[Amps] FL2100 572B Failure Mode

[2]

>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. 

**  A melted hole seems unlikely to have been caused by too much grid 
current.  My  wild and crazy guess is that the filament may have touched 
the grid where the hole is burned,

>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.

**  Burned usually =s overload.  Cracked usually indicates damage from a 
glitch..
>
>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.
>
** Newer 572B reportedly do have more gain than those produced 20 years 
ago.  However, bias V seems unlikely to affect VHF gain.

>...

-  R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K, 
www.vcnet.com/measures.  
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