The most difficult to stabilize tubes are generally those
with the longest and thinnest element leads. The 811/572
family falls into this category.
If you look at older amps like the Heath Warrior and Gonset
(?not sure of number?) 811 amps they neutralized the PA by a
third winding in the filament choke. You might get some
ideas from them. Since Ameritron already had fixturing for
winding the filament chokes, and since a dedicated inversion
transformer worked better, the Ameritron amps used that
method of providing phase inversion.
Other things to watch for in Dentrons are low voltage rated
resistors across diodes and 2W 100k carbon resistors across
electrolytics. Carbon resistors are notorious for shifting
value with prolonged heat and often tend to go open from
moisture that corrodes internal connections from metal leads
to the carbon element. The worse thing is they often cycle
downwards in resistance and that can cause thermal runaway
as the increased dissipation pushes resistance lower.
Carbons should never be used for critical applications like
HV dropping resistors or equalizing resistors. Sylvania set
more than a few houses on fire by using a few 2W carbon
resistors to drop B+ down to supply semiconductors in the
color circuits.
73 Tom
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