Hi Han, it seems something in the output circuit is changing value. The
most likely part is in the plate loading circuit as that is where maximum rf
current flows.
Many amplifiers switch in extra capacitance on 160m and the quality of the
capacitor(s) is critical. Many ceramic capacitors have a dielectric
material that cause a change in capacitance as the temperature changes. Any
loss
in the capacitor causes heating that is related to current squared times the
loss. Using several smaller capacitors in parallel is often more stable
than a single large value capacitor. Smaller capacitors seem to have less
dielectric loading and lower losses.
I have seen 500 and 1000pF door knob style capacitors with these drift
problems. On the other hand, I have seen no 100pf capacitors with drift
problems.
I have not seen drift problems caused by a bypass capacitor; perhaps others
have. They are removed from the higher rf currents because of the plate rf
choke presence.
I would be very suspicious of the 500 pF capacitor you mentioned.
73,
Gerald K5GW
In a message dated 12/16/2011 11:35:15 A.M. Central Standard Time,
higasa@plum.ocn.ne.jp writes:
hi gents,
Recently I was asked to repair a Heathkit SB-1000 amp
from a local friend. At first the Eimac 3-500Z installed was
arcing pink like stabilo instead of orange.
For many hours I have baked the tube with Ep of 1600V
with 2k-ohm 100 W resistor in series of the plate as to
W8JI's page and now it sustains up to 300 mA of Ip without
the ark. Also, I had to exchange a faulty bias zener diode
to a small zener diode + NPN boost transistor to suppress
extra zero-signal Ip - it worked nice.
On the other hand I have a Chinese graphite plate 3-500Z
and it works nice on 80 m-10 m band with Ip <= 420 mA,
Ig<150 mA, 60 W drive and nearly 850-900 W out.
The problem is the power-down of its 160m band.
Seemingly it has 800 W out but within seconds the power
goes down to 400 W or less when key-down.
No burning or heating, or degradation of input SWR can be seen.
I have replaced three adding capacitors of its output
tank circuit - two 170 pF and one 500 pF but seemingly
no improvement obtained.
Or, are the capacitance of bypass- or coupling capacitors
insufficient on 1.8 MHz band?
Anyone please tell me the possible cause and suitable
solution of this trouble?
de Han JE1BMJ
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