>
>> Rich wrote
>>
>> >>Transistors that C/E short offer zero protection.
>>
>> Ok so sense the voltage drop across a resistor across the screen supply,
>> feed the transistor via an opto isolator,make the transistor trip a fast
>> relay in the primary of the supply transformer.This is better and more
>> reliable than a fuse surely.
>
>Better still, use a screen supply at least two times the desired
>voltage. Get that voltage from the same transformer that supplies
>anode voltage, and use a shunt regulator.
Shunt regulators are not suitable for tubes that use over 800v on the
screen.
>Use the bias and blower
>voltage (with an air pressure switch) to turn on the HV system.
>Sense the regulator current with a trip system that turns off the PA
>if regulator current exceeds a safe margin over the quiescent
>current.
>
>1.) Under any condition of positive screen current, you can never
>have more dissipation than the quiescent dissipation set in the
>regulator.
>
>2.) Negative screen current will be limited to a lesser value than
>positive screen current, preventing overdissipation from negative
>screen current in most cases.
>
Tetrodes with handles usually do not have negative screen current.
Negative screen current does not cause overdissipation. The problem is
voltage runaway.
>3.) Everything is interlocked.
>
>4.) Protection is instantaneous.
>
>5.) If any semiconductor shorts or if HV or bias dumps, the system
>fails safe.
A fast acting fuse worked well for me using an 8171 and an 8169.
>
>Then you can use the fuses to prevent wire and transformer fires,
>which is a job they are suited for.
Such was not the case. The 3AG screen fuse would typically open in
seconds if I inadvertently started with too-light loading.
later, Tom.
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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