Dear Morris,
The SB-220 should be modified so that it limits the inrush
current to the filaments. I used a switch and resistor in the filamanet XFMR
primary that put the resistor in series and then bypassed it
when the filaments lit up.
The SB-220 needs to be parasitic-proofed with nichrome-and-resistor
plate chokes just near the radiant plate caps.
As I built my '220 when they were in vogue, the filter section was equipped
with 470K resistors and .01mF caps around the diode, and
1.2M resistors around the series-wired filter caps to equalize the
charge, but I think technology offers a better solution nowadays.
There was also a HV standby switch added, too, if I recall correctly, to
isolate the tubes when I wanted to leave the unit inoperable.
However, there's a website and there's a Mr. Measures around this
forum who has studied much of the mods on '220's and may offer
good ideas.
Best of luck: It's a great amp.
Hal
KA1XO/4
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