Rich Measures wrote:
>>2. Sometimes the arc isn't any kind of RF problem at all. Many of mine
>>have been due to insects (or loose parts shredded by the blower) being
>>carried in the airstream until they lodge in the chimney between the
>>anode and the screen ring.
>>
>â?¬ How did the insects get inside the tubes?
We are talking about power supplies, and the power supply doesn't care
where the arc is.
> Direct screen grounding is how large
>>>tetrode amps are normally built.
>>>
>>Yes, you can do that... but there are many more medium-sized amplifiers
>>that use the SK610-20-30-40 series of sockets with screen bypass
>>capacitors.
>>
>Why trash a socket with a shorted bypass?, Ground the screen with
>copper ribbon and the socket is still useable.
Only after you have provided a cathode bypass capacitor and re-wired the
entire power supply.
Although DC grounding of the screen eliminates the problem of the screen
bypass capacitor, it creates new problems in other parts of the circuit.
The B-minus rail is now negative of the chassis, so the G1 and anode
current meters are no longer close to chassis potential. Likewise the
grid bias and heater supplies have to be floated negative. This is
likely to require total re-wiring of the power supply, lifting all the
existing chassis connections and converting them to well-insulated
rails. The insulation requirements are not too difficult with normal
screen voltages - but think what happens when an arc from B+ hits the
chassis. The arc effectively clamps the B+ rail to chassis potential, so
the B-minus rail is pushed hard negative, taking with it the meters, the
cathode and the G1 and heater supplies.
Oh yes, that cathode bypass capacitor... When you DC-ground the screen,
you have to bypass the cathode instead. That capacitor faces exactly the
same problems as the screen bypass cap you thought you had eliminated!
DC-grounding the screen can certainly cure some RF stability problems,
but it doesn't cure any DC and flashover problems - it merely shuffles
them into different places in the circuit.
Sorry, still no easy answers...
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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