[Amps] SB220

[ka1xo at juno.com]

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