On 12/14/2011 12:38 AM, BJ VOUTE wrote:
Just my opinion:
First: What do we cool on the tubes with forced air?
On glass tubes which are cooled primarily by radiation, we are mainly
cooling the seals.
With external anode tubes we are cooling both the anode and the seals.
On both types those seals get *hot*.
What about warm up?
With warm up we are concerned about thermal shock to the seals, inrush
current, and the cathode reaching temperature for proper emission .
On many indirectly heated cathodes (most of the type used by hams) the
inrush is very small but it takes time for the cathode to come up to
temperature for proper emission.
Leads on these tubes are typically short, small, and have little mass so
the pin seals come up to temp in a hurry. The larger seals only come up
to temp during operation.
It typically takes about 3 minutes for the cathode to come up to temp
and stabilize. Few of these tubes have an inrush current large enough
to require a step, or soft start ... or worry about. That is not
ignoring on/off cycles. Few of these have an inrush % more than a tiny
fraction of incandescent light bulbs or tubes with directly heated
cathodes. The venerable big, heavy, and very popular YC156 takes 6 to 8
minutes for the cathode to come up to temp
Then there are the "instant on" tubes such as the 3-500, 4-400, 4-1000
and the larger tubes that are becoming popular with homebuilders such as
the 3CX3000, 4CX3000 and up. These larger tubes are in a different
class than the former. We are looking at filament currents of 50 amps or
more, much in some cases.
In some cases the filament for the final uses more power than the
typical ham station.<:-))
Again the glass tubes are cooled primarily by radiation while the seals
are cooled by conduction, both forced air and conduction through the
leads into the sockets. Directly heated cathodes also run more current
than do the indirectly heated so they put more demand on the tube seals.
The only reason I can see for the delay on power up is waiting for the
pin seals to stabilize as it might be quite a shock for them to go from
ambient to operating in just a few seconds.
To "Step Start or not"?
Some amps do not have the capacity of delivering enough current to the
filament for that to be a problem. OTOH step start and ramp up is
becoming more common and although originally applied to the plate
voltage is showing up on the filament(s) as well.
Does it do any good? Does it prolong tube life? Maybe...probably, but
most of these tubes do not have a tremendous inrush compared to a "light
bulb" or the big iron.
OTOH it certainly does not hurt to ease the voltage up over a few
seconds. You only need delay the full voltage long enough to prevent
that big pulse of current.
Cool down?
I'm not normally critical of amplifier companies, but:
The statement from QRO strikes me as a bit...strange. What do they mean
"after prolonged operation?" Those tubes reach full temp after a 30
second to 1 minute transmission and they are *HOT*. The power supply is
a different matter.
In general it's a good idea to leave the cooling air on after
transmitting for a few minutes prior to shutdown. I let the fan run
until the exhaust air is no longer hot.
It would be even better, but probably make little if any difference in
most amps if the fan would run for a minute or two after the filaments
were turned off.
"The way I do it" which is not a suggestion<:-)) : Once the timer says
the tube is ready to go, I go. With my old 2K4 I turned it on and
figured it was ready to go by the time I could pick up the mike. For
cool down, I wait until the exhaust is no longer hot before powering down.
When I go out to the shop and fire up the rig I turn on the amp. If I'm
going to be gone more than half an hour I turn it off because that
usually means I'll be gone for several hours.
It's my opinion that other than the "real QRO" amps that warm up has far
less to do with tube life than does the way the amp is operated and in
some cases ... designed. Some amps are designed to run "full power", but
many are not. Most of the older amps are not! They are designed for 1KW
DC input average or 2 KW PEP input. At 60% efficiency 1500 out is on the
order of 2500 input or as much as 2 1/2 times the power the older amps
were designed to handle! Some have insufficient tube dissipation,
insufficient emission, insufficient plate voltage, insufficient plate
current, insufficient cooling, insufficient iron in the PS, or
insufficient AC power from the wall. Exceed any of these and something
is likely to fail.
The industrial and broadcast pulls are becoming more popular with
serious builders because you can pick up one that will likely last a
lifetime for a comparatively reasonable price. (half to less than a
quarter of a new 8877) Often the sockets cost more than the tubes. Some
like the YC-156 don't even require sockets.
I'd bet Carl has seen many of the old 1KW amps where the op has tried to
make them run today's legal limit...unsuccessfully <:-))
BTW I was listening to a group of "old guys" (I can say that at my age)
complaining about some of the local hams who had been offering
suggestions on how to run their rigs but were splattering all over them.
The one was particularly hot about three who lived on his block wiping
his receiver out on all bands. <:-)) I doubt my 5000MP would handle my
neighbors KW on the same band either<:-))
73
Roger (K8RI)
> This is what I gather:
>
> The AL-82 manual specifies warm up time as "approximately 30 seconds" while
> the AL-80B manual states "10 seconds". I am guessing that the chimney type
> cooling VS the muffin fan cooling method accounts for the different warm up
> times?
>
> W8JI states that to obtain maximum life from an 8877 tube, not to cycle the
> heater (filament) excessively. http://www.w8ji.com/8877_failures.htm It
> stands to reason that this would apply to a 3-500 tube as well.
>
> The QRO HF-2000 manual states: "After prolonged operation, let the Amplifier
> run for several minutes without drive applied so the fan will cool the tubes
> before you turn the Amplifier off."
>
> When I plan on operating an amplifier for 5 hours I leave it on for 5 hours.
> I don't think turning an amplifier off and then back on after 30 minutes
> constitutes excessive cycling(subjective?), but I consider a tube filament to
> be like a light bulb and it is said that the life of a light bulb is
> shortened by cycling.
>
> I as well warm up my tube amplifiers for about 5 minutes and let them cool
> down to what physically feels like idling temperature to me before shutting
> down. Whether this is necessary or to what extent the tube needs to cool
> down may likely be a topic of debate but common sense tells me it does not
> hurt anything.
>
> I'm looking forward to more info about this topic.
>
> 73, BJ
> k0cwo
>
>
>
>
>
>> From: dave@nk7z.net
>> To: amps@contesting.com
>> Date: Tue, 13 Dec 2011 07:30:30 -0800
>> Subject: [Amps] Warm up and on time question...
>>
>> Hello,
>>
>> I have an AL-82, my habit is to turn it on, allow it to warm for about 5
>> minutes prior to use, use the amp, then turn it back off. Sometimes I
>> find that I will be using the amp again within 10 or 15 minutes, maybe
>> 30 minutes. Is it better to leave it on for the half hour, or turn it
>> off then re power it 30 minutes later.
>>
>> Does this cause any issues over the long term? Should I just leave it
>> on for the half hour? Worried about repeated heating and cooling of the
>> tubes.
>>
>> Dave
>> NK7Z
>>
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|