> CT. . . The Kenwood 922 has a tendency to intermittently oscillate at
> its anode-resonance freq. of c. 130MHz. Such oscillations may cause
> intermittent spitting at the Tune-C and or bandswith, large changes in the
> R of the vhf suppressor resistors, and bursts of high grid current which
> place lateral electromagnetic force on the hot filament-helices. With
> repeated oscillations, the filament may eventually be bent far enough to
> touch the grid cage. When the filament touches the grounded-grid, the
> +110v power supply shorts to ground. If the amplifier is not switched
> off, the filament transformer will melt down. One fix is to cut the wire
> on the bias relay that connects from the coil to the bias contacts. For
> purists, the existing 100k resistor can be connected across the NO bias
> contacts on the relay. . - later, Court
Rich gave good advice, except for the parasitic stuff. He has a
fixation and pet theory that almost any failure is do to a "parasitic".
Sorta like ethnic cleansing applied to PA's, where those nasty
conventional suppressors have to go.
The bulk of arcing failures in the 922 (and other PA's) are caused
by improper relay sequencing, improper loading, driver transients,
or load faults.
Anytime you drive a PA without proper loading, the tank voltage
soars and can reach several times the normal operating voltage.
That's an effect we are all familiar with in SB-220's, Viking Valiants,
Rangers, DX-100's and other tube equipment. In MOSFET PA's, it
just wipes out the FET's from drain to gate punch-through.
A common problem in the 922 is they never bent the relay contacts
slightly, to be sure the antenna connected before the drive could be
applied. Another problem is rigs like the TS950, or IC775, that slam
the PA with a few hundred watts of drive. That extra drive switches
the tube hard, and tank voltages momentarily build up to many kV
because the tube is misloaded for that drive level.
Another common cause is operator error, where the operator tunes
the amp at lower power and never loads the amp at full drive.
Also, lightning arrestor sometimes arc, and that disconnects the
load allowing the tank to build up high voltages.
While I can't say for certain an amp could never oscillate and
cause arcing, it is probably one of the least likely causes of
failures. I've never been able to get one to oscillate and arc,
however, even when I intentionally introduced VHF oscillations.
73, Tom W8JI
w8ji@contesting.com
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