Hello all..
One month ago I bought a used Alpha 91B amplifier, but after some days i
realized that something was wrong.
I was getting very poor rapports on SSB.
I was told that it sounds a bit like "RF feedback", and it sounds like the gain
was stepping on a strange way in the Amplifier.
Almost like a "click" on top of the modulation.
It is diffecult for me to decribe, but my English is not good enough to explain
it in another way...Sorry.
My own idea about this issue, is some kind of defect or bad alignment at the
"Dynamic bias" ESB board.
I believe if this circuit were defective, I would get problems like described.
I have been measured some DC levels on the amplifier;
G2 is +360V without PTT and also when I PTT.
G1 is -127V without PTT and -82V when I PTT.
When I measure the voltage at the 2 cathode resistor's I got this result:
Across resistor 1 (13 Ohm measured) I have 0,0V at standby, 0.055V at PTT and
3.6V @ 1000W.
Across resistor 2 (12 Ohm measured) I have 0,0V at standby, 0.430V at PTT and
4.1V @ 1000W
The current in tube 1 is then, 0mA, 4.2mA and 277mA.
The current in tube 2 is then 0mA, 27mA and 341mA.
Some difference in the bias, but I don't think this is my "bad sounding"
problem, or.....?
Now I could use any good information about this amplifier.
I am not very familiar with tubes, and I would be grateful for any information.
I was thinking of trying to disable the "Dynamic bias" ESB system, so this
could show me if the system was ok without this "power down" in speech pauses.
The ESB is, as I understand it, changing the G1 voltage from -82 to about -55
when the amplifier sense some RF.
Does anyone know how to disable this ESB ?
Or maybe I could just align the -82V down to -55V,and then I should have -55 in
both "step".
I am not living in a tropical place (Far from it), so I think it would be safe
to do this.
Maybe if I could get some kind of "alignment manual", or another "tune up"
instructions on this issue.
Can anybody advise me what to try and do from here?
I would be grateful for any tips and tricks.
Btw I think the tubes are "very used" in this amplifier, but I have fine gain
and effectivity is also ok.
Any mods. for this amplifier would also be very interested to have.
Regards,
Thomas OZ1JTE.
www.oz1jte.dk
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