[Amps] SB-1000 160 m power down

[Han Higasa]

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