Mike,
Thank you very much for the explanation. I completely agree.
The question is how a tube, showing maximum reverse screen current leading
to an excessive screen dissipation could be used in an amateur amplifier.
Yes, the screen is in a danger only when the amplifier is moderate driven,
but the PA is supposed to work at any drive level from 0 to max.
73, Val LZ1VB
> Val,
>
> With a tetrode your screen supply must be able to source and sink current
> wile maintaining regulation of the voltage at the screen. One way of
> doing this is with a series-regulator and a bleed resistor at the screen,
> but this approach consumes quite a lot of power. Another approach is to
> use a shunt-reguoator, of which one of the best designs for this type of
> tube is the G3SEK Tetrode boards project.
>
> At low levels of drive (and power out) is is normal to see negative
> (reverse) screen current due to secondary emission and without a bleed
> resistor (or better, a shunt regulator) this would result in rising screen
> voltage and the potential for a run-away condition leading to a flash-over
> and possible damage of the tube and/or socket.
>
> There will be a point when you increase the drive where the screen current
> will go less negative, if you continue further it will come back to zero
> and if you continue much beyond this point it will start to go positive.
> At this point what typically is happening is that the peak RF voltage is
> such that when it is considered on the low side of the cycle (subtracting
> from the HT) the instantaneous anode voltage is dropping down to the
> screen voltage.
>
> Generally when you get to this point you adjust the loading (and/or
> tuning) to increase the power transfer to the load (and reduce the screen
> current). From here on you can carefully maximise the drive while
> minimising the screen current by adjusting for maximum power output. Some
> positive screen current is okay but you need to keep well within the
> screen dissipation.
>
> Mike G8TIC
>
>
>
> On 28/01/2012 17:57, Val wrote:
>> Don,
>>
>> How the bleeder resistor would help, if the reverse current exceeds 37
>> mA?
>> At 325 V screen voltage this makes 12 W - the maximum screen dissipation.
>> I have seen many brand new tubes with much higher maximum reverse
>> current,
>> which they use to achieve at moderate power levels. They are good only to
>> work at full power in industrial devices.
>>
>> 73, Val LZ1VB
>>
>>
>>
>>> A bleeder resistor to ground at the tube screen connection should help.
>>>
>>> Don W4DNR
>>>
>>>
>>>
>>>> I forgot to mention that most of these tubes run reverse screen current
>>>> with low drive and power levels. Mine (4CX1500B/FU728F) only goes to
>>>> positive screen current near the US legal limit output. At that point
>>>> I
>>>> tune for max screen current.
>>>>
>>>> 73
>>>>
>>>> Roger (K8RI)
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