[Amps] Cheap auxiliary amp cooling

[hs0zed at csloxinfo.com]

I have a bit of a problem understanding this too. Maybe I'm missing something
but of all the amps I own they all draw only filament power when not transmitting.

As soon as I start transmitting I'm speaking and the amps plate current is up
and down like the proverbial. When I stop speaking I stop pressing the PTT. I
never see ZSAC.

Is this perhaps a scenario looking for a problem?

Cheers
Martin, HS0ZED
 

Quoting Vic K2VCO <k2vco.vic at gmail.com>:

> 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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> > Amps at contesting.com
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> 
> -- 
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco/
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