>
>Paul Christensen wrote:
>>
>>Regarding the Alpha PA-77...
>>
>>I just reviewed the schematic and here is my understanding of the
>>transistorized grid over-current circuit:
>>
>>The cathode-side of a 8.2 V zener is coupled to the 8877's cathode through a
>>15 uH choke and 0.02 uF bypass cap. The anode side of the zener is then
>>brought to a discrete darlington transistor pair (Q4 & Q5). The darlington
>>serves two purposes: 1) it functions as the electronic bias switch (EBS),
>>and 2) the voltage dropped across the darlington's emitter resistor (R20 -
>>3.3 ohm @ 1/2-watt) to ground is then sent to the base-bias resistor of the
>>grid over-current transistor (Q2). The collector of Q2 drives a relay
>>which in turn forces a manual reset of the amp into standby.
>>
>>If Q5 develops a CE short and subsequently opens R20, this could expose the
>>base of Q2 through a 100-ohm resistor directly to the anode of the zener.
>>Not sure what damage (if any) could develop from here. Anyway FWIW...
>>--------------------------
>>Parts used:
>>
>>Q2 - 2N5184
>>Q4 - 2N5184
>>Q5 - 2N5321
>>
>Thanks for providing some concrete information, Paul.
>
>So the way it's normally meant to operate is that grid current flows
>down through R3, and when the voltage drop exceeds about 0.6V it turns
>on Q2 which trips the protection relay. That would happen at about 200mA
>grid current, which makes pretty good sense (200mA x 3.3 ohms = about
>0.6V).
>
>Note that no grid current flows through Q2, and Q2 does not "interrupt"
>grid current. It simply operates a relay which switches the amp to
>standby.
>
>What happens to Q2 in a major surge depends on the other things
>connected to R20. If the circuit is at all conventional, R20 is the grid
>current metering resistor, so there must be a meter connected across it,
>and hopefully also a protection diode (anode to ground). Then the
>B-minus rail and anode current meter connect to the top of R3.
>
>R20 is in the path of any major current surge from B+ to ground, as it
>flows up through R3 to return to B-minus. If the protection diode is
>connected across R3, the voltage drop will not exceed say 0.7-0.8V, so
>both R3 and Q2 should survive OK. Q2 *will* survive OK if the base
>resistor is increased from 100 ohms to say 10k (and maybe change Q2 for
>a device with higher current gain... I don't know the 2N5184).
>
>So, as usual, the survivability of the DC/control circuit - and the
>meters - depends on getting several design features right.
>
>NONE of this is any excuse for disabling a grid current protection
>circuit! Yesterday I tuned up a 3CX800 amp I've been restoring. Since I
>don't have four hands, three eyes and the reactions of a fighter pilot,
>the grid current protection operated about five times before finding the
>optimum combination of the tune, load and drive controls. No problem - I
>knew that whatever I did, the grid would be in no danger.
>
>As for grid fuses (either control grid or screen grid), think what
>happens when they blow? If they let the grid float free in the electron
>stream, that puts the whole tube into even more danger.
>
\\ However, when the cathode electrons strike the floating grid it
acquires a negative charge, which should reduce anode current. With the
grid floating, there should be no grid current.
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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