>
>Rich Measures wrote:
(I did not write:)
>>>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.
>>>
(This I wrote)
>>?Ǩ 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.
>
? The "entire power supply"? The screen power supply connects between
the cathode (-) and the screen (+). This is true in either case.
>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.
? RE: Figure 5. The screen current meter is no more than 0.2v above
gnd. The anode current meter floats at the screen 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,
? chortle
>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.
>
? see Figure 5. In the event of an arc, "Ds" shorts to protect the
meters, et cetera.
>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!
>
? Cathode bypasses are soldered in. Replacing them is easy.
Socket-type screen bypass caps are an integral part of the socket. They
seem to be not-replaceable.
cheers, Ian
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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