Thu, 6 Dec 2001 09:02:18 -0500
By comparison, my PA70-V uses the same darlington EBS circuit to the cathode, but Q2 does not exist as this was a feature
"enhancement" in the PA-77 which later replaced the PA70 series. Instead, a 1/4-amp grid fuse is placed in series with the 8.2 V
zener diode to the tube's cathode.
Now, to Rich's point (I believe)...the fuse may in fact blow faster in this arrangement than in the case of the PA-77 where a
sampled switching transistor operates a relay to force a standby condition. The relay uses no special acceleration circuit. So, it
would be an interesting test to measure the "break" time of the fuse versus a transistor/relay combination.
----- Original Message -----
From: "Paul Christensen" <firstname.lastname@example.org>
To: "Ian White, G3SEK" <email@example.com>; <firstname.lastname@example.org>
Sent: Thursday, December 06, 2001 8:22 AM
Subject: Re: [AMPS] AL1500
> > 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).
> That is correct Ian.
> > 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.
> Right again. In essence, Q2 "samples" the emitter current on the darlington pair.
> > 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,
> Yes. I failed to mention that the R20 sample point also routes over to the multi-meter circuit for grid I monitoring.
> > and hopefully also a protection diode (anode to ground). Then the
> > B-minus rail and anode current meter connect to the top of R3.
> Could be...I didn't look this far.
> > 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).
> I agree...perhaps going from 100-ohms to 10K in order to increase input isolation and changing Q2 to a darlington-pair may be
> -Paul, W9AC
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