The 3cx800 requires 19 CFM @ 25C per tube at sea level to have 800 watt rating. At this rating the data shows .5" dP (Pressure drop) is developed - this is for a pressured chamber below the tube socket.
Yes it would be best to supply a blower and pressurize the lower chassis.
However one design limitation is to keep the MLA2800 compact enough to still fit
in a carry on bag that will fit under an airline seat. I did not want to use the
stock Dentron muffin fan as I could not verify it's ratings. I picked a Comair /
Rotron MX2B3 (or MX2A3).
Next note the fan curve for this model and 50 CFM at 0.2" dP
Now note the fan curve for typical muffin fan - at 0.2" dP it's around 28CFM.
The design of the MLA2800 pressurizes the Anode chamber - not the case below the socket. This reduces the pressure requirement. The 3cx800 requires 11 CFM each for 600 watt rating - producing a 0.2" dP This is with the pressured case below the socket. With the MX2B3 we are operating above the 600 watt rating but perhaps not at the full 800 watt rating. It is important to remove any resistors in the fan circuit that may exsist in the original MLA2500. It might be possible to run the fan at a slower RPM during receive to help with noise but I have not operated it that way. Some DC muffins produce more air flow (study the fan curves) and could be used if a DC supply was so installed in the MLA2800.
Note the 50Hz curve of the MX2B3 is OK at 600 watts per tube should you travel to such locations. It is also possible to stack the fans for more cooling which would still fit in the carry on bag. copyright 2003