[Amps] FW: Air temps leaving the amp.

[Carl]

I suppose you mean that the 8877 filament is 10A.

Carl
KM1H


----- Original Message ----- 
From: "Matt" <maflukey at gmail.com>
To: <amps at contesting.com>
Sent: Monday, February 03, 2014 10:56 PM
Subject: [Amps] FW: Air temps leaving the amp.


> Air temp rise = (total watts dissipated x 3.41) / (1.08 x cfm)
>
> Total watts dissipated = heater watts + (1-efficiency) x (input power +
> exciter power)
>
> 8873 heater spec is 10A @ 5.0 = 50w
>
> Assuming your tube is biased fully off at idle, then back calculating your
> fan cfm is as follows:
>
> Cfm = (50w x 3.41) / (1.08 x (82-68 F) = ~11.3 cfm
>
> On a good SSB pileup assume you are on the key ~50% of the time and your
> modulation duty cycle is ~50% running ~60% plate efficiency:
>
> Total dissipated power = 50 + (1500 x .5 x .5  x .6) = 275w
> (assume 1500 PEP at the output here includes the exciter)
>
> Expected temp rise would then be:
>
> Rise = (275 x 3.41) / (1.08 x 11.3) = ~76 F
>
> 68  + 76 = 144 F, whereas you measured 142 F, 11.3 cfm seems to check
> out....
>
> So to answer your question, given the Eimac spec....
>
> 1500w x (11.3 cfm / 38 cfm) = 446 watts total dissipation limit @ 11.3
> cfm...
>
> 446w - 50w (heater) = 394w net dissipation @ 11.3 cfm
>
> 394w / .6 efficiency = ~657w output or ~1050w input...
>
> It would seem that there could perhaps be a shortfall when holding a long
> dead key tune up or running a RTTY QSO.
>
> Matt
> KM5VI
>
>
>
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Gary Smith
> Sent: Monday, February 03, 2014 2:10 AM
> To: amps at contesting.com
> Subject: [Amps] Air temps leaving the amp.
>
> Before working the 160M contest last week I put the digital temperature
> probe I use for cooking meat in the oven into the air flow from my amp 
> just
> to see what temps I generate when running a frequency.
>
> The room temp is 68F and at idle the temp sits at 82 as it exits the amp.
> Running pretty steady CQ and replying, the air temp gets up to
> 142 when I'm hot (pun intended) & heavy into the contest. According to the
> specs n the Eimac sheet it gives the amount of air that needs to be moved 
> to
> handle 1500W anode dissipation as 38CFM to hold the temperature below 225C
> (437F) at 50C (122F) ambient temp.
>
> http://www.ok1rr.com/tubes/eimac/3CX1500A7.pdf
>
> Since they don't give air exit temps in the data I don't know if my 142F
> means I need to speed up the blower or if all is well as is. I'd rather 
> keep
> the noise level low but would rather have more fan noise than a burnt 
> tube.
>
> Idears?
>
> 73,
>
> Gary, KA1J
>
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