[Amps] Tube Cooling

[Robert Briggs]

I am experimenting with a new 2 x GI-7B amplifier using muffin fan types 
to establish whether I can get reasonable air flow...Test 1 was using a 
pair of 240 volt 3.5 inch fans mounted beneath the tubes blowing over 
the cathodes exiting through chimneys and out the top of the 
amplifier...Result is pretty dismal..Muffin fans don't like restricted 
air flow...

Experiment 2 was mounting a pair of the fans above the tubes sucking the 
air through the coolers...At first I thought the flow wasn't much better 
but discovered that the exiting air is thrown radially rather than  
straight upwards...Of course as there is no concentration of the output 
upwards one can only guess how effective the flow is...

Tomorrow, I am going to add a pair of 12 volt 3 inch muffin fans under 
the base plate cathode compartment and try a push pull effect with the 
240 volt fans on top..If this improves the flow I will then replace the 
two top 240 volt fans with a more powerful single 5 inch muffin 
fan...The fans certainly run quieter than any blower I have 
tried....Suitable blowers are virtually unobtainable in VK and have to 
be sourced overseas which with shipping make them very expensive..

I figure that in service running 700 watts ssb output, dissipation would 
not be greater than 400 watts of heat from 2 tubes all up but this is 
just a generalisation on my part...I don't have a means to actually 
measure anode heat..

Running data modes for meteor scatter etc would probably strain the 
friendship past 300 watts output in 30 second bursts but worth looking at..

I would be interested in other opinions..

Bob..VK3ZL..