Doubts about pentode screen voltage - UK Vintage Radio Repair and Restoration Discussion Forum

Miguel Lopez · 2nd Sep 2014, 5:45 pm

Hi all

I intend to use several 6J5P or 6J38P pentodes in future RF experiments, but I have a couple of doubts.

My intention is to run the pentodes on 50 to 60 V DC, and to increase stability I thought to connect the screen to a regulated (via Zeners) voltage.
6J5P and 6J38P are equivalents of 6AH6 and 6BH6 respectively.

http://www.radiomuseum.org/tubes/tube_6j5p.html
http://www.radiomuseum.org/tubes/tube_6j38p.html

First: Would that connection increase stability? (less hum, less noise.............nothing at all?)

Second: What happen if the anode voltage falls bellow that on the screen?

Thank you very much in advance.

kalee20 · 2nd Sep 2014, 6:15 pm

If the anode voltage fell below the screen-grid, it should not matter. Bear in mind that output pentodes and beam tetrodes generally have their screen-grids tied to HT+, so about 250V or so, but the anode swings downward to 30V. Only when the anode voltage is so low that it goes round the 'knee' of the characteristic and current falls significantly, is there a problem.

With simple tetrodes, it's another matter and you get the negative resistance effect.

G8HQP Dave · 3rd Sep 2014, 12:16 pm

No need for Zeners, unless you are building a low power RF PA. Zeners can be noisy too.

Just use the usual resistor to supply rail, and capacitor to ground (or cathode).

2nd Sep 2014, 5:45 pm	#1
Miguel Lopez Heptode Join Date: Dec 2011 Location: Mayabeque, Cuba Posts: 617	Doubts about pentode screen voltage Hi all I intend to use several 6J5P or 6J38P pentodes in future RF experiments, but I have a couple of doubts. My intention is to run the pentodes on 50 to 60 V DC, and to increase stability I thought to connect the screen to a regulated (via Zeners) voltage. 6J5P and 6J38P are equivalents of 6AH6 and 6BH6 respectively. http://www.radiomuseum.org/tubes/tube_6j5p.html http://www.radiomuseum.org/tubes/tube_6j38p.html First: Would that connection increase stability? (less hum, less noise.............nothing at all?) Second: What happen if the anode voltage falls bellow that on the screen? Thank you very much in advance. __________________ When electrons move, things happen. There's nothing you can do that can't be done.

2nd Sep 2014, 6:15 pm	#2
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Doubts about pentode screen voltage If the anode voltage fell below the screen-grid, it should not matter. Bear in mind that output pentodes and beam tetrodes generally have their screen-grids tied to HT+, so about 250V or so, but the anode swings downward to 30V. Only when the anode voltage is so low that it goes round the 'knee' of the characteristic and current falls significantly, is there a problem. With simple tetrodes, it's another matter and you get the negative resistance effect.

3rd Sep 2014, 12:16 pm	#3
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Doubts about pentode screen voltage No need for Zeners, unless you are building a low power RF PA. Zeners can be noisy too. Just use the usual resistor to supply rail, and capacitor to ground (or cathode).