4-400b tube in SB220

[nomadradio]

The SB220 has a sneaky way to push a breeze across the undersides of the sockets.

The tips of the fan blade extend below the chassis deck. The portion of the blades that extend below deck level blow some air beneath the sockets. This modest airflow is sufficient for the base of a 3-500. But the metal base ring on the 400 will partly block it.

Probably okay for intermittent duty. Would never be sufficient for continuous duty, but neither is the rest of the amplifier.

73

 

[Shadetree Mechanic]

The 4-400 should have cooling air coming up through the socket, otherwise the pin seals will overheat. I'm not familiar with the SB-200 in that regard.

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