I got an NOS 6A8 from Antique Electronic Supply.

On my Heathkit TT-1 it tests OK except for grid current. I retested several times, making sure that tube was as hot as under operating conditions, but got the same result. Gassy tube?

I have two used 6A8’s which show lower Gm, but no grid current.

Truth be told, when the tube is in use I would probably never suspect that anything was wrong.

What do you think, should I return the $12.55 tube or just pretend that I never tested it?

I say plug it in and use it and see what it does.

It'll be a while before my current project is ready for that, but you reminded me that I have a set downstairs that uses a 6A8 to try it in.

Thanks.

Yeah, definitely try. Put it to good use for a day or so and if it performs well then you got what you wanted. However, I would look into the supposed condition to see if it shortens the life of a tube. I have seen a few tubes do strange things on testers and still be fine. But yeah, put it to the heat and return it if it doesn't perform well over way more than a few minutes of time. Tubes actually often have a very long lifespan compared to popular opinion.

To put this short thread to bed:
A week after the last post, I was ready to give the tube a long run. First I retested it. No grid current.

Thanks for saving me from returning a perfectly good component, and I'm glad I saved the several tubes I had set aside previously due to grid current.

To put this short thread to bed:
A week after the last post, I was ready to give the tube a long run. First I retested it. No grid current.

Thanks for saving me from returning a perfectly good component, and I'm glad I saved the several tubes I had set aside previously due to grid current.

Two things come to mind ...

If you have cleaned your switches in the tube tester by hosing them down with contact cleaner ...

Or if you have the tube tester in a very humid location (read basement of an non-air conditioned house), that will definitely cause an erroneous grid current reading.

Not knowing what's going on here..... All I can say is tubes generally
don't have grid current..... They have plate, and cathode current....

.

Not knowing what's going on here..... All I can say is tubes generally
don't have grid current..... They have plate, and cathode current....

.

I want to see if I can get this right. There is normally a grid bias voltage which is usually negative with respect to cathode and placed between the cathode and plate in a basic triode to control current. That current would normally be measured between cathode and plate.

That I feel pretty good about but I really never thought about the flow of current there (if any) unless there is a problem with the tube. Is the original poster saying that he had no grid leakage on the tester. Which would be good. Right? Checking my own knowledge....:thmbsp:

Disclaimer: I have no idea what I'm talking about. This is the concept I've got from reading information from sources which I cannot vouch for.

The "grid current" which is the issue here results from electrons hitting gas molecules which produce positive ions. The positive ions flow to the (negative) grid which reduces the negative bias of the grid.

So, the grid current measurement is supposed to indicate the presence of gas in the tube. Maybe, if the gas condition (quantity, type) is minimal, the electron bombardment reduces it to something which no longer produces grid current ("burning off" the gas).

So, the grid current measurement is supposed to indicate the presence of gas in the tube.

Yep!

Simply stated, think of it as the gas allows the control grid to go too positive and this makes the tube conduct too much current and shift the grid cut-off-point.

The Triplet design emission tube testers (Knight 600 and unmarked Knight look-a-likes, Eico 600's and look-a-likes, Heath TC -1, -2, & -3 and many other inexpensive brands) do not check grid current directly. Instead they measure grid voltage (not current) created by opening the control grid circuit. This voltage is indirectly measured by reading the increase in emission of the tube. This is why the bad-good scale is goes in the opposite direction for this test.

Unlike the Knight and Eico, the Heath pin select lever switches are slightly more reliable, but, unfortunately, they are more prone to leakage due to high humidity and/or absorbing excessive amounts of contact cleaner and this can cause grid current and shorts tests to be slightly more sensitive than the designer intended. (If leaky, so can the .1 MFD capacitor on the short test.) On some of the later tube testers Heath switched to the same simple switch that Knight and Eico used.

I was always taught that once a tube gets gassy, it will only get worse.

True then, but not always true now, when one tests an old tube that has not been used for 50 years. The heat from the filament can, over several hours or days running, actually improve emission and reduce gas.

James

Yep!

Simply stated, think of it as the gas allows the control grid to go too positive and this makes the tube conduct too much current and shift the grid cut-off-point.


I was always taught that once a tube gets gassy, it will only get worse.

True then, but not always true now, when one tests an old tube that has not been used for 50 years. The heat from the filament can, over several hours or days running, actually improve emission and reduce gas.

James


Tubes generally do not have current flow at the control grid. It is a control
point of a near perfect infinite impedance amplifier. Typically -7 volts DC bias
at G1 to the cathode. It's always one of the first, and easiest checks to see
if a tube circuit has a problem. Plate, Cathode, and G1, should typically have
a reasonable range to them. I never looked into what the "gas" was, and
what it did. Since 'early film' has a description of how the tube testers check
for a leakage current at the G1, it would seem that as the tube ages some
gas is developed inside the tube and behaves like neon or some other non
inert gas would at voltage. It begins to conduct. So it would allow an
unexplainable shift in G1 bias. You may see it there...

I was also always taught that gas will always get worse, and never get better.

If something is turned off 50 years and it gets better with use, I would assume
it is because the cathode gets waken up and it's emissions get better with
time because it's getting back into the heat - reheat cycle - the environment
it was designed to be operating in....

I would imagine the gas condition to be an out-gas of materials used inside the
tube that time and heat, and high current have produced, and once this occurs
it generally can't be undone....

.

Tubes generally do not have current flow at the control grid. It is a control
point of a near perfect infinite impedance amplifier. Typically -7 volts DC bias
at G1 to the cathode. It's always one of the first, and easiest checks to see
if a tube circuit has a problem. Plate, Cathode, and G1, should typically have
a reasonable range to them. I never looked into what the "gas" was, and
what it did. Since 'early film' has a description of how the tube testers check
for a leakage current at the G1, it would seem that as the tube ages some
gas is developed inside the tube and behaves like neon or some other non
inert gas would at voltage. It begins to conduct. So it would allow an
unexplainable shift in G1 bias. You may see it there...

I was also always taught that gas will always get worse, and never get better.

If something is turned off 50 years and it gets better with use, I would assume
it is because the cathode gets waken up and it's emissions get better with
time because it's getting back into the heat - reheat cycle - the environment
it was designed to be operating in....

I would imagine the gas condition to be an out-gas of materials used inside the
tube that time and heat, and high current have produced, and once this occurs
it generally can't be undone....

.
The Getther is there to collect that out-gasing. If a tube has sat stagnant for decades and the gas has collected away from the getther then running it/ heating it (which will increase the kinetic energy and motion of the gas atoms) may move them into contact with the getther and cause them to be absorbed.

A tube that has been in regular use (as in a tube in a set that a service tech would have see in say the 50's) that has gas probably has a used up getther or a leak too large for the getther to deal with and likely would not improve with use.

I was defining "gas" as the tube tester defines it, something that causes static charges, almost always positive, to form on a disconnected element.

This usually is not a problem on an amplifier, but on a detector (AGC keying, Sync Sep, AM audio detection, etc.) that has a sharp cutoff, that small change is often a problem.

Squirrel Boy's cathode and Electronic M's getter & getter splatter are the most probable two things that a little heat cures.

James