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@contesting.com] on behalf of peter chadwick
[g8on@fsmail.net]
Sent: Wednesday, March 27, 2013 10:56 AM
To: amps@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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