On 5/10/2012 5:43 PM, Ron Youvan wrote:
> 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.
"(+24 kW of aural power)" -
Sounds like my wife when she gets together with her sisters and the big
black kettle...
My SDR receivers have response pretty much down to DC, and their audio
mixes to a system that has a -3dB point below 30hz. That may sound
atypical, but we are many - at least more than you might think.
If I hear 60 or 120hz in the signal of anyone I'm talking to I let them
know. Often all that does is make me unpopular, so I don't go out of my
way to mention it unless I'm actually in QSO with the hummy ham.
73,
Jim N7CXI
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