>
>> 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.
>
? A photograph of a grid/filament shorted 3-500Z appears on p. 15 of the
September, 1990 *QST*. The title of the article is "Parasitics
Revisited".
>The bulk of arcing failures in the 922 (and other PA's) are caused
>by improper relay sequencing,
? The 922 was designed for the TS-820S transciever. The RF relay
switches so slow that hotswitching takes place with modern transceivers.
> improper loading,
? I have neither seen nor heard of this problem.
> driver transients,
? For fundamental freq. arcing to occur at the 922's 6kV Tune-Cap, the
3-500Zs would need to develop over 6000w pep. Does this seem possible?
>or load faults.
>
? true enough. An arc in the antenna tuner can arc the 922's tank.
Same thing for an arc in a damp coax connector or an arc in a defective
lightning arrestor.
>Anytime you drive a PA without proper loading, the tank voltage
>soars and can reach several times the normal operating voltage.
? My SB-220 will not arc the tank when it is fully driven and fully
mistuned. The Tune-C in a SB-220 typically has a withstanding potential
of approx. 4kV. During the Grate Debate, someone actually measured
maximum Tune-C potential in a mistuned sb-220 at 3600v-peak. Normal
Tune-C potential is approx. 2600v-peak on SSB.
>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.
? I agree that current transients brought about by hotswitching
stimulate parasites. . However, bending contact supports will not
prevent hotswitching if RF comes down the pipe before the relay contacts
have stopped bouncing. The fix for hotswitching is either to buy a
TS820S, or to install high speed switching (approx. $60).
> 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.
>
? ..... if you happen to have a pair of 3-500Zs that can deliver over
6000w output - and you remember to disconnect the load. .
>Another common cause is operator error, where the operator tunes
>the amp at lower power and never loads the amp at full drive.
>
? It is true that this malpractice temporarily increases the potential
across the Tune-C. A maximal increase of perhaps 25% seems possible. In
a 922, this is more than 2000v shy of what is needed to arc the Tune-C.
>Also, lightning arrestor sometimes arc, and that disconnects the
>load allowing the tank to build up high voltages.
>
? The tank typically arcs. The load is still there. The arc in the
arrestor limits the potential across the coax. .
>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.
>
? As I recall, during the grate parasitics debate you stated that you
were able to make an AL-80 oscillate at c. 155MHz.
. Dick Erhorn claims that no Alpha has never sustained a
parasitic-oscillation -- even though he has reportedly seen plenty of
gold-sputtered tubes.
- cheers, Tom.
The long excursion on the wide river in the Land of the Pharoahs
continues.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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