On 8/16/2010 4:27 PM, Rob Atkinson wrote:
> The dissipation spec for a tube is what it is. cooling doesn't change
> it. Cooling matters for the pin seals.
That's not quite true for external anode tubes that require a specific
air flow for a given dissipation. Replace the anode radiator with water
cooling and it's even higher...with some limitations such as cathode
emission. IOW if the tube goes into saturation no amount of extra
cooling is going to give the ability to run more power although it will
make the tube run cooler.But if it has the emission capability you can
increase the plate dissipation until you reach saturation, even if you
have to resort to water. Ultimate dissipation also depends on the tube
design and whether it uses thoriated tungsten or indirectly heated,
oxide coated cathodes.
Often the emission capability of the cathode is as much a limiting
factor as the plate conditions.
However if you take a tube like the YC156/3CX5000A7 is really not a true
3CX5000A7. It us a 3CX15000A in a 3CX5000A package in that tube the
ultimate power is quite dependent on the amount of cooling.
73
Roger (K8RI)
> The mfr. spec. for 3-500 pins is a bit above 200 degrees C. (I can't
> recall the exact temperature but I know the plate and bottom pins are
> over 200 C.) That's pretty high for what hams normally do and one
> reason why Ten Tec gets away with 80 CFM fans in the Centurion amps
> for what is marketed as a SSB CW amp. They do de-rate it for RTTY
> for 10 minutes at 600 watts. Yes it is true that they have a low
> volume fan in there because it is quiet and there are so many idiots
> who piss and moan if an amp is too loud--just read on-line Centurion
> reviews where all these guys are raving about how quiet the Centurion
> is--that they and other manufacturers are driven to this if they want
> to make a product that hams will buy.
>
> But there are other factors to consider too with that amp. The stock
> h.v. p.s. uses a 500 ma. CCS transformer and the fan is positioned to
> draw air in over it and blow it out through the RF deck. The current
> spec. for the plate transformer is part of the reason for the 600 W.
> continuous RTTY spec. I believe. As I recall if you do the math, 600
> w. is right around 1/2 amp on the p.s. So you also help it when you
> increase air volume. OTOH, the filament choke sits directly under
> the tube socket deck right in front of the lower arc of the fan,
> blocking a lot of air flow to the pins. Not good in my opinion, but
> it is hard to do anything about it without increasing the length of
> the leads from the choke to the cathode pins.
>
> The air moves over the envelopes but it also moves over the anode heat
> sinks and plate choke. It is a pretty good cabinet cooling design
> except that as usual with commercially made amps, the cabinet is a bit
> too small and the fan diameter could be more if you ask me. Of
> course it is not as good as having the tubes in chimneys in a large
> rack with a bigger 1 A plate transformer.
>
> To the origninal questioner, I operate a Centurion on AM with 250 to
> 300 w. for lengthy transmissions (10 minutes or more) until I get a
> plate modulated AM rig. I installed a 180 CFM dc fan and also opened
> up the cover by cutting out the vents and covering the rectangular
> holes with expansion steel. When the air is roaring through the amp
> (and at 180 CFM it is pretty loud) you need to open up the cabinet so
> it can freely move in and out. On a hot summer day the air coming out
> of the amp hits 115 deg. F. in an 80 degree shack. Not too bad.
> (MIne also has a Dahl h.v. transformer.). The graphite plates get
> orange. You are probably okay with your new fan but I'd start out at
> 800 w. and keep an eye on things. Observe your plate current demand
> and keep in mind your CCS spec. on your h.v. supply is your other
> limiting factor and may be more critical than your tubes--I don't know
> what kind of stock B+ tran they use now.
>
> 73
>
> Rob
> K5UJ
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