Roger,
The fan IS blowing up and the case temp is actually substantially lower.
Pushing the exhaust air back down into the case wouldn't make much sense??
73,
Jim N7CXI
On 3/30/2012 6:44 PM, Roger wrote:
> On 3/30/2012 11:08 AM, Vic K2VCO wrote:
>> 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),
>
> My commercial amp uses EBS on CW break-in just fine and when using VOX
> on SSB.
>
> I really don't like the idea of using the PTT line though as I tend to
> hang onto the mike while EBS would save a lot of heat.
> OTOH I don't consider 300 watts a lot of power compared to the over 1000
> that Jim was talking about.
>
> AND I view just placing a fan on top of amps with more than a bit of
> suspicion. At the least I'd have the fan blowing up, not down. The
> case might be hotter but the tubes should be cooler. Still as some one
> else said to the effect, don't have the extra fan blowing against the
> air flow as many of them end up doing.
> Just because the cabinet is cooler doesn't mean the tubes are.
>
> 73
>
> Roger (K8RI)
>
>
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