Does anyone have a procedure for neutralizing the FL 2100? My amp needs
the tubes replaced and I was thinking of using the Svetlana 572B's. I
read that they recommend the mod even if your using the same brand of
tube that it came with. I do not want a band aid fix.
Thanks
Bert KD7JEH
>a while back someone said that there are problems with svetlana
572b
> tubes??
>
> hank, n6buu
The Svetlana 572 tubes have two problems. They require more bias
to cut the tube off. In some amplifiers with marginal cutoff bias (the
FL2100) the tubes do not cut off.
This causes a problem because the anode and input of the tube
are unloaded when the relay transfers to the receive mode. The
poor anode to filament shielding in the 572 creates a feedback
path, and the amplifier will oscillate on higher bands while on
standby.
The "band-aid" fix is to increase cutoff bias in the original circuit by
changing to a voltage doubler in the bias. A better fix is to open the
cathode return path and let the tubes self-bias into cutoff. The
ultimate fix would be to neutralize the tubes, and open the cathode
path.
The second problem relates to anode voltage breakdown. In a
traditional class AB or B PA the peak anode voltage when the PA
is properly loaded and driven with correct drive for that loading is
just under two times the anode voltage.
In abnormal operation....like when the tube is being momentarily
overdriven by a transient from the exciter, the antenna presents a
mismatch of the right angle at the antenna port due to an arc or
other fault, or the antenna is not transferred to the amplifier the
peak anode voltage can be several times the normal operating
voltage. Generally a switch or capacitor will arc and limit the
voltage, but if the tube has inadequate hold-off it can flash over
internally.
About 2/3 of the new Svetlana 572B's failed a dc breakdown test of
5000-6000 volts. Some cleaned up after running, but then a large
percentage of them failed in the field. Almost none of the tubes
handled 8 kV. 5000 volts would be typical in a properly loaded
amplifier running the 572 within ratings. IMO, the tube should have
headroom of about 50% over the maximum expected peak voltage
in normal operation. For example, good 3-500Z's will take about 3
times the rated dc operating voltage in a dc test, which is about
50% more than the typical peak voltage in normal operation.
The 572 will always be a good sub for an 811A because they have
enough headroom for 1500 volts or so dc anode voltage. Any
vendor should work.
Some Chinese tubes are good, some aren't, depending on the
manufacturer in China and how the tubes are tested. You have to
trust the retailer to know the difference, and select the reliable
sources, since they can be sent out in whatever name the
customer wants printed on the tube.
73, Tom W8JI
w8ji@contesting.com
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