[ARC5] Regulator noise. (was BC-454 - screen voltage)

[Leslie Smith]

Hello Dennis
Quote: " Zeners are often even noisier than an ionized gas tube.  But
the standard
> fix is easy; an RC feeding a load."

I have a well informed associate who says the common 3 terminal
regulator family  eg LM7812 are noisier than zener diodes.
This idea has always intrigued me.

I'd be interested to hear your observation on the relative noise output
from each device.  Is the noise from a VR tube radiated noise, or passed
along the supply line?
If the noise is "line-coupled" will an lowpass "LC" kill the noise, eg
1mH choke plus 10nF to common?

73

  Leslie Smith
  vk2bcu at operamail.com


On Sat, Mar 9, 2013, at 13:53, Dennis Monticelli wrote:
> Zeners are often even noisier than an ionized gas tube.  But the standard
> fix is easy; an RC feeding a load.
> 
> Dennis AE6C
> 
> On Fri, Mar 8, 2013 at 3:56 PM, Brian Clarke
> <brianclarke01 at optusnet.com.au>wrote:
> 
> > Hello Ken,
> >
> > The tube characteristics written in tube manuals (whose?) are for
> > establishing other characteristics, like life, gain and emission. They are
> > not set in stone.
> >
> > If you want a quiet receiver, I would leave the Voltage regulator tubes
> > out - they rely on ionisation of the enclosed gas = very noisy. I know 'coz
> > I tried it once - with a BC-454.
> >
> > If you want to experiment with tube element Voltages, set up an octal
> > socket on a breadboard, and measure noise and distortion vs Voltage. Trying
> > to do that inside a built receiver makes it very difficult to discover
> > what's causing what.
> >
> > 73 de Brian, VK2GCE.
> >
> >
> > On Friday, March 08, 2013 8:14 AM, you said:
> >
> >
> >  Looking at the tube tables, I see that the screen voltage to the 12SK7s
> >> is 150 VDC when the
> >> plate voltage is 250 VDC. When the plate voltage is 100 VDC, the screen
> >> voltage should also
> >> be 100 VDC, according to those tables.
> >>
> >> Both ratings are for when the tube is operated in Class A.
> >>
> >> This receiver is still using the original screen-voltage voltage-divider
> >> (the two black wire-
> >> wound resistors standing up in the back). Screen voltage is 75 VDC to all
> >> the tubes in the
> >> receiver, except the 12A6. Plate voltage, with the power supply I am
> >> presently using, is 275
> >> VDC.
> >>
> >> I wish to reduce the plate voltage to no more than 180 VDC. I plan to use
> >> a VR-75 and a VR-
> >> 105 in series for this.
> >>
> >> What is the consensus concerning screen voltage to the RF and IF amps?
> >>
> >> My plan is to raise it to at least 100 VDC via a resistive voltage
> >> divider across the 180 VDC
> >> regulated input.
> >>
> >> In your opinion, what might be the result, both pro and con?
> >>
> >> In my opinion, this will result in greater gain (due to the higher
> >> relative SG voltage) and lower
> >> noise (due to the reduced plate voltage) at the RF and IF amps.
> >>
> >> Ken W7EKB
> >>
> >
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-- 
  Leslie Smith
  vk2bcu at operamail.com

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