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Old 14th December 2004, 02:37 AM   #1
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Default Modifying (Reducing) Screen Voltage

I,m wanting to know about reducing the DC voltage on screens of pentodes and beam tetrodes in Ultralinear Mode or Triode Strapped. So I turn to my On-Line learning facility -

I have a Ultralinear Amp (EL34) with 480V on the Anodes and Screens - a 1K screen resistor is incorporated. 480V on the screen seems to me to be asking for trouble.

I did some searches and suggestions ranged from just using a series zener to drop the screen volts to a complicated scheme which used a LED in the emitter of a transistor and then arranged the whole thing as a Vbe multiplier (or in this case a Vbe + LED volts) multiplier which come to think of it is just a glorified zener anyway.

Also I could'nt see any firm opinion as to whether a bypass capacitor (for the voltage dropping element) was required or not. My guess is that its essential.

I want to change the current amp from Ultralinear to Triode Mode BUT the 480V worries me.

Any references / opinions on reducing screen voltage in Ultralinear and Triode Mode gratefully received.

Thanks (in anticipation)
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Old 14th December 2004, 06:10 AM   #2
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1. If it ain't broke, don't fix it. It might take a few months for it to break though. If it ever breaks at all. In which case, the tubes don't care the slightest about their screen voltage.

2. A gas tube (not recommended), zener or Vbe multiplier will certainly work. I would recommend a capacitor across it to dampen the impedance and reduce noise (not like you'll get much noise pickup on a screen anyway).
This shows triode limits as 500V, with a derating to half dissipation at 600V. I'd say #1, your tubes are not in danger.

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