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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