[Amps] instability issue

[Fuqua, Bill L]

It does not have to be TGTP. 4CX tetrodes can have a negative plate resistance under the right 
conditions. I have experienced a couple of times where this has produced problems with smaller tetrodes. 
Once had a very unusual problem with a low frequency (Hz) with a solid state amplifier.
I was really scratching my head on that one until I realized what was happening. 
  The solid state single stage amplifier 300 Watt amplifier, copied from Motorola Application Note, 
appeared as a negative resistance on the power supply line. It had a fixed power gain due to the 
negative feedback and when the supply voltage increased, its efficiency improved thus reducing
the current demand. Thus, the power supply was seeing a negative resistance. The power supply 
used a inductive input filter. And the whole system oscillated at the resonant frequency of the 
LC in the filter. I damped the filter ckt a bit it stopped oscillating. 

73
Bill wa4lav

________________________________________
From: Amps [amps-bounces at contesting.com] on behalf of peter chadwick [g8on at fsmail.net]
Sent: Wednesday, March 27, 2013 10:56 AM
To: amps at contesting.com
Subject: Re: [Amps] instability issue

The classic cause of LF oscillation (LF parasitics) is a grid choke and an anode choke going off in a TPTG fashion because the neutralisation isn't right at the low frequency. If the plate choke has a bypass to ground and then another choke off to the HV, the bypass may be too small to be effective at LF. If we're talking grounded grid, the filament choke could be in the game, especially if the grid is not directly grounded.
An old trick is to use a small neon lamp on a well insulated stick. Run round by the plate circuit and a VHF parasitic will lead to the glow being very dependent on how far the bulb is from the tube: it also frequently has bluish tinge to the glow. An LF parasitic generally has a much darker red colour, and the brightness doesn't change so much as you run along the plate circuit RF components.
73
Peter G3RZP
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