Re: [Amps] TL-922A - Loud Pop, Blown Fuses

[R.Measures <r@somis.org>]

On Dec 5, 2004, at 6:03 PM, Clay Curtiss W7CE wrote:

> Hi Rich,
> I didn't remove the tubes, but I did remove the anode connection to 
> both of
> them for the quick HV test.  The fuses don't blow when the anodes 
> aren't
> connected.

Clay -- This is good news.
> I figured the sound wasn't inside the tube for the reason you mention. 
>  Any
> idea what could be causing the loud pop?

My guess is that it would likely be from the HV to chassis-gnd.  I was 
never able to observe the arc that causes a big-bang, but recently 
someone just happened to be looking in the right direction at the right 
time and saw an arc from the anode cap up to the RF deck lid.  At c. 
1000V per mm, the distance of the arc was several times greater than 
was possible with the normal approx. 5800V peak at the anode (during 
the half-cycle when anode-I is cutoff and the tank is flywheeling).  
Thus, the whatever it was that caused the big-bang/pop must have been 
some kind of anomaly. My guess as to whatever it is that causes the arc 
is that it is not HF energy since HF would pass on through the 922's 
tank circuit and go to the load.
-  My free, unsolicited advice at this point in your detective work is 
to install a suitable glitch R in the +HV just before the main HV-RFC 
in order to protect the tubes from grid-filament short during a 
big-bang.  --  note -- a grid-fil short is produced by the middle part 
of the (1830ºK) tungsten filament helix bending sideways during the 
big-bang.
-  The glitch R should be able to withstand a +HV to ground short.  A 
glass-coated 10 - 15 ohm 10w WW resistor will usually do the job.  
Since it is wirewound. this resistor can replace L2, the 12uH VHF choke 
at the base of L1, the main HV-RFC.   Also, when I work on a 922, I 
take aviation snips and cut off the HV-shorting interlock spring.  I 
use the remaining part of the spring as a ground solder-lug for adding 
another 2000 to 3000 pF of +HV bypass C to keep 160m RF from eating up 
the un-RF-bypassed +110vdc PS electrolytic filter C.  Oddly enough, 
Trio-Kenwood RF-bypassed the HV electrolytics, but neglected to protect 
the LV PS electrolytic.
-  Another precaution worth taking with a 922 is to cut the wire that 
puts +110V on the fil-CT (cathode-connection) during Rx.  Thus, if a 
tube grid-fil shorts, T2 will not have a meltdown.  note -- if such a 
short occurs, there will be 200 - 300 mA of Ip on Rx.

> The first time I heard it, I
> figured it was the spark gap on the T/R relay, but after thinking 
> about it
> that doesn't make sense since I wasn't transmitting and the amp was in
> standby.

Good thinking.  If Trio-Kenwood had tried connecting a 
spectrum-analyzer to a 922, they might have discovered something that 
at first seems to be impossible -- i. e., RF at the anodes with zero RF 
drive - and yet no RF at the output!

good luck
> 73,
> Clay
>
> > Hello, Clay -- An internal tube short makes very little sound.  A loud
> > pop is in the atmosphere, not a vacuum
> >
> >
> > Does the 922 blow fuses with the tubes removed?
> > >
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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