[Amps] SB-1000 160 m power down

[TexasRF at aol.com]

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 at 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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