[AMPS] TL-922 Filament Transformer Protection

[Tom Rauch]

> 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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