Right--I should have included the envelope Carl and I recall you
mentioning in some other thread that the Eimac glass envelopes were
thicker than the Chinese so I assume they would hold up better but
either way I would not want to push the envelope (groan) hi hi
73
Rob
K5UJ
On Mon, Aug 16, 2010 at 5:09 PM, Carl <km1h@jeremy.mv.com> wrote:
> 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@gmail.com>
> To: <amps@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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>
>
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