[Amps] Parasitics & Filament Sag

[Robert B. Bonner]

I have only had one 3-500Z amplifier take off on me when I was purposely
testing PC suppressors back in the early 1970's.  I was attempting to
improve 15 and 10 meter performance. 

For some stupid reason it had become fashionable to lift the grid leads off
the chassis providing a feedback circuit in a faint attempt to improve
linearity in the Drake L4's and this carried into the L4B's amplifiers.
Since the Heath SB-220 was a poor man's copy it also included the resistors
and bypass cap on the grid leads to ground.  This actually caused more
problems than it cured.

During my tests the amp would self oscillate in the low VHF range, probably
around 75 MHz at about 3000 Watts input.  It was pretty obvious as the plate
and grid meters were PINNED HARD and the tubes were busy trying to melt
everything inside (grids, filaments, plates).  The only cure was switching
off the HV power supply as the tuning was very broad once she started
running on her own.

I found by eliminating these extra components and concerning myself with
input and output isolation the amplifiers actually became more stabile.  I
could actually run WITHOUT PC suppressors just as Tom did with his Ameritron
designs.

Bob K0DD