All good points Ron. In seriously HV broadcast Tx systems damaging arcs are
an important design consideration and reason to restrict stored energy.
However, the discussion here started out in the context of a traditional
single-phase scenario with a simple reservoir capacitor on the output of the
full-wave rectifier. No filter chokes; just a solitary filter capacitor /
series cap bank.
I.e. what's found in the majority of commercial and homebrew 1 to 3 kW class
ham radio linear amplifiers. Reducing the filter capacitance down to the
aforementioned minuscule 2 uF may not cut the mustard in many RF amplifiers.
On the other hand having super-sized HV power supply C is folly. There is a
pragmatic and sensible value of capacitance beyond which nothing is gained.
A 3-phase power supply inherently has a low ripple component and at 3X the
AC frequency; only a few percent, even without a filter capacitor. Hence a
Pi-section choke filter with small C will clean it up very nicely.
Yes 3-phase HV power supplies are very nice from several points of view;
hence their universal use in broadcast transmitters and some high-end ham
amplifiers.
We note that many modern receivers, particularly the SDR architecture type
have audio responses that extend quite low and would readily reproduce any
incidental hum modulation on an errant signal.
Leigh
VK5KLT
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of KA4INM
Sent: Friday, 11 May 2012 12:31 AM
To: amps
Subject: Re: [Amps] The power of an over sized electrolytic capacitor / how
much PS filter C is enough?
Leigh Turner wrote:
/*snip*/
> 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.
We also need to take into account the fact that many HAM receivers do not
reproduce 120 Hz well.
When we started the earphones were connected between the plate of the AF
amplifier and the 90 Volts. (battery?) so it was DC coupled, but that is no
more.
I have maintained many "low power analog TV transmitters" over the years
and a little 60 or 120 Hz hum is a big problem, it causes hum bars to roll
up the screen for everyone, but the hV power supply was always a Pi network
with two 10 mFd "oiled" paper capacitors and a sizable choke, a low pass
filter passing DC and less than 60 Hz. Arcs in the tube or elsewhere are a
great concern in broadcasting so the stored energy in 10 mFd is way less
that the kind of energy stored in the stack of electrolytics found in most
HAM power amplifiers.
I maintained a 110 kW visual TV transmitter (+24 kW of aural power) that
had a 1.0 mFd filter capacitor (only) possible because it's
transformer+rectifier had 6 X 60 Hz ripple. (360 Hz.) (at 30 kV) All this
was to have a minimal energy storage to minimize damage from arcs.
BTW: If you dropped ONE strand of fine wire stripped from zip cord across
it, it would not melt.
--
Ron KA4INM
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|