[Amps] AL-80B Trouble

[Tom W8JI]

what was happening.  He most emphatically admonished me to 
write this
guy in California for the nichrome supressor kit or I'd lose 
another
tube in short order.  I'd never even heard of nichrome, but 
I ordered
it.
   From the moment I put that kit in the amplifier it has 
not uttered an
un-invited sound and is still with that 16 year old tube.>>

In the midst of all that nichrome work you also did 
something that fixed the actual problem with that amp.

It's like selling a kit for tire repairs where the lug nuts 
are changed along with sweeping the nails out of the 
driveway. It's easy to think magical lug nuts cured the 
problem.

 about running high power. (no neighbors to QRM)  So I hand 
carried it
to a local ham repair shop.   He checked the switch again, 
cleaned and
re-soldered it, fired it up,  "Pfffttt".   He sniffed around 
and sure
enough found a strong 110.4 mhz oscillation around the tank 
circuit
when I was transmitting on 80 meters. (field seems strongest 
around 15
meter pi-section)  He pulled one of Rich's kits out of his 
stock on
hand, put it in, and the amp has never arced since.  He only 
cautioned
me not to use it on 10 meters... or run it at reduced power 
there.  The
suppressor also caused another anomaly to disappear.  Often 
when I'd
tune the amp on 80 meters the reflected power would jump 
slightly as I
tuned back and forth close to resonance.  Doesn't do that 
anymore. >>

I'd like to have seen that amplifier Dennis to see what 
actually caused the problem. It's well known the highest HF 
voltage appears on the tap to the bandswitch for the 160 
padder caps, and that that is a weak spot in the switch 
because of that voltage. There also was a run of switches 
that left out a critical connection that clamped unused 
portions of the tank coil to ground.

How did the guy measure the 110MHz and how did he measure 
the field intensitry?

What else was changed besides the nichrome?

The fact is nichrome doesn't do much at upper VHF. N7WS 
measured this, and circuit theory confirms it. The primary 
effect of nichrome is it induces the largest change in Q at 
the lowest frequency, and that change diminishes as the 
frequency is increased.  This is why nichrome was popular in 
breadboarded amplifiers using tubes with very long leads and 
poor ground connections to the ground buss. If you wanted to 
stabilize an amplifier at HF, adding nichrome could be a 
workable solution.

73 Tom