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FW: [AMPS] Bypassing Screen of 4cx800a for 50 MHz

To: <amps@contesting.com>
Subject: FW: [AMPS] Bypassing Screen of 4cx800a for 50 MHz
From: 2@vc.net (2)
Date: Thu, 20 Dec 2001 15:36:59 -0800
>
>2 wrote:
>>
>
>>>> As Rich says, all the components on the cathode/G1 side have to be
>>>> insulated to withstand a large negative voltage in the event of an arc
>>>> or other current surge  - including the anode and G1 meters and all the
>>>> bypass caps. A varistor between cathode and screen (chassis) will limit
>>>> the negative voltage surge, but it has to be rated to handle the full
>>>> surge current... which is why you still need a limiting resistor in the
>>>> B+, to limit the maximum current that the Varistor has to handle.
>>>>
>>//  I tried the metal-oxide varistor approach.  Even though the varistor
>>was substantial, it would destruct during a glitch,
>
>That's how you find out that the Varistor wasn't substantial enough.

//   Indeed
>
>>The slam-dunk fix
>>was a spark gap set to 500v more than the screen V (with a high-pot
>>tester).
>>
>The choice between a Varistor and a spark-gap protector isn't  always so 
>clear-cut. The Varistor can start to conduct and provide protection only 
>a little above the normal operating voltage, but it might be destroyed 
>in a big hit. On the other hand a spark gap is much more robust and has 
>a low-voltage clamping action when triggered, but it triggers less 
>reliably.

//  The anode supply was c. 9kV.  The screen supply was 1300V. The spark 
gap fired at c. 1800v.  It worked ok.   The cathode bypass caps were 
rated at 3kV.
>
>500V above the normal screen voltage is leaving a *lot* of headroom 
>before any protection kicks in. Also an open-air spark gap may not 
>create a reliable low-voltage arc which effectively clamps the screen to 
>the cathode, and holds it there. A sealed spark-gap protector does that, 
>but it requires a specific gas (argon? xenon? mixture?) at a specific 
>pressure - and they wouldn't do that if they didn't have to. Also the 
>breakdown voltage is more reliable than an open-air gap, so you don't 
>have to leave as much headroom above the normal operating voltage.
>
>Another alternative is a thyristor crowbar, like they use in low-voltage 
>power supplies but with a higher voltage zener chain. When fired by 
>excessive voltage across the zeners, this will very firmly nail the 
>screen to the cathode.
>
//  The filter C stored >1kJ.  

>>>> Another problem with grounded-screen is that all your RF bypass
>>>> capacitors are grounded to the chassis, not the cathode, and when you
>>>> switch to TX there is a surge of current through the screen connection
>>>> to charge these bypass caps.
>>
>>//   Normally, RX cutoff is achieved in a grounded-screen tetrode
>>amplifier by increasing negative grid V, not by switching screen
>>potential off and on.  Thus, screen potential is unchanged between RX and
>>TX.
>
>That is probably a better way.
>
//  For VOX - surely.  Diagram at Figure 4 on my Web site.

>>//   The above scenario was for a g-g triode, not a tetrode.  It is my
>>opinion that the heater should always be connected to the cathode and the
>>heater should float -- unless one has a freebee supply of new tubes.
>>
>You're probably right, for both triodes and tetrodes, because in both 
>cases an arc condition can pull the cathode potential a long way from 
>the (grounded) filament potential.
>
cheers,  Ian

-  R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.  
end


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