[1000mp] Very Hot Final Heatsinks

[Rob Atkinson]

i stay out of class A entirely.   i heard a new PA costs $700 in the 
u.s.  for the 200 w. version.   i almost fried mine anyway one time 
when i put the cover back on and the speaker leads obstructed the fan 
and kept it from turning.   just by luck i touched the heat sink after 
about 15 or 20 minutes and was it ever hot!   pulled the cover off and 
saw the speaker wires down in the fan.   after that close call, plus 
hearing over the past year or so, all the stories about class A doing 
in the PA, I stay on AB.   If in AB ur heat sink still gets too hot, 
I'd just aim a second (external) fan down on it.

with the increased popularity of high duty cycle modes, plus a renewed 
interest in AM, the finals on rigs are getting a workout they did not 
receive back in the days when a transceiver was made to do ssb and cw.  
   i wish manufacturers could make the new rigs more like ccs 
transmitters, but i guess the cost would by sky high.

rob / k5uj



On Monday, November 28, 2005, at 03:16 PM, Hans Remeeus wrote:

>
> Class-A uses a constant current. This constant current is important 
> for a lower TX distortion. When you decrease the power output from 75 
> to e.g. 25 Watts this has to be absorbed by the heat sink. So the 
> heatsink has two jobs: handle the constant (HIGH) current and handle 
> the absorbtions of the 50 Watts in less power output. This is too much 
> for the finals. When an amplifier is operated in Class A, it will be 
> hottest at zero output power, because all of the DC power input will 
> be converted to heat.
>
> This is the main reason why I sold my Mark-V 200 and bought the Mark-V 
> Field with 25 Watts Class-A, which is a very good power input for my 
> linear amplifier Acom 2000A. It then gives 800 Watt power output, 
> which in most situations is more than enough.
>
> To be honest; who ever will notice that you are transmitting in 
> Class-A instead of Class-AB...? IMHO Class-A is nice to have, but not 
> need to have.
>
> 73,
> Hans Remeeus (PA1HR)
> http://www.remeeus.nl
> Communication is about people, the rest is technology.
>
>
>
>
> William Shadid schreef:
>> Norm,
>>
>> Hans is correct. I long ago learned this with my Mark V -- leave the 
>> power
>> level at maximum on Class A. I note a higher temperature level in 
>> Class A
>> but nothing like what I hear about on the reflectors. Up to now I had
>> thought this to be a problem with selected radios -- but Hans' note 
>> gets me
>> to thinking that it is really a lack of understanding with the radio. 
>> Yaesu
>> should be more clear about this -- but they certainly do not have an 
>> edge on
>> things omitted from the manual!!
>>
>> I drive an older Drake L-7 with my Mark V and use the Class A 
>> selection
>> simply to keep my drive power low. I do not use Class A when I run 
>> PSK.
>> While I don't often run the amplifier, I can say that I note only 
>> somewhat
>> higher heat sink temperatures. The fins are far cooler than running 
>> older
>> domestic, solid state radios such as an old Cubic Astro 103 with its 
>> large
>> rear heat sink. They are also much cooler than the single heat sink 
>> on my
>> Ten-Tec Omni VI+ after a long operating session. If you are 
>> experiencing
>> "scalding" temperatures at 75 watts output, Class A, I would say that 
>> a call
>> to Yaesu is in order.
>>
>> 73,
>>
>> -Bill, WA9MXQ
>>
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