Fan cfm

[Jan Ditzian]

OK, while I am plotting modifications to the SB-201, I am also thinking 
about the fan.  I have already disconnected the noise generator that is 
built into the bottom of the chassis, and I am using an external muffin 
fan (junk box, 117VAC, circa 25 years old) on top of the rig to cool the 
tubes.  The amp is nearly dead silent.  However, in the spirit of not 
laving things well enough alone, I am considering installing a fan into 
the case.

I measured, and there is barely 25 mm clearance above the Chinese 572B 
and the top cover.  This means that a 25 mm depth fan might touch the 
glass envelopes, which is probably not a good plan.  I do not know the 
required cfm for these tubes, so perhaps someone can guide me here. 
There are smaller fans (e.g., 40x40x20 mm) that run on 12VDC and would 
provide 8 cfm.  My original plan was to use a big 40x40x25 117 VAC fan 
and get 81 cfm.

If someone has gone through this exercise and concluded that you really 
cannot do it safely inside on top, please let me know.  I do not think 
that the pusher approach works as well, and I would be willing to 
compromise and put a permanent outboard 117VAC fan on top of the the top 
cover.  I figure that it can be screwed through the holes with really 
skinny screws.

I expect to power the fan from 117 VAC in the chassis.  I am running on 
220 VAC (249 actually), so I could also use a 220 VAC fan and bypass all 
internal power.

The top internal plate was removed long ago and far away.

73,
Jan, KX2A

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