Hi. I something of a beginner.
Often it can be the passing of unwanted grid current that makes a tube undesireable for service. That would often be associated with a tube described as "soft" or "gassy".
If a tube is seriously gassy, I believe plate current will tend to "run away". It's also possible, I think, to consider a tube gassy if grid current amounts to too many microamps.
I think that with a tube like a KT88, some small unwanted grid current could be acceptable, but at any rate I am assuming that anyone wanting to purchase such a tube would like to know, if the data is to hand, how many microamps of unwanted grid current is flowing.
My KT88 seems to be passing (at the moment - I may "cook it") some unwanted grid current. And I want to know how to measure it correctly.
I want someone to please explain how to measure grid current and why it is measured the way described.
I have a test setup. Originally created to "cook" the tubes if required.
I have 310V feeding a triode connected KT88. I have put in a 270R cathode bias resistor. Grid is connected to the negative end of the bias resistor, via a 1K0 resistor, or directly. I have a switch that can short out the 1K0.
With the setup:
Plate voltage: 310V
Plate current: 90mA
Cathode bias: 24.4V
Since the curent stabilises at 90mA, I presume I don't have a tremendously gassy tube.
When I switch out the 1K0 resistor in the grid circuit, no change in plate current occurs.
Now, I understand that grid current is often calculated by measuring voltage across a resistor in the grid circuit. Whether this is the only acceptable way I don't know. I have a 200uA meter and currently it is in series with the grid circuit. In other words the grid connects to one end of the cathode resistor via the meter. I muse this is perfectly acceptable way of measuring the grid current. If not I want some to say so.
So, a meter in series with the grid circuit is how I'm measuring unwanted grid current. And actually it comes out the same whether the 1K0 is in circuit or not. And the KT88 seems to passing about 4uA grid current.
As to this 4uA value: If it stays at this value, I don't whether it makes the tube unfit for purpose - or what.
Anyway, I am interested in the measurement of grid current.
Also, why does grid current flow if g1 is -24.4V negative WRT k?
Often it can be the passing of unwanted grid current that makes a tube undesireable for service. That would often be associated with a tube described as "soft" or "gassy".
If a tube is seriously gassy, I believe plate current will tend to "run away". It's also possible, I think, to consider a tube gassy if grid current amounts to too many microamps.
I think that with a tube like a KT88, some small unwanted grid current could be acceptable, but at any rate I am assuming that anyone wanting to purchase such a tube would like to know, if the data is to hand, how many microamps of unwanted grid current is flowing.
My KT88 seems to be passing (at the moment - I may "cook it") some unwanted grid current. And I want to know how to measure it correctly.
I want someone to please explain how to measure grid current and why it is measured the way described.
I have a test setup. Originally created to "cook" the tubes if required.
I have 310V feeding a triode connected KT88. I have put in a 270R cathode bias resistor. Grid is connected to the negative end of the bias resistor, via a 1K0 resistor, or directly. I have a switch that can short out the 1K0.
With the setup:
Plate voltage: 310V
Plate current: 90mA
Cathode bias: 24.4V
Since the curent stabilises at 90mA, I presume I don't have a tremendously gassy tube.
When I switch out the 1K0 resistor in the grid circuit, no change in plate current occurs.
Now, I understand that grid current is often calculated by measuring voltage across a resistor in the grid circuit. Whether this is the only acceptable way I don't know. I have a 200uA meter and currently it is in series with the grid circuit. In other words the grid connects to one end of the cathode resistor via the meter. I muse this is perfectly acceptable way of measuring the grid current. If not I want some to say so.
So, a meter in series with the grid circuit is how I'm measuring unwanted grid current. And actually it comes out the same whether the 1K0 is in circuit or not. And the KT88 seems to passing about 4uA grid current.
As to this 4uA value: If it stays at this value, I don't whether it makes the tube unfit for purpose - or what.
Anyway, I am interested in the measurement of grid current.
Also, why does grid current flow if g1 is -24.4V negative WRT k?