paul Kraemer
Cc: Amps reflector
Subject: Re: [Amps] Using an oil capacitor
Several respondents to my message have said the same thing. This capacitor
cannot be used
at its voltage rating in filter service and needs to be de-rated to about half
its spec'd
voltage in that case.
I actually have two of these, so I could probably use them in series for a 26.5
uf filter
capacitor. This would be adequate for a cw transmitter, although I'm not sure
if 6% ripple
wouldn't be audible on SSB. However, two capacitors make a very big package, so
I'm
thinking about going back to the conventional string of electrolytics.
Any recommendations for a source of electrolytics? There are lots of
inexpensive ones on
ebay, but who knows if they are factory rejects!
On 6/19/2011 10:10 AM, Paul Kraemer wrote:
> Vic BE CAREFUL. BE VERY CAREFUL!!!
> From the size and capacitance you describe it is probably an "energy storage
> capacitor"
> which is not suitable for power supply filter at even it's published ratings.
> You will be
> picking pieces of capacitor out of places you really don't want them.
> Paul K0UYA
>
> ----- Original Message ----- From: "Vic K2VCO" <vic@rakefet.com>
> To: "Amps reflector" <amps@contesting.com>
> Sent: Sunday, June 19, 2011 12:59 AM
> Subject: [Amps] Using an oil capacitor
>
>
>> I have an oil capacitor rated at 53 uf 3.5kv. I would like to use it in a
>> power supply
>> with a transformer delivering 2.4 kVAC. The peak voltage is thus close to
>> 3400V.
>>
>> I've never used an oil capacitor before. My line voltage is pretty constant
>> at 240V, but
>> if it were to rise to 250V, the voltage rating of the capacitor would be
>> exceeded. What
>> kind of safety factor do capacitors like this have?
>>
>> The capacitor is 4-1/2" x 4" x 10" and is marked CSI 4W308T. The CSI website
>> <http://www.csicapacitors.com/csite/cms/149.htm> doesn't list this
>> particular one, but
>> other 3.5kV capacitors have similar part numbers. There's no detailed spec.
>> There is this
>> statement:
>>
>> "The WM series of capacitors is designed for general purpose pulse
>> applications in power
>> supplies for lasers, flash tubes and other capacitor discharge circuits
>> where moderate to
>> long life is required."
>>
>> I'm not sure if this is appropriate for power supply filter application.
>> Anybody familiar
>> with these?
>> --
>> Vic, K2VCO
>> Fresno CA
>> http://www.qsl.net/k2vco/
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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