Phil Clements wrote:
>
>Subject: Re: [AMPS] How To Design HV Glitch Protection Circuits
>
>
>> While Eimac suggests a value of R in the screen ckt, experience
>> indicates that a value of R in the screen ckt during a flashover will
>> allow the screen voltage to ramp above the rating of the screen bypass
>> capacitor. My experience is in VHF and UHF using the 4CX250 and 4CX300
>> tubes. Thus sockets with screen bypass capacitor built in is
>> necessary. The end result, get the check book out, cause a new socket
>> is needed.
>>
>> 73
>> Bob K4TAX
>
>I have found Harris to be very innovative in their designs of broad-band
>no-tune
>high powered amplifiers. Such as plate chokes with no resonances, etc.
>A very nifty solution to your problem is also included in the RF-110A amp
>that uses a pair of 4CX1500B's. It consists of an SCR, four diodes, and three
>resistors. They call it an arc suppressor. When there is a very momentary arc
>between the plate and screen the arc suppressor protects the screen power
>supply components in the event of such an arc. When an arc occurs, the
>current passing through the screen regulator diodes causes a voltage drop
>across a resistor which fires the SCR. This shorts to ground two points:
>1. The final amplifier screens at their sockets.
>2. The highest voltage point on the screen supply.
>This shunts the arc currents around the screen supply components to
>ground, and shunts the screens of the tubes to ground.
>I hope this is helpful to you tetrode folks.
>
>(((73)))
>Phil, K5PC
What Bob describes can easily happen with 4CX250/350 sockets, if you
don't take the trouble to protect them.
Eimac's advice about a series resistor in the anode supply is good, but
their advice about a series resistor in the screen circuit is *bad*.
I never like going against the tube manufacturer's advice, but there are
two very good reasons to ignore it here:
1. It increases screen voltage variation on normal operation, which
makes IMD significantly.
2. As Bob says, if there is an arc from anode to screen, this resistor
will allow the arc to drive the screen voltage more strongly positive,
and will probably destroy the bypass cap in the socket.
3. The resistor only protects against an arc from the screen supply to
ground or cathode. But what's to protect? The screen supply is only a
few hundred volts in amateur-size tetrodes (even Rich's) so the risk of
the screen ever arcing is very low.
The solution is to use the resistor in the B+ supply, and to limit the
screen-cathode voltage using a VDR (MOV, varistor), a spark gap
(Siemens) or a thyristor crowbar like Phil describes.
Bottom line: DO use the big series resistor in the anode supply. DON'T
use any series resistor in the screen - use VDRs etc instead.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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