[Amps] once again: plate current rising problem...

[corneliuspaul@gmx.net]

hello again,

as I had only one answer (TNX N7RT!!) to my question, 
I would like to ask for a hint once more:

what is your guess, is the described problem caused 
by something in the circuit responsible for the tube bias
or could it also have other causes?

I will swap the 7.5V zener diode responsible for the bias
stabilization with 10 standard diodes but if thats not the 
faulty part then I dont know where else to look... 
(and I hope its not the tube itself...!)

73s Con DF4SA



--------------------------former post------------------------------------------
Dear Group,

I am trying to repair my SB-1000 which died 3 years ago when 
was transmitting on 7mhz while the antenna broke down. 

(my earlier post asking for a spare part for the bandswitch 
is for the same reason)

The amp is equipped with the QSK board option. One transistor 
on this board had died: during transmit it switches the zener diode 
to ground which is connected to the center tap of the filament 
transformer. This zener diode sets the operating bias and 
carries the idle plate current (around 80mA)
The transistor had become a short, so as soon as I switched 
the amp from "standby" to "operate", the idle current started to flow.
I put in a new transistor and that problem is solved. I carefully 
tested the amp and the output power easily reached at least 500W.

But there still seems to be a problem:
When I transmit, the plate current starts to rise slowly after a 
few seconds. A few seconds later the rising becomes faster. 
I stopped transmitting because I dont want to kill
any more parts in the amp... The process is repeatable after a 
few seconds. I have even noticed that the current starts to rise 
when I only let the plate idle current run for a while. 
The effect is just much more pronounced and faster when transmitting.

Can anybody help me, what would be the reason for this problem?

Many thanks,
73 Con DF4SA