Gentlemen,
During the last weeks, we have got some fine and nonsense statements. Let me
tell you my personal experience in building a PA 2 3-400z, c. 20 years ago.
When I switched the linear on first time - bang ! Like a gun shot, tuning
cap arced, bandswitch arced after several times.
Then I put more units R in parallel, more damping - bang again.
BTW the bang came in only during keying never in cut off with HV on. And
a scope from Tektronix (old 454?) showed that a bang comes 100-200ms behind
the leading edge of the rf.
I considered it must be a gassi tube, I took out a new Eimac of the yellow
box heating the filament for a couple of hours - bang (150 Ohms in B+ 3.2 kV).
I made several modifications now in the suppressor design until oscillation
stops.
The PA was completely stable with all combinations of tuning detuning open
output open input with both tubes (during bang series I lossed one tube, grid
filament short).
I closed the cabinet for about 2-3 years made some contest and figured up
the confirmed in DXCC.
Later on I want to increase the tank efficency, more output/less heat.
Decrease loaded Q - less C and more L on 10 is the clue. The main reason
for doeing that, after 3 minutes of full out (measure IM distortion) a tap
was desoldering.
At first I changed the air cap to a vac and second I put the old suppressors
in because the strips are smaller calculated they have less area, meaning
less c to the top cover, no tank L change to this time. Key down - bang.
So, I turned in the old suppressors, the bang stopped again and I modified
the 10m coil.
Today, completely stable, all components look like new and I am satisfied
with this black painted box until today. Output looks like 'brand new'.
Each of the 4 suppressors have 3 pieces 33 Ohms in parallel, short leadings,
simple Carbon, no blowing no changing today. The 'bang' traps have 2
and the 'quiet' traps have 5 windings at small diameter.
Without personal attacks every discussion statements rebuttals are fine.
Let me say, the AMP is a good arena for combattants.
NiChrom is not the only way, but may can help in a effective manner
- nothing else.
Best wishes, Uwe, DL9NC
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