[Amps] 89 Alpha schematic

[MU 4CX250B]

Clearly there’s an error in the schematic. So far as I can tell, the
fuse should be in series with R10, the surge-limiting resistor. It’s
purpose is not to protect R10, but to protect the amplifier in case
the soft-start relay (or associated timing circuit) malfunctions and
doesn’t close. In that (unlikely) event, the amplifier will try to run
with R10 in line. The operator may not notice anything awry until the
amplifier starts to draw plate current. Eventually R10 will burn out,
but in the interim the filament voltage and bias voltage will be too
low, and the plate voltage will sag. The fuse, if properly placed,
will blow and shut the amplifier down before any damage could can
occur. We can debate the wisdom of including the fuse in the
amplifier, but I’m inclined to leave it out. It reminds me of the fuse
designers used to place in the cathode lead of grounded grid
amplifiers. Again, it’s best left out.
73,
Jim w8zr


Sent from my iPhone

> On Apr 4, 2021, at 12:22 PM, gudguyham via Amps <amps at contesting.com> wrote:
>
> If the relay fails the fuse should blow before the resistor burns out.
>
> Sent from the all new AOL app for iOS
>
>
> On Sunday, April 4, 2021, 2:20 PM, Robert W5AJ <woodr90 at gmail.com> wrote:
>
> This probably same for Alpha 86..
>
> photo of schematic showing incoming line and soft start at this link
> http://pages.suddenlink.net/w5aj/89Fuse2A.jpg
>
> Traced single wire from the 2A fuse to R10 - no other connections with that
> wire
> Indicates to me that the 2A fuse is in line with the resistor, should be
> shown in line above where it is shown.
>
> now to the question:
> decades ago, some designer for Alpha must have thought that fuse would serve
> a purpose.
> I can think of a few pointless purposes but nothing that justifies adding in
> the fuse.
> ANY Thoughts ?
>
> PS:  Can't find other published designs that fuse the limiting resistor
>
> 73 W5AJ
>
>
> Robert
> Midland, Texas
> http://pages.suddenlink.net/w5aj/
>
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