The best thing you can do for the TL-922 is get rid of the series
connected filaments by replacing the filament choke and
transformer.
Even though the series-connected filaments have a "voltage
equalizing" connection that connection has enough resistance to
allow one filament to have more voltage and dissipate more power
during turn on.
A second problem with the series filaments is if tubes become
unbalanced in dynamic characteristics the dc anode current return
path can superimpose a little hum on the signal.
Series connected filaments are bad enough in Christmas lights,
they are dumb in a PA with 150 dollar tubes..
Lacking that mod, a slow start for the filament is a good cure,
because the voltage and current will rise slow enough that the
filament start-up dissipation in each tube stays reasonably close..
73, Tom W8JI
w8ji@contesting.com
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