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 Post subject: Screening for grid emission
PostPosted: Jan Tue 30, 2018 12:39 am 
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Just found what appears to be my third case of grid emission. In all cases, the tube in question tested good in my tester (Precision 10-12).

The symptoms are pretty consistent---with correct voltages, all resistors in spec, and no leaky caps, the plate voltage goes too low, indicating the the plate current is too high.

So---is there any established way of screening for grid emission?

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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 1:43 am 
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Try another tube tester. Something other than a dynamic or emission type.

Chas


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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 2:22 am 
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When I worked at a TV shop we had a tube tester called something like a Sencore Mighty Mite. It was an emission tester, but it had a very sensitive grid leakage test. It had a tube inside it which was used to preform the grid leakage test.

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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 2:26 am 
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I've got a B&K 747 transconductance tester. It has a grid emission test. I find quite a few tubes that have that fault, however depending on the circuit grid emission doesn't always cause a problem.

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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 2:29 am 
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The excellent leakage testing reputation of the Sencores is why I have the TC-109 which I use most often and the TC-162 secondarily.

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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 3:56 am 
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A way to test for grid emission or gas is to ground the grid and read plate current. Then lift the grid connection, let it float, plate current should drop a little. When there is grid leakage plate current will increase.

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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 5:05 am 
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I just threw together a kludge to test the tubes with my lab B+ supply. Put in plate and cathode resistors, based on an assumed plate current. Need some tweaks, but even at 30 volts on the plate it sorts out tubes that are drawing too much current.
Curious if it might be better to monitor the actual grid current.....

Thanks for all the other replies---I can see that a better tube tester would be a big asset,,,,but I really like the Precision for the quick tests.

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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 4:38 pm 
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One of the telltale signs of grid emission is, it is not present when the equipment is first powered up. It appears some time (say 10-15 minutes) after powering up. The grids in the tubes have to get hot enough before they start emitting electrons and this takes time since they are not directly heated.

If the effect is apparent when the circuit comes on cold then it is more likely to be internal or external leakage resistance, or gas. Gas current will not appear until the voltages on the various electrodes are sufficient to ionize the gas. Leakage resistance may be measured at any convenient voltage, though it is usually preferable to have the heater on as some heater-to-cathode problems don't become apparent unless the heater is hot.

All tubes have some leakage resistance, gas, or grid emission if you test them hard enough. Sencore and B&K had to answer accusations from consumer advocates that their leakage tests were unrealistic gimmicks intended to sell unnecessary tubes to consumers. In all cases, what matters is the bias conditions in the circuit. If a tube has a very high value grid return resistor, grid emission, gas current, or leakage are more likely to cause trouble. If the grid return resistor has a low value, or fixed bias is used, then the emission, gas current, or leakage will probably be swamped and won't have much or any impact on circuit operation.

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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 5:57 pm 
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Here is some actual data:
Test setup**:
---supply B+: 250 volts
---plate load: 390 Kohms
---grid resistor: 1 Mohm
---cathode resistor: 2 Kohm
measured voltage on grid and converted to current

The first 2 samples are branded "Fender"---made by Sovtek. Sample 9 is a "JJ" that appears new. The others are all Fisher-branded, made in W. Germany
Attachment:
12AX7_grid-leakage.jpg
12AX7_grid-leakage.jpg [ 12.73 KiB | Viewed 676 times ]


**the test setup is similar to the circuit where I first found a problem, but with the grid resistor added
(tube was sample #1, and the symptom was low plate voltage)

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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 6:02 pm 
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Responding to Chris's comments:
My test data just posted is after maybe 1 minute. For the problem that started all this (with tube #1), the symptom (garbled audio), showed up a soon as the set came alive.

After much Googling, I'm now saying "grid leakage", and not "grid emission". In my two previous experiences, the problem took longer to appear.

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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 6:58 pm 
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As your test klooge shows, depending on the circuit, you can often check for emission or leakage by simply checking the voltage across the grid resistor. The Mighty-Mites and others held the tube in cut-off by making the grid very very negative and then monitored the grid voltage across a high resistance (10 megs or so). They would easily detect emission (or leakage) of over 100 megs effective, sometimes causing rejection of tubes that would work fine in a lot of circuits. B&K 707's had calibration instructions for either 100 or 20 megs.

My first experience with grid emission was a radio that would play fine for about 10 minutes and then slowly fade out. My 707 showed one of the tubes as good until it had warmed up for 10 or 15 minutes and then emission went off the scale. The tube was in a circuit that had a high grid resistor (megs). It worked fine in another circuit that had only a few hundred ohms to ground.

As previously mentioned, all tubes have some level of leakage or emission. Maximum grid resistance is often specified to account for this, with 470K or less common for power tubes.

Grid circuit testers show up on eBay fairly often.

RRM


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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 7:48 pm 
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I'm actually a big fan of those little Seco grid circuit testers that were popular years ago. They still turn up on eBay fairly regularly. The older ones used eye tubes and only checked for grid leakage/emission. The idea was that you'd use your regular tube tester to check for things like shorts and emission or transconductance, then use the Seco if a grid leakage/emission test was needed. You'd set the bias control to put the shadow of the eye tube in a certain area. If the shadow couldn't be set properly it meant the tube had leakage; if it had grid emission the shadow would change after 10-15 minutes. Those testers would respond to about 25 megohms of leakage or about 200 pA of grid current. The interesting thing about them was that they would show up positive or negative changes in grid current, so regardless of where the problem was coming from, you'd see it.

It should be noted that grid circuit testing is normally only done on small signal tubes. Most power tubes inherently have too much grid leakage or emission, and/or residual gas to be tested this way, especially after they accumulate some operating hours. It's why tube manufacturers often prescribed low values of grid return resistance like 220-k or 470-k for them.

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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 9:18 pm 
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All of a sudden this topic has gotten my attention.

Last Summer I finished a Zenith 53, which has several 24's, 27's and 45's. After several weeks operation I noted that upon turn on and first moments of play volume was good as well as sensitivity, then a rapid fade to relatively poor sensitivity and a bit of noticeable distortion, a "gravelly" sound... The radio has gotten the obligatory capacitors.

I have not had time to get back into it...

My first thought is deterioration of the original resistors, all well within tolerance when completed.

I want to touch all bases and throughly check the tubes, again, it may be the TV-7D/U or the Triplett may be missing a grid emission issue. At the time such a problem was not evident...

I would like to see a bench set-up for grid emission that I could do for the three or more candidates: 24a, 27 and the 45. I have vintage power supplies and and stand alone D'Arsonval meters handy.

Such a set-up would enable other users to do this obscure test to help clear said elusive performance issues.

Chas


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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 9:43 pm 
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OK, you asked for it!!----behold the kludge:
Attachment:
kludge.jpg
kludge.jpg [ 55.81 KiB | Viewed 641 times ]

Attachment:
PS.jpg
PS.jpg [ 30.19 KiB | Viewed 641 times ]

Power for filament and B+ only....
grid leakage measured as the voltage on the "appropriate" grid resistor. I listed earlier my values for a 12AX7 (in a low-level circuit with plate current way under 1mA). The values will depend on the tube to be tested.

In my case, I just emulated the circuit that had the issue. Let's guess how to do it more rigorously.
1. determine the desired operating point (plate current and voltage)
2. find the right cathode bias resistor
3. find a plate resistor that puts the plate voltage in the right place with a supply voltage that is feasbile.
4. find the value of the grid resistor. (I just GUESSED for mine---my measurements might have be easier if it were lower.
I think the right way to do it it to first determine how much grid leakage would be a problem, then sizing the resistor to give an easy to measure voltage.

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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 9:51 pm 
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Or you could just swap the tube with another one and see if that's the problem. :roll:

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 Post subject: Re: Screening for grid emission
PostPosted: Jan Tue 30, 2018 9:58 pm 
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Chris108 wrote:
Or you could just swap the tube with another one and see if that's the problem. :roll:
Wise man has stated profound truth......last time around this bush was a 6Q7 in a Crosley Fiver. Plugged in a different 6Q7--done.
Today's little adventure is a Fisher 400 FM Stereo Brick Schoolhouse Receiver---different rules.

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 Post subject: Re: Screening for grid emission
PostPosted: Jan Wed 31, 2018 1:41 am 
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You can make a simple emulation of the Mighty Mite circuit function by tying the grid to ground through a DMM and all the other elements to a positive voltage source. Sencore used 45 volts for older testers, 8 volts for newer ones. If the DMM is 10 megs in, each volt is 0.1ua; if it's 1 meg (like the small red ones from you-know-who), each volt is 1.0ua.

Sencore calibrated for leakage resistance by connecting test resistors from the grid pin to one of the other element pins (no tube installed). For example 45 volts, a 10 meg meter and 100meg test resistor will give a reading of 45V/110 megs or approximately 0.41ua. 20 megs (one of the B&K calibration points) will give 45V/30 megs or about 1.5ua.

None of this is anywhere near an exact science but it does give some idea of good/bad. As has been previously mentioned, these tests were aimed at small-signal tubes; primarily in TV set front ends or IF's where small leakage could cause big problems. Power tubes are a different matter and generally allow much more leakage and have grid resistors specified accordingly. The Langford-Smith Radiotron books have sections discussing all this in detail.

RRM


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 Post subject: Re: Screening for grid emission
PostPosted: Jan Wed 31, 2018 4:08 pm 
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Mark, what do the colors represent in your 12AX7 grid leakage chart?

Matt


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 Post subject: Re: Screening for grid emission
PostPosted: Jan Wed 31, 2018 5:12 pm 
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skylerca wrote:
Mark, what do the colors represent in your 12AX7 grid leakage chart?

Matt

9 tubes were tested---two triodes per tube. I displayed it that way so I could see how tubes compared in terms of one triode section leaking more than the other.

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 Post subject: Re: Screening for grid emission
PostPosted: Jan Wed 31, 2018 6:34 pm 
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Got it. Looks like they don't make 'em like they used to!

I noticed you picked up a few more 12AX7 from the classifieds. Let us know how those stack up!

Matt


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