Patrick enscribed:
Hi All,

I'm about to build a 4CX1500B amplifier for 2m.

Most, if not all, descriptions are blowing air into the enclosure on the
base side of the tube and the air goes through the socket and finaly
through the anode chimney.
For several reasons i would like to blow into the anode enclosure
directly onto the anode, the air would then go out through the anode
chimney. Only a small part of the air would cool the base of the tube
through small holes in the anode/cathode separation.

Is that a stupid idea ? Is there any reason not doing so ?

73, Patrick

No, not a stupid idea at all. That's the way I do it on my current
4CX800 2m amp, and I have used that method on several 4cx250 class amps
in previous years. I think its was popularized, if not initiated, by
K2RIW some decades ago in his classic 432 amp.
Three things to watch for:
1) make sure there is a good seal from the outer part of the plate
resonator to the outside world; I use 25mil teflon sheet as a chimney
with clamps made from brass strip.
2) ensure there is an exit in the grid compartment so that the air can
flow through adequately.
3) put some mesh on the blower inlet to the plate cavity, otherwise the
resonance will be affected by the fan rotation. Gives a subtle, but
noticeable AM buzz.

Good luck with the project.

73 Alf NU8I (aka G4ABB,ON8KL,VK6GA,PZ5AG, etc)
Scottsdale AZ DM43an