>
>
>
>> ----------
>> From: Ian White[SMTP:G3SEK@ifwtech.com]
>> Reply To: Ian White
>> Sent: 20 December 2001 10:22
>> To: Peter Chadwick
>> Subject: Re: [AMPS] Bypassing Screen of 4cx800a for 50 MHz
>>
>> Peter - please can you forward this to AMPS. My messages are currently
>> being blocked by contesting.com's paranoid Spamcop robot!
>>
>>
>> 2 wrote:
>> >>If I were going to build a single Tetrode amp on 6m or 2m I'd consider
>> >>running the screen at deck - RF simplicity at (slight) HT complexity
>> (ie.
>> >>need for a B- lead).
>> >>
>> >// A chassis-grounded/earthed screen solves the screen's bypass-C
>> >resonance problem, however, an anode (B+) to ground arc can briefly
>> >drive the HV negative lead to 1000s of volts. Unless precautions are
>> >taken, the arc may cause damage to components that are connected to the
>> >HV negative
>>
>> Even for grounded cathode, you still need a separate B-minus lead
>> (unless you want to do the anode current metering up there at B+
>> potential) but that isn't normally a problem because B-minus is clamped
>> very close to chassis ground by protection diodes.
>>
>> Rich is correct that the advantages in RF performance of grounded-screen
>>
>> are balanced by disadvantages on the DC supply side. Basically you have
>> to float all the anode, G1 and G2 power supplies, which must be commoned
>>
>> at the cathode - not the chassis. The *only* chassis ground for the
>> entire power supply must be through the screen current meter.
>>
>> 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, The slam-dunk fix
was a spark gap set to 500v more than the screen V (with a high-pot
tester).
>> 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.
>This makes screen-overcurrent protection
>> more difficult... still possible, but more difficult.
>>
>> > In a typical filament-type tube amplifier, such as 2, 3-500Zs, such an
>>
>> >arc usually will not breakdown the filament-transformer secondary
>> >insulation, but it will often damage the grid-current meter shunt
>> >and/or the grid current meter. In a typical heater-type tube amplifier,
>>
>> >provided that one side of the heater is grounded, a HV+ arc to chassis
>> >can cause the cathode to briefly rise to several kV negative - which
>> >causes an arc between the cathode and the grounded heater (typical
>> >cathode-heater rating: 150v-max). The secondary arc can burn the
>> >heater wire in two, thereby destroying the tube.
>>
>> Also very true. If the screen is at chassis ground, the cathode will be
>> at -350V or more. Unless you tie the heater to the cathode, the
>> heater/cathode voltage rating will be exceeded even in normal
>> operation... and with an arc it will be even worse.
>>
// 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.
cheers, Ian
tnx, Peter
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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