[Yaesu] FL2100 amp problem

[Darren Collins G0TSM]

Greetings fellow DXers.
I have an FL2100 amp and I`m having some sort of bad oscilation
problem. (this is the original 2100 not the `b` version)

The amp uses a pair of 572B`s tubes but one is a Penta and the
other is a Centron. I have previously installed the Yaesu mod
to increase the bias voltage to 35V.

The amp used to loaf along at 400W on 20+40M but one
day I decided to tune her up on 10M. All seemed fine running
about 300W for 35W drive but only after a few couple of CQs 
the plate IC meter pegged off the scale, the plate voltage 
dropped to 1.5KV due to the load and before I could hit the off 
switch the 13A fuse in the shack`s extention lead blew.
I found that the 22ohm resistors across the anti-parasitic coils
on the anodes had fallen off due to extreme overheating but at
the time the amp was cool and didn`t even smell hot (I have 2x
90mm fans mounted on the top of the amp).

I replaced the anode resistors and checked *every* componant
in the amp, bias, psu, alc etc.
At switch on all is well with 2.4KV plate voltage but if the
amp is keyed, using the rig or a screwdriver or a resistor the 
plate IC meter pegs off the scale, and the two tubes have a 
bluey glow in them.
If this happens the +35V bias voltage rises to about 80-100V
but I assume this voltage is being generated from the tubes
and not the bias/relay circuit.
If I keep `flicking` the TX occaisionally I will see the bias
current on the meter of 0.03Amps and the tubes look ok but If
I try all four combinations of using only one tube I still
get the current problem so I presume either boths tubes have gone
faulty or there is still a fault in the bias/alc/relay circuit
which I have missed.

I know it`s easy to say try another set of valves but I`d rather
try and save my £112. I believe another alternative is to use
811A`s which cost about £40 for the pair and I`ve been told these
just drop straight in but obviously have lower power output.
Or, is there an easy way of checking a 572B valve?

I`d appreciate any advice from you tube guru`s..

73 Darren G0TSM

g0tsm@qsl.net  www.qsl.net/g0tsm/

(message also sent to UK packetclusters)


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