[AMPS] 572B and an old FL2000B

[Tom Rauch]

> Yes it does, what it is actually about is not the bias per se, but the
> cut-off voltage, which has to be higher (more negative!) for the more
> lively Svetlana tube.

The power gain of the Svetlana 572 is not higher, the problem is the 
cut-off bias requirement is higher.

In amplifier that are not neutralized, this can cause the tubes to 
oscillate when the amplifier is on standby. The reason this happens 
is the input and output circuits are totally unloaded because the 
antenna relay transfers to bypass, yet the tube is still not in cutoff.

Despite claims in the past on this reflector, that problem was never 
addressed. The Svetlana tubes that were sold were all new old 
stock tubes, and could not be modified because they were not 
being produced any longer.  

 I do not recommend you to use the this tube, I went
> through two sets in fairly rapid succession, before I threw the towel in
> the ring. My FL2000B is now happily clicking along with a single

Ameritron had the same problem. They ramped up and build a few 
hundred amplfiers with 4 572B's, and filled the first 100 or so with 
Svetlana tubes. Out of those, about 60% of the tubes had 
excessively low anode voltage breakdown.

If you think your odds of having a problem were high using two 
tubes, imagine using four at a time!!

Fortunately some of the Chinese tubes are much better, although 
they all need tested before use.
73, Tom W8JI
W8JI@contesting.com 

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