[Amps] Cheap auxiliary amp cooling

[Vic K2VCO]

I often wonder why so many amplifiers are designed so that the full ZSAC flows when the 
user isn't actually transmitting. I always install a circuit in my QSK amplifiers which 
cuts off the plate current when the key is not closed.

This applies to VOX or PTT voice and data modes, too, although the heat reduction there is 
smaller. A 3-500Z that is idling at 3kV with ZSAC of 100 mA is dissipating 300 watts just 
sitting there!

I know that RF-drive operated circuits to do this are 'controversial' to say the least (I 
wouldn't ever use one for either CW or SSB), but it's even simpler to do this with the 
QSK/VOX/PTT line. I would think the reduction in heat would increase the life of many 
components, not to mention decrease the electric bill.


On 3/30/2012 12:29 AM, Jim Barber wrote:
> While playing with my new toy, (QRO HF2500DX) I found that I was able to
> confirm that it does indeed need auxiliary cooling for high duty cycle
> modes. Even at relatively low  or no power output - just the ZSAC is
> 450ma @ 2400V, or over 1000 watts of just plain old heat.
>
> I decided to check my computer parts supplier to see if there was
> anything that looked like it might work for a "booster" exhaust fan, and
> it turned out that a 250mm (10"!) 12V PC case fan has a brushless (no RF
> noise) DC motor, turns slow and makes little audible noise, but manages
> to move 105 CFM of air. (at zero back pressure, of course) They cost
> $26.99 USD in single units.
>
> I haven't built a sealed mount yet, but just sitting the fan on top of
> the case above the tubes dropped the exhaust temp 40-50F during
> 375/1500-watt AM transmissions. The noise of the "booster" fan is so low
> compared to the internal 50CFM blower that I can't really hear it.
> Legal-limit SSB conversations (with modest compression) get the
> airstream only lukewarm now, instead of 130F+ .
>
> I also bought a 243 CFM 8" 12VDC Conair-Rotron hurricane generator that
> I want to build a temperature-ramped PWM controller for when I get A
> Round Tuit. (probably 25khz, 30-100% duty cycle to copy the newer PC
> controllers, which work well) In the meantime, the cheap PC case fan
> seems to be getting the job done.
>
> 73,
> Jim N7CXI
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-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/