On Aug 28, 2004, at 5:46 AM, Alek Petkovic wrote:
A friend of mine Has recently bought a FL-2100Z and it is fitted with
Svetlana 572B tubes.
He has had a couple of bangs so far on 20m resulting in the loss of
the 22 Ohm parasitic suppressor resistors.
This means that the tubes do not have low gain. Unless the filament
broke off during the osc., I would use the tubes.
Also, a previous owner has put 33 Ohm wire wound resistors in the grid
circuit. The originals were of course non inductive resistors which
are very hard to source here in Australia.
I am sending him some new 33 Ohm resistors and some nichrome wire to
make up some new suppressors plus a glitch resistor.
The new grid resistors should be no more than 0.5w, and the total grid
bypass C should be c. 1500pF to protect the grid fuse resistors from
10m RF.
I am also sending him some new Cetron tubes and have advised him to
get rid of the Svetlanas and put some extra resistance in the filament
line to reduce the voltage to a safer value.
Has anybody had any experience with Svetlana 572B tubes? I was under
the impression that they spelled trouble in hf amplifiers. Is this a
fact?
Alek -- Rumour has it that they have more gain than the old RCA or
Cetron 572Bs, so naturally they are a tad harder to tame.
...
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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