On Mon, 29 Mar 1999 10:15:04 +1000 "Graham Jones" <g.jones@bom.gov.au>
writes:
>G'day Carl,
>
>In reply to Greg's query on loading, you wrote:
>
>> Loading Heavy actually means to DECREASE the Load C value, it is
>> confusing. On top of that the 0-100 panel labeling is one way on
>some
>> amps and the opposite on others.
>>
>> I prefer to tune for absolute max on the Bird and then increase
>loading (
>> Less C-2) until power drops about 5%. This will be the point of best
>IMD.
>> Any reduction in power should then be done at the exciter. A MLA
>with
>> perfect tubes is capable of over 1500W out @ 100W drive but that is
>> really pushing them.
>>
>> 73 Carl KM1H
>>
>
> I've been curious about a related subject for a while. I
>believe I more or
>less understand the mechanism by which heavy plate loading reduces
>IMD
>(i.e. by limiting negative peak anode voltage excursions going into
>the
>non-linear region). But how does it increase tube life? The Eimac
>data
>sheet for the 4CX250B states in regard to VHF operation: "Such
>operation
>should be conducted with heavy plate loading, minimum bias, and the
>lowest
>driving power consistent with satisfactory performance. It is often
>preferable to operate at a sacrifice in efficiency to obtain increased
>tube
>life."
>
> To Carl and other knowledgable persons on the reflector -
>could you please
>explain how heavy plate loading and reduced efficiency can increase
>tube
>life.
I believe you picked up on two items and left out the remainder Graham.
The minimum bias and drive power are very important parameters in the
bottom line.
In a tetrode heavy loading also decreases Isg which may contribute to
life by limiting secondary emission which is often a factor in 4CX250B
VHF operation.
I'll leave the scientific explanation up to a qualified tube engineer.
73 Carl KM1H
>
>I apologise if this subject has been covered previously on the
>reflector.
>
>Thanks in advance.
>
>73 Graham VK3BKG
>
>
>
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