>
>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.
>
\ Perhaps grid-current does not stop soon enough? The 77 has a
history of gold-sputtering 8877s.
>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...
Beats me.
tnx, Paul
>Parts used:
>
>Q2 - 2N5184
>Q4 - 2N5184
>Q5 - 2N5321
>
>-Paul, W9AC
>
>----- Original Message -----
>From: "Paul Christensen" <paulc@mediaone.net>
>To: " AMPS" <amps@contesting.com>
>Sent: Wednesday, December 05, 2001 14:04 PM
>Subject: Re: [AMPS] AL1500
>
>
>>
>> > // Some Alphas apparently C-E-short their interrupt transistor.
>>
>> If I am not mistaken, the original Alpha 77 (model PA-77) uses such a
>>grid-protection circuit. However, I don't recall how well the
>> switching transistor is buffered and isolated from the grid. If I get a
>>chance tonight, I'll look at the schematic.
>>
>> On the other hand, my Alpha PA-70V (with a single 3CV1500A7) uses a
>>factory-designed 1/4-amp/3AG grid protection fuse. The fuse
>> seems to work well and blows quite nicely when I accidentally apply full
>>exciter power at 100-watts. What I don't know is the
>> long-term detrimental effect on the tube caused by these brief and
>>occasional periods of excessive grid current. Of course, the
>> fuse blows well before the grid ammeter has an opportunity to display the
>>excessive condition (due to the meter ballistics). But,
>> how fast is fast enough?
>>
>> -Paul, W9AC
>
>
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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