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Old 14th October 2010, 12:35 PM   #1
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Default Measuring unwanted grid current

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?
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Old 14th October 2010, 01:58 PM   #2
JohanB is offline JohanB  Sweden
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Hi,

Good you're checking this and investigate. You'll learn a lot when doing so and understand a little more about tubes.
Everything you're doing and your measurements are correct.
But you must determine if the grid current is negative or positive. The grid current caused by gas ions makes the grid positive.

The normal grid current caused by the space charge around the catode, makes the grid negative.
That's the reson they specify a max "grid leak" resistor in the datasheets, so this current not interfere with the normal bias.

100KOm I think is minimum for a fixed bias KT88 and some 270KOhm for a cathode bias, were the gid current induced extra bias is of less importance.

If you want to include a gas test in your rig, you can keep the 1KOhm fixed in place (to ensure no HF oscillations) and add a switchable 470K resistor in series with the grid.
If the current rises when switched in, you have gas in the tube. If nothing happends or the anode current is slightly lower, the tube is fine.

Don't throw away a little gassy tube. The getter will still work when you're burning in the tues in your rig. So a gassy tube can "repair it self".
But to much gas and if the getter mirror turns white in the edges, nothing can save that tube.

I've seen many cheap chinese power tubes, that are eating up the getter mirror in a few hundred hours. This i probably not caused by a leaky tube, but gas is released from not clean and gassy components inside. Like gas trapped in the micas or in poor quality anode metal sheet. This gas evaporates when the tube get varm and shortens the life substantially.

JohanB
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Old 14th October 2010, 02:28 PM   #3
DF96 is offline DF96  England
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4uA is a bit high for an output valve, but it may improve with use. 4uA would be very high for a smaller valve, where 0.1uA or less would be more usual.

As Johan says, 1K is far too small for detecting grid current but it is ideal as a grid stopper to prevent oscillation - put it right at the grid pin.
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Old 14th October 2010, 02:34 PM   #4
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Let me see if I can understand unwanted grid current:

Non gassy tube
Electrons from the cathode are speeding out towards the plate. Since the grid is negative (we have cathode bias), these electrons pass by the grid, so do *not* produce an electron flow from cathode to grid, through external grid circuit, back to cathode. Thus no unwanted grid current.

Gassy tube
Electrons speeding out towards plate, hit some gas molecules, knocking off electrons. This creates positive ions. These positive ions are attracted towards the grid and make the grid positive.

Problem: I cannot see how the ions make the grid positive, when it's significantly negative WRT cathode because of cathode bias.

There are two examining situations: a) where there is no bias whatsoever, I mean no cathode or fixed bias. Then who have a situation where we have fixed or cathode bias.

I'm trying to measure unwanted grid current with cathode bias. If this is possible, then I don't understand how these positive ions create a current.

Last edited by richard8976; 14th October 2010 at 03:01 PM.
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Old 14th October 2010, 03:25 PM   #5
DF96 is offline DF96  England
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Positive ions make the grid positive when they land there because the current flow goes through the grid bias resistor. This is typically 100's of K. Ohm's law!

This might not be a good time to introduce a complication, but electrons can still land up on the grid even when it is a bit negative. Electronic grid current disappears below about -1V to -1.5V on the grid. How can a negative grid attract electrons? Two reasons: contact potential (the grid and cathode are made of different metals), heat (the electrons have random thermal energy from the hot cathode so some can overcome a small repulsion).
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Old 14th October 2010, 05:20 PM   #6
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Lets say there is no fixed or automatic bias. A gassy tube means positive ions land on g1. g1 is connected to k, via a resistance.

Will electrons flow through the external resistance? I guess. From k to g1. Probably also from k to g1 inside the tube as well.


Since g1 becomes positive wrt to k, electrons will want to flow towards the grid through the external grid circuit. g1 and k will be at different potentials, because g1 and k are not connected by a low resistance wire.

If g1 and k are connected with a wire, then current, in theory, wants to pass from k to g1 as before. But g1 and k would be at the same potential. So, I guess current flows, but you never really get any potential between g1 and k.

But, anyway, lets assume we have cathode bias.

Positive ions land on g1. But g1 never becomes posative, because of the negative bias on g1. So all that gassyness results in is a reduction of negative bias on g1. You cannot measure any grid current due to positive ions landing on g1.

Well, at least, that is what is bothering me. How can you measure unwanted grid current due to gassyness, when you have cathode bias?

Last edited by richard8976; 14th October 2010 at 05:33 PM.
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Old 14th October 2010, 06:02 PM   #7
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There has to be an explanation for this.

g1------------k___/\/\/\__ Cathode bias resistor
|
|_____/\/\/\____________|
Grid resistor

I'm seeing the end of the grid resistor connected to g1 as being at a negative potential, due to voltage developed across bias resistor. Ions landing on g1 are not making g1 positive, just less negative.

That's how I'm seeing it. But, I'm seeing it wrongly, I must be?

Last edited by richard8976; 14th October 2010 at 06:19 PM.
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Old 14th October 2010, 06:41 PM   #8
JohanB is offline JohanB  Sweden
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Quote:
Originally Posted by richard8976 View Post
Lets say there is no fixed or automatic bias. A gassy tube means positive ions land on g1. g1 is connected to k, via a resistance.


Positive ions land on g1. But g1 never becomes posative, because of the negative bias on g1. So all that gassyness results in is a reduction of negative bias on g1. You cannot measure any grid current due to positive ions landing on g1.

Well, at least, that is what is bothering me. How can you measure unwanted grid current due to gassyness, when you have cathode bias?
You don't need to maesure the positive or negative grid currents. As they have different polarity, they can be there but can equals each other so there will be zero current resuling.

As I wrote, the best way to detect a gassy tube is to see if there is any positive anode current change when you're open/shorting the 470KOhm resistor from G1 to ground (cathode)

In a tube there is a cloud of electrons surrounding the cathode due to the heat and the cathode emission coating. This is called "space charge". This charge will transfer to G1 and make a negative grid current through the grid leak resistor. The closer the grid is to cathode, the larger the initial current will be, but it should be close to zero at 1-1,5V. (We were discussing this in an other thread)

I'm also a little confused that you could measure 4uA (seems much) with -24 v on the G1, but we still don't yet know if it is a positive or negative current.

4uV in a 470 KOhm resistor will give nearly 2V difference in the grid voltage and will absolutely be visible in the anode current, even with a catode resistor bias!


Maybe you've seen in diagrams, that many microphone and tape-head triode input stages are "missing" the cathode resistor to make a bias. Only a 10MOhm resistor from grid to ground and a capacitor to the signal source. This capacitor is charged by the "space charge" to a negativ bias of arond -1 to 1,5V
Over that knee, the "space charge" current is to low in f.ex. an ECC83/EF86 and leaks trough the 10MOhm resistor to keep it on around 1V.

But if you send a high input voltage or transients from a scratchy record, it will be rectified by the grid-cathode "diode" and give a high negative voltage to the grid/capacitor, blocking the tube for a time, by a time constant of 10MOhm and the capacitors value.
Besides saving on components, this also leaves the cathode on ground potential and it will be less disturbed by the heater voltage noise and hum.
Easy to do, but not desired in HiEnd equipment.

JohanB
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Old 14th October 2010, 06:50 PM   #9
JohanB is offline JohanB  Sweden
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Quote:
Originally Posted by richard8976 View Post
There has to be an explanation for this.

g1------------k___/\/\/\__ Cathode bias resistor
|
|_____/\/\/\____________|
Grid resistor

I'm seeing the end of the grid resistor connected to g1 as being at a negative potential, due to voltage developed across bias resistor. Ions landing on g1 are not making g1 positive, just less negative.
Yes you're right. Less negative and that will show up in an increase in the anode current. Then you know that the tube has a little gas.
But after a day in your burn-in rig, it could be gone (absorbed by the getter) and perform without problems.
The getter is working all the life time to catch gas, but as I wrote before, if there is to much gas, it will be poisoned and the tube burns out.

JohanB

Last edited by JohanB; 14th October 2010 at 06:52 PM.
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Old 14th October 2010, 06:54 PM   #10
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Do we have a situation where positive ions landing on g1 is simply said to give rise to positive grid current, a current opposite to any current caused by a negative bias, but no-one actually does measure any positive grid current?

Being a newbie and reading a about grid currents, I thought people were actually measuring unwanted grid current.
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