[Amps] Neutralizing Dentrons

[Tom W8JI]

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