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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