Zener diode replaces cathode by-pass

1/14/2008 1:58:51 AMPeter G. Balazsy

I left this post at ARF as well ....but for those here who do not frequent there I repeat it here:

I played with this a bit last year but tonight decided to confirm the concept.

In an AA5 radio most use a 50L6 or such and employ cathode bias with a 150 ohm resistor from the cathode to B- .

Typically they use a small 5uf or 10 uf cap across that cathode resistor called a "cathode by-pass" cap.

That cap is actually there to help stabilize the cathode voltage so the resulting grid bias will not vary (causing negative feed back) when the varying input grid signal causes the tube to conduct more or less.

In normal operation when the tube is at (class "A") quiescence... it will draw somewhere about 30ma or so resulting in about a 5 or 6 volt drop across the cathode resistor.

I decided today to try replacing both the cathode resistor and the by-pass cap with a single little .4watt 5.2volt Zener diode.

Works like a charm.

Any feed back ?

1/14/2008 7:27:43 AMTonyJ

Bear with me if this is a silly question, but haven't sat down yet to dig into tube theory. In looking at an AA5 schematic, I see the resistor/cap combination you mention on some of the tubes. Am I correct in thinking that this is done so that grid signals will appear as negative with respect to the cathode? In the older sets I have, the center tap of the HV winding goes through a resistor network before the ground reference, so that a 'negative' voltage is present for biasing I assume.

:I left this post at ARF as well ....but for those here who do not frequent there I repeat it here:
:
:I played with this a bit last year but tonight decided to confirm the concept.
:
:In an AA5 radio most use a 50L6 or such and employ cathode bias with a 150 ohm resistor from the cathode to B- .
:
:Typically they use a small 5uf or 10 uf cap across that cathode resistor called a "cathode by-pass" cap.
:
:That cap is actually there to help stabilize the cathode voltage so the resulting grid bias will not vary (causing negative feed back) when the varying input grid signal causes the tube to conduct more or less.
:
:In normal operation when the tube is at (class "A") quiescence... it will draw somewhere about 30ma or so resulting in about a 5 or 6 volt drop across the cathode resistor.
:
:I decided today to try replacing both the cathode resistor and the by-pass cap with a single little .4watt 5.2volt Zener diode.
:
:Works like a charm.
:
:Any feed back ?

1/14/2008 10:21:59 AMLewis L

:Bear with me if this is a silly question, but haven't sat down yet to dig into tube theory. In looking at an AA5 schematic, I see the resistor/cap combination you mention on some of the tubes. Am I correct in thinking that this is done so that grid signals will appear as negative with respect to the cathode? In the older sets I have, the center tap of the HV winding goes through a resistor network before the ground reference, so that a 'negative' voltage is present for biasing I assume.

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Correct on both assumptions. The grid stays negative with respect to the cathode so the tube will conduct all 360 degrees of the input wave without attracting electrons from the cathode.

Lewis

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:
::I left this post at ARF as well ....but for those here who do not frequent there I repeat it here:
::
::I played with this a bit last year but tonight decided to confirm the concept.
::
::In an AA5 radio most use a 50L6 or such and employ cathode bias with a 150 ohm resistor from the cathode to B- .
::
::Typically they use a small 5uf or 10 uf cap across that cathode resistor called a "cathode by-pass" cap.
::
::That cap is actually there to help stabilize the cathode voltage so the resulting grid bias will not vary (causing negative feed back) when the varying input grid signal causes the tube to conduct more or less.
::
::In normal operation when the tube is at (class "A") quiescence... it will draw somewhere about 30ma or so resulting in about a 5 or 6 volt drop across the cathode resistor.
::
::I decided today to try replacing both the cathode resistor and the by-pass cap with a single little .4watt 5.2volt Zener diode.
::
::Works like a charm.
::
::Any feed back ?