John,
Have you ever looked at Surface Mount Power Film Resistors? A 25 ohm/
25 watt is available from Mouser at around $6. I used one of these as
the input resistor on a tetrode amp several years ago and it has work
without any issues.
http://www.mouser.com/ds/2/62/MP725-2439.pdf
73, George
W2GS
On 1/18/2016 3:13 PM, John Lyles wrote:
If you get a strong enough wirewound resistor, this can suffice as the
surge current limiting resistor for a spark over in a tube. With 250
amps at the beginning of the overcurrent (2500 V/10 ohms) spike, there
is enough magnetic field to break a wirewound R as the windings have
mechanical force between turns. I have always used bulk carbon for
series R, like Kanthal Globar makes. They are exactly the right part
for this.
http://www.globar.com/ec/bulk-ceramic-resistors/globar-bulk-ceramic-non-inductive-resistor-applications.html
http://www.globar.com/ec/bulk-ceramic-resistors/axial-leaded-resistors.html
Finding them through distribution might be a problem, though. Has
anyone else used these?
John
K5PRO
Date: Sun, 17 Jan 2016 13:19:20 -0500
From: "Carl" <km1h@jeremy.qozzy.com>
To: "Tom Hellem" <tom.hellem@gmail.com>, <Amps@contesting.com>
That blown resistor may have been underated and blew during a tube
arc. The
idea of a surge suppressor is to not fail and allow the milliseconds
for a
fuse/breaker to open.
A 10-15 Ohm 20-25W Real wirewound resistor will cure that problem.
Look for
Vitreous Enamel in the description.
Carl
KM1H
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