Jeff Blaine AC0C wrote:
<snipped some comments>
Thanks for the info, Jeff.
You mentioned the tube temps..., the 3CX1200A.
I had looked at them, and had some questions so I'll take the
opportunity to throw them out here...
The full datasheet is here:
http://www.ok1rr.com/tubes/eimac/3CX1200A7.pdf
says effective 3/85. Scanned copy.
Has a lot of good info.
page 3 has text requirements for cooliing.
"Forced-air cooling is required to maintain the base seals at a
temperature below 250 deg. C. and the plate seal at a a temperature
below 250 deg C.
Air flow requirements to maintain the above max. temp. are shown on the
cooling data curve (page 5).
At 1200 W of plate dissipation an airflow of 30 cfm with a back pressure
of 0.5 in. of water should be applied and directed through the fins of
the tube.
A minimum of 5 cfm must be supplied to the base of tube whenever the
filament voltage is on.
In all cases, the only criterion of proper cooling is the temperature of
the tube seals. Tube temperatures may be measured with temperature
sensitive paint, spray, or crayon.
I don't know what back pressure the AL1200 cabinet plus blower creates.
The chart on page 5 (of the above linked pdf) shows no CFM numbers for
1.5 kw. I assume this is 100% duty cycle.
It has two curves. I think the arrows on the curves are showing a range
of allowable CFM + pressure bounded by the two curves?..Does anyone know
for sure what those two curves are trying to show?
i.e. 1.35KW is bounded by 36 CFM 0.36 in. pressure, and 60.1 CFM 0.61
in. pressure.
1.2KW is more sensibly bounded by 30.1 CFM 0.31 in. pressure and 46 CFM
0.46 in. pressure.
This is at 25 deg C ambient, 29.92 in. Hg. (sea level?).
I guess 1.2KW is the max they spec because of the 1200W plate
dissipation, and they're talking about 100% duty cycles.
So lets look at the stock AL1200 blower specs: G2S085-AA19-15
http://www.ebmpapst.us/allpdfs/G2E085.PDF
says the -15 blower (it has the CFM vs pressure curves in a graph) is
about 26 CFM at 0 pressure.
This is at 115 VAC. I don't know what voltages the 4 taps give the
blower in the AL-1200. Assuming 115/120 max?
The -15 blower has no airflow (0) at ~.41 in. pressure. So it can't
generate air flow with much pressure.
At 0.16 in pressure, it's airflow is only 19 CFM or so.
The drop in replacement blower, the G2E085-AA05-21 is much better. At
0.16 in pressure, it has 48 CFM or so.
at 0.32 in. pressure, the -21 has about 40 CFM.
The -21 is 57 dBA vs 38 dBA for the stock -15.
Curiously the -15 says "Max ambient Temp C: 40"
while the -21 says "Max ambient Temp C: 70" ..so that's interesting. I
don't know why the -15 has such a low spec of 40 deg C.
So for any kind of high duty cycle, it seems like you don't really want
the -15 blower, but the -21.
I suspect the -21 blower used on the lower taps would be good/better
airlflow too, even at < the peak noise.
It is capacitor operation though, from the table? No capacitors on the
-15. I don't know if that's an issue. The -21 uses 28 W vs 22W for the -15
Without testing the tube seal temps, it sure sounds like a -21 blower
would make the AL1200 less marginal at high duty cycles, ...i.e. match
the spec sheet better?
I know this has been said before. But the data sheet details weren't
provided. Also, I think talking about a single CFM number (zero
pressure) for a blower makes no sense. It makes the -15 blower sound
better than it is.
I know people have talked about measuring tube seal temps. But I've seen
no data.
The cost comparison at Digikey is $86 vs $122 for quantity 10. I've not
gotten the single quantity prices.
I've seen quantity one price of $169 for quantity one for the -21 elsewhere.
Comments?
-kevin
ke6rad
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