GS-35b Air Cooling
by Bill Jones, K8CU
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Some Russian data sheets show the GS-35bís anode cooling requirement to be 2500 liters/minute. Other specification sources include information on the cooling requirements of the tubeís cathode and grid. By definition, one cubic foot equals 28.327 liters. Converting to cubic feet per minute (CFM), the anode cooling requirements are, then, about 88.3 CFM.

Refer to the airflow diagram to the right. In this amplifier, the under chassis area is pressurized. A number of screened airflow holes are placed in the grid compartment and main chassis mounting area. An inspection of the photograph for the grid compartment will show some of the eight one inch diameter screened inlet air paths. Two 2.5 inch diameter holes in the top chassis plate provides an air path towards the bottom of the tube radiator. Providing a large airflow surface as possible will reduce the back pressure requirements of the external air blower fan. Passing all of the airflow directly through the under-chassis shielded grid compartment will easily satisfy the cathode and grid cooling needs. Some light duty requirements will allow a smaller blower to be used. The only certain way to know the tube is being adequately cooled is to measure the tube seal temperature to insure compliance with the manufacturers maximum specification.

It is very important to not ignore the smaller cathode and grid airflow cooling requirements. The table below shows GS-35b element cooling requirements in various units of measure:

GS-35b Cooling requirements
Volumetric Units Anode Cathode Grid
   ft3 per Min (CFM) 88.3 1.8 10.6
   m3 per hr 150 3 18
   liters per min 2500 50 300

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