Here's another example of the old wisdom that the road to Hell is paved with good intentions.
Old timers will recognize the rear panel of the venerable Heathkit SB220 or SB221 amplifier.
This trick probably started with a good intention like "This fan will push more air than that old one".
Just one problem. This fan only blows air above the chassis deck onto the tubes above the sockets.
The original fan has extra-large blades that require a slot cut in the chassis deck. The blades extend below the deck and push some breeze across the underside, and onto the bottom sides of the sockets.
And onto the pins of the tubes.
The fan in the pic won't. Just blows onto the tubes above the deck. This will tend to cause the pins on the tubes to get too hot. The heat can melt the solder inside the tube's pins, especially the two filament pins. The heat will oxidize the contact surfaces on the socket's spring contacts and cause even more heat from the poor connection this makes to the surface of the pins.
Becomes a downhill snowball until the filaments start going dark. By then the only cure is new sockets AND tubes.
Both.
At the SAME time.
So, if someone offers you a SB220/221 that looks like this in the rear, run don't walk the other way.
73
Old timers will recognize the rear panel of the venerable Heathkit SB220 or SB221 amplifier.
This trick probably started with a good intention like "This fan will push more air than that old one".
Just one problem. This fan only blows air above the chassis deck onto the tubes above the sockets.
The original fan has extra-large blades that require a slot cut in the chassis deck. The blades extend below the deck and push some breeze across the underside, and onto the bottom sides of the sockets.
And onto the pins of the tubes.
The fan in the pic won't. Just blows onto the tubes above the deck. This will tend to cause the pins on the tubes to get too hot. The heat can melt the solder inside the tube's pins, especially the two filament pins. The heat will oxidize the contact surfaces on the socket's spring contacts and cause even more heat from the poor connection this makes to the surface of the pins.
Becomes a downhill snowball until the filaments start going dark. By then the only cure is new sockets AND tubes.
Both.
At the SAME time.
So, if someone offers you a SB220/221 that looks like this in the rear, run don't walk the other way.
73
