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
-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
>
>
>
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