[Amps] Plate modulation from power supply ripple?

[Fuqua, Bill L]

  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 at contesting.com] on behalf of Markku Oksanen [ww1c at outlook.com]
Sent: Thursday, October 03, 2013 1:25 AM
To: garyschafer at 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 at comcast.net
> To: ww1c at outlook.com; amps at 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 at contesting.com] On Behalf Of Markku
> > Oksanen
> > Sent: Wednesday, October 02, 2013 12:40 AM
> > To: amps at 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 at contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
>

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