[Amps] TL-922 Revisited.

[Richard]

>Hello all,
>
>I am looking for explanations for an occurrence with my
>TL-922. Let me state a couple of things up front, because
>this could easily become another endless thread about
>parasitics.
>
>1. This amp has had every known mod on the Rich
>    Measures site done to it already. Don, KI6SZ, did
>    the modifications. Thanks to Don and Rich for their
>    help/contributions.
>
>2. After the initial mods were done, I suffered another
>    grid short on one of the tubes. These tubes are the 
>    3-500ZG's from RF parts. After getting and re-installing
>    another set of tubes Every thing has been FB for about
>    four months.
>
>
>Well, maybe... Yesterday, I had tuned up as usual into
>the dummy load at about 14.240. So far so good. I answered
>a CQ at 14.310 without the amp, again so far, so good.
>Conditions were fairly bad, so I put the amp on line at
>about 500 watts. Again, OK. But, when I upped the
>power to full, about 1200 watts or so, I heard a POP!
>Followed by the sound of my voice coming from the 
>amp and accompanied by a bright white light.
>
>Not good... Upon examination, I found that the two 1-ohm
>resistors that are part of the new parasitic suppressor had burned.
>This  suppressor is formed from nichrome wire and forms a
>series-parallel network.
>
>I replaced the resistors, and re-installed the suppressor.
>Apart from an initial sput or zap sound from the amp, all
>seems normal. After the initial zap sound, I did remove and
>inspect the suppressor, suspecting that I might have burned
>another resistor, but it looks normal. I also installed another
>3-500ZG to check against the first. This was done to see if
>the initial tube had developed a fault. So far so good. I then
>re-installed tube number one and tuned the amp on 10,15 and
>20 meters. Power out, plate and grid current, HV, input SWR,
>all appear OK.
>
>So now to  the 64 dollar question: Was this just some kind of glitch, or
>was I visited by the dreaded VHF parasitic demon?
>
Resistance-wire parasitic suppressrs are not a magic cure-all.  They can 
reduce 120MHz-3-500Z gain to about half of what the stock supppressors 
yield - but only if they have enough L in the suppressor coil.   A 
suppressor which runs cool at 29MHz is not going to be very 
suppressive/lossy at VHF.  
-  The usual cause of open 1-ohm [Ra, anode fuse] resistors is an 
anode-circuit parasite at c. 120MHz.  I own a needs-fixin' TL-922 that 
does the same thing that yours did every 5 or so years.  The next time I 
repair it, I'm increasing the suppressor coil's [L-supp] inductance to c. 
120nH.  This reduces the vhf gain of the amplifier with the tradeoff of 
more 10m dissipation in R-supp.  I'm also going to install a 
cavity-damper loop to vhf-load the enclosure that contains the output 
section.  When/if I do this, I will publish the results.  
--  If you have not switched the cutoff bias on Rx from V-cutoff to 
R-cutoff, a shorted tube can quickly destroy the 922's [unfused] filament 
transformer.  The same thing applies to the SB-220.

>Should I just keep on operating until something else fails, or
>should I beat the rush and run screaming into the hills?

If the 10-ohm, 10w glitch R was installed in series with the +HV --  and 
the 30-ohm fuse resistors were installed at the grids in place of the 
orig. 1mH chokes, the tubes would be unlikely to short during an 
anomaly/glitch.  
>
good luck, Gary

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
end