[Amps] Tube cooling

[Carl]

I dont believe glass tube pins will ever exceed 200C under any condition as 
long as they have the required air flow.

The plate seal is critical as well as the envelope temperature, there have 
been many tubes where the envelope side itself  became molten and sucked in 
during a runaway condition.

While air wont do anything to improve an anodes dissipation it will help the 
envelopes ability to maintain integrity with the result the tube will be 
able to exceed its non air cooled ratings. The 572B and 4-250A are a few 
that fall into that category. There is a pratical limit to how far you can 
stretch the limit. Ive run a Cetron 572B at 700W output with a Coleman 
lantern chimney and pressurized air flowing from underneath and no short 
term harm. That was just a test to see what would happen back when they were 
still in production and cheap.

Carl
KM1H


----- Original Message ----- 
From: "Rob Atkinson" <ranchorobbo at gmail.com>
To: <amps at contesting.com>
Sent: Monday, August 16, 2010 4:27 PM
Subject: [Amps] Tube cooling


> The dissipation spec for a tube is what it is.  cooling doesn't change
> it.  Cooling matters for the pin seals.
>
> 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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