[Amps] Cheap auxiliary amp cooling

[Leigh Turner]

Hi Jim,

A quiescent power dissipation of circa 1.1 kW in an amplifier is insane; an
energy wasteful desk-top blast furnace!  Such a design also places undue
stress on the tube(s) and reduced service life.

Such an amp ideally needs a well designed and implemented Electronic Bias
Scheme / EBS circuit to minimise quiescent plate current and excessive power
dissipation when used in SSB mode. It offers little practical advantage in
high duty cycle modes though.

A tri-state bias scheme works extremely well; tubes initially cut-off; tubes
move to a moderate pre-bias state of say 80 mA when PTT is activated, then
moves to full ZSAC only when say a few hundred mW of RF drive from the
exciter is present; or even better use the xcvr's DC+ V on Tx control line
to activate the bias controller's full 450 mA / deep AB1 linear bias current
condition.

Such a progressive bias scheme is completely benign in its action and
introduces no leading edge distortion artefacts, maintaining a spectrally
clean amplifier SSB output signal.

Some commercial amplifier manufacturers have implemented such clever EBS
schemes and they work a treat.

Many purist folk disparage EBS systems, but when implemented properly they
can achieve excellent results. It would not be difficult to retrofit such
efficacious bias control measures to your HF2500DX amp.

73

Leigh
VK5KLT


-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Jim Barber
Sent: Friday, 30 March 2012 5:59 PM
To: Amps at contesting.com
Subject: [Amps] Cheap auxiliary amp cooling

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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