Hi Rich, Phil, Peter et al
The QY5-500 is a bit of a turkey of a valve -
it's described as a 'radiation-cooled tetrode'
- needs about 5kV on the anode to deliver
any real power, and it's a class-C valve really.
Still it works fairly well in this linear which was
my first-ever HF amp., built in 1978.
The QY5-500 is about 7" tall by 6" diameter,
and I'm cooling it with 2 x 120mm square fans,
the lower one mounted so that half of it is
cooling the cathode compartment + valve pins
etc. The 2 fans are only about 3" from the
side of the valve, and give a strong flow of air.
The only reason I discovered the difference in
temp. from one side to the other was during testing
I removed the valve from its socket about 3 minutes
after running at full power, and the glass was still
very hot on one side, and cool on the other.
No temperature measurements were made, just
holding the valve.
So - this is clearly a case of badly-designed cooling,
and the fact that the air is moving fast is probably
adding to the difference in cooling, not helping.
Also, I have not painted the adjacent metalwork
black, so a lot of the I.R. is reflected around the
anode circuitry.
You can see a photo (outside only) of this amp.
and others at www.qsl.net/gm3woj/
73
Chris GM3WOJ
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