Ian White, G3SEK
Ian White, G3SEK" <email@example.com
Fri, 7 Dec 2001 12:03:28 +0000
Paul Christensen wrote:
>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.
>would be an interesting test to measure the "break" time of the fuse
The break time of a fuse depends crucially on the size of the overload.
Remember that a fuse operates in a very simple way, by heating up a
piece of very thin wire until it melts. For the fastest break times, it
requires a huge overload that flashes the wire to vapor almost
That's why fuses are great for protecting DMMs against the common
mistake of trying to measure voltage when the meter is still set to
measure current - the fuse blows very quickly because the overload is so
big. That's what fuses are very good at.
But an overload of only 50% can take a v e r y long time to melt the
wire - even an "ultra-fast" fuse can take several seconds. As a reality
check, I just tested a 250mA F-rated fuse (a spare for my DMM, but you
might use that same fuse for grid protection) at 50% overload. Well, the
fuse got warm, but after 60 seconds it still hadn't blown!
If a fuse is only protecting something like a line cord, a 50% overload
for several seconds doesn't mean much - but it means a helluva lot to
the grid of a tube! That's why most amp designers prefer an electronic
trip circuit with a sharp threshold.
>versus a transistor/relay combination.
Just tested that too. The circuit is a bipolar transistor sampling grid
current, into a CMOS trigger that drives the cathode bias relay. There
is a 1 millisecond delay on the input to the transistor, to prevent
nuisance triggering, and the data sheet for the relay (nothing special,
a regular Potter & Brumfield / Schrack RTE240xx 8 amp power relay) says
the break time is 2 ms.
I hit the tube with barely enough drive power to take it above the trip
threshold. Sure enough, about 3 ms later the tube was safely into
The important thing is that the delay of an electronic trip can be only
a few milliseconds for *all* levels of overload. This is what an
electronic trip can do, but a fuse can not!
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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