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.
--
73 Ron KA4INM - All E-mail sent to this address shall linger in the Google
cloud forever!
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