Why Not Supersize the Resistors?
Question: "If being a bit heavy-handed during tune-up and operation of your Kenwood Hybrid rig can 'smoke' the resistors in the cathode and screen circuits, why not replace those resistors with larger units?"
Answer: I have been asked this question any number of times. Some Kenwood web sites even promote the idea of increasing the wattage of these resistors, and one even suggests using wirewound ceramic power resistors up to 10W!
The Reason NOT TO is Simple: Kenwood purposely selected these wattages to act as "fuses" in the event someone did not properly tune and load their rig, and then keyed it for an extended period. Better to lose a 20 or 40 cent resistor, than a $125 power transformer! I have seen many burned resistors in rigs I have repaired, but I've never seen a burned up power transformer in a rig where the resistors were not "supersized." Keep the resistors sized exactly as Kenwood designed them, and your rig will easily last another 30 years.
About Wirewound Resistors:
Most of you probably already know that wirewound resistors are
inductive, and this is a no-no for use where a pure resistance is
called for, such as dummy loads, and the like. As frequency goes
up, so does the impedance of these wirewound resistors (they
appear as inductors because they are made of coils of
wire), and they simply don't behave properly! Do not
arbitrarily replace carbon composition or film resistors with wirewound
units under any circumstance!
Kenwood Design Philosophy: The Kenwood
hybrids are a balance of form and function - Everything in them has
been sized for a reason. They have lasted a mighty-long time in
their original form, don't feel obligated to improve upon that success.
Example: Here's pictures of a rig I received a few weeks ago,
where someone apparently bought into the "supersize" theory. The
first picture shows the "improved" final screen resistor a previous
owner had installed, and the second picture shows my properly sized
replacement. I'm sure the current owner would much rather replace
that resistor instead of the power transformer and final tubes should
his coax fail, for example.
It takes a pretty severe overload to destroy
the original Kenwood parts. Always replaced damaged
parts with similarly rated components, and trust the original
Kenwood designers for having done their job properly.
