Folks,
Some time back we had failures of 572Bs in the Yaesu 2100 amplifier we used
at the club station. First indication was excessive plate current and red
hot graphite plate in one of the 572Bs. Even in Standby. The FL2100B has no
grid current meter. Removal of one of the tubes showed maybe a 1/4"
diameter hole in the grid, visible just up from the bottom of the tube, just
inside the bottom of the plate. The grid wires involved were about 4 or 5,
and had a hole melted almost symetrically midway between the supports. The
grid wires that vaporized left small balls of molten wire at the tip of each
wire at the perimeter of the hole.
Also, the carbon resistor connected to the grid ( I think about 33 ohms) was
cracked, apparently due to heat.
I read about a problem with this amp, of oscillations caused by marginal
bias voltage, combined with newer 572Bs which may have had some extra gain.
I now wonder if a different event happened. The operator of the amp tuned
plate and load for absolute maximum output, without adding a small amount of
loading, watching for a very slight decrease in output power, as oft
recommended.
I notice that when the Drake L4B is tuned only for max output without the
slight extra loading applied that the grid current sometimes soars, and
comes right back down when loading control is advanced slightly from max
output.
I wonder if the FL2100B problem was simply excessive grid current, (which we
cannot monitor without the grid current meter) due to tuning for max power
without a little extra loading?
BTW, anybody have an FL 2100B basket case with the small PC bord containing
the biasing diodes? We need that one to put ours back together.
Any ideas?
Pat aa6eg@hotmail.com
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