It is a matter of degree. If the tube is operating in class-C it is swinging
from saturation to cut-off similar to a switch. The voltage that appears on the
plate swings about the range of
the power supply voltage and 10 percent ripple would produce 10 percent
modulation.
However, if the tube is operating in a linear mode things are quite
different.
The extreme case would be a tetrode or pentode. A change in plate voltage
produces little change in plate current.
This, by the way, drives plate modulators crazy. To get around this the screen
is modulated so that from the plate modulators view point the impedance is
reasonable and the tube appears to be a triode.
The screen voltage changes makes the plate current change nearly proportional
with the plate voltage.
Triode tubes operating in a class AB2 configuration do not saturate.
The effects of small plate voltage changes have a lesser effect on the output
power.
When operating SSB you don't have a carrier and this effect is not very
noticeable.
It tends to modulate the SSB signal rather than producing a hum just as in the
old AM radios that had
a electromagnet in the speaker that served as the power supply choke. There is
an effect but one
that is of a lower order you may say.
When operating CW it would be noticed but it would be perhaps 3 to 4 percent
ripple.
73
Bill wa4lav
________________________________________
From: Amps [amps-bounces@contesting.com] on behalf of Markku Oksanen
[ww1c@outlook.com]
Sent: Thursday, October 03, 2013 1:25 AM
To: garyschafer@comcast.net; Amps
Subject: Re: [Amps] Plate modulation from power supply ripple?
Hi Gary, All
I was thinking about your statement and looking a typical tube anode
characteristics (curves).If I read this correctly, change in plate voltage does
cause a change in plate current.Is this not plate modulation? I am not sure if
the operating class changes this.Perhaps I need to simulate this too...
Thanks
MarkkuWW1C
> From: garyschafer@comcast.net
> To: ww1c@outlook.com; amps@contesting.com
> Subject: RE: [Amps] Plate modulation from power supply ripple?
> Date: Wed, 2 Oct 2013 12:36:06 -0500
>
> You don't hear the ripple because it does not modulate the linear amplifier.
> If you were to run the amplifier in a non-linear mode such as class C then
> it would plate modulate it.
>
> 73
> Gary K4FMX
>
> > -----Original Message-----
> > From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Markku
> > Oksanen
> > Sent: Wednesday, October 02, 2013 12:40 AM
> > To: amps@contesting.com
> > Subject: [Amps] Plate modulation from power supply ripple?
> >
> >
> > All
> > I was simulating (with LTspice) a three phase power supply where the
> > secondary is in star configuration with full wave rectification. You can
> > get also a second voltage from this by connecting to the center of the
> > star, this voltage is half of the full wave rectified voltage and is
> > only half wave rectified (per phase), 3 pulse in stead of 6 pulse for
> > the higher voltage.
> > It turns out that even relatively high value (tens of uF) filter
> > capacitors would leave some 10% of ripple on the lower voltage. Now the
> > question is: How much plate modulation this make? The ripple frequency
> > is 150 Hz and to me this looks like your regular high level, plate
> > modulated AM transmitter. Why do we not hear this on a typical signal
> > if it is there?
> >
> > MarkkuWW1C
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
>
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