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 Post subject: Seco grid leak tester circuit
PostPosted: Aug Sat 13, 2022 9:42 pm 
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Joined: Jan Thu 07, 2021 9:41 am
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Location: Melbourne, Victoria, Australia
Does anyone have a SECO GCT-8 grid leak tester and can verify the socket wiring for me please.
On the schematic it shows pins 3 and 6 of the 12AU7 going to the cathodes of the test sockets, but the description about wiring a new socket seems to indicate that these pins also go to the anodes.

One example given is for a DR7 socket (image attached), and is says to wire pins 1,6,8 and 9 (Both cathodes and anodes)to the white lead which I presume goes to pins 3 and 6 of the 12AU7.

Also, the grids connect to the junction of the 4.7m and 1.5m resistors, the DR7 only has a control grid, but for other socket types is it still just the control grid or all grids that are attached.


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 Post subject: Re: Seco grid leak tester circuit
PostPosted: Aug Sun 14, 2022 2:22 am 
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It would just be the control grid that would be an issue.

Grid leakage in most tubes is when cathode material manages to get onto the control grid wires. As the tube heats up, this material begins to emit electrons which affects its charge particularly if the grid circuit has a high impedance.

I had a Philco 95 which would play great when first tuned on cold, but gradually, the volume would drop as the tubes warmed up. This would continue until the radio was deaf as a stump. Turn it off and let it cool, then it would play again.

Turned out a 24A had grid leakage which would affect the AVC voltage and draw it down making the radio insensitive. In another radio without AVC the tube may work just fine.

-Steve

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 Post subject: Re: Seco grid leak tester circuit
PostPosted: Aug Sun 14, 2022 4:09 am 
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Joined: Jun Sat 09, 2007 7:14 am
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Location: Melbourne, Florida
"Grids" on the diagram means the control grids of tubes to be tested. All the other elements are tied together and go to the "cathodes" connection (including all grids other than the control grid).

RRM


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 Post subject: Re: Seco grid leak tester circuit
PostPosted: Aug Sun 14, 2022 4:28 am 
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Joined: Jan Thu 07, 2021 9:41 am
Posts: 235
Location: Melbourne, Victoria, Australia
Retired Radio Man wrote:
"Grids" on the diagram means the control grids of tubes to be tested. All the other elements are tied together and go to the "cathodes" connection (including all grids other than the control grid).

RRM

Thanks, so in effect all pins except the filament and control grid have the aprox 47v applied and the tester reacts to any leakage back to the control grid.
I’m currently building a similar circuit into an existing valve tester that only measures shorts and emission test.
Kevin


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 Post subject: Re: Seco grid leak tester circuit
PostPosted: Aug Sun 14, 2022 2:09 pm 
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I have a couple of these, they were made when TV came into use to check IF, RF and sync tubes for a very small leakage.


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 Post subject: Re: Seco grid leak tester circuit
PostPosted: Aug Sun 14, 2022 11:44 pm 
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Joined: Jun Sat 09, 2007 7:14 am
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Location: Melbourne, Florida
Here's a link to a similar tester made by Senco (Sencore):

https://bama.edebris.com/download/senco ... C2_LC3.pdf

RRM


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 Post subject: Re: Seco grid leak tester circuit
PostPosted: Aug Wed 17, 2022 7:39 am 
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Joined: Jan Thu 07, 2021 9:41 am
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Location: Melbourne, Victoria, Australia
Retired Radio Man wrote:
"Grids" on the diagram means the control grids of tubes to be tested. All the other elements are tied together and go to the "cathodes" connection (including all grids other than the control grid).

RRM

Thanks again, I now have this working in my TST valve tester. You may be able to help me with another question if you don’t mind.
It is recommended that tubes be pre heated before testing as some faults won’t show up when the tube is cold. Will the tube preheat if there is filament voltage but no b+ to the other pins?

I’m hoping that is the case because in the TST tester the filament voltage is available whenever a tube is in the test sockets, even though the b+ is only available when the emission test is selected.


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 Post subject: Re: Seco grid leak tester circuit
PostPosted: Aug Wed 17, 2022 11:44 am 
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Joined: Jun Fri 19, 2009 5:34 pm
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Location: Floral Park, New York
What the Seco instructions say is that some tube problems, grid emission in particular, take time to evidence themselves. The grid wires have to get hot enough for the emissive material on them to activate, before there is any emission from the grid. Gas may cause similar problems, since gas molecules ionize more easily when heated. So what Seco recommended was if you had a TV or an FM radio that worked for say 10-15 minutes from cold, then the AGC voltage went haywire, you would put each IF tube on the tester, set the eye tube, then wait the same 10-15 minutes to see if the tube acted up on the tester too.

One might think, "what's the big deal about that, one could just change the tube!" But when tracking down intermittent issues in equipment it is nice to have an independent confirmation that you actually did find the problem before you give the set back to the customer. In the service business you usually only get paid to fix something once; call-backs and repeat visits for the same problem ruin your day and the customer's real fast. It's been decades since I had to fix an AGC problem in a tube TV, but I still use a Seco tester to check VTVM tubes for leakage, gas, and grid emission.

In a tester where the B+ is not applied except when the "test" button is pressed, it will take longer for the tube to heat up fully, but it will eventually. The GCT circuit is only intended to be used on small signal tubes--not on power tubes--and in small signal tubes the plate dissipation is usually pretty small compared to the heater dissipation. The reason GCT testing is not done on power tubes is because those types inherently have more grid leakage and gas to begin with, and because they are almost never used in circuits with more than half a megohm of external grid resistance. Since the external resistance shunting the grid is much lower than the leakage resistance inside the tube, the external resistance dominates. Consequently minor amounts of leakage, gas, and grid emission are acceptable in power stages.

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 Post subject: Re: Seco grid leak tester circuit
PostPosted: Aug Wed 17, 2022 7:53 pm 
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Joined: Jun Sat 09, 2007 7:14 am
Posts: 5806
Location: Melbourne, Florida
kevwr wrote:
Retired Radio Man wrote:
"Grids" on the diagram means the control grids of tubes to be tested. All the other elements are tied together and go to the "cathodes" connection (including all grids other than the control grid).

RRM

Thanks again, I now have this working in my TST valve tester. You may be able to help me with another question if you don’t mind.
It is recommended that tubes be pre heated before testing as some faults won’t show up when the tube is cold. Will the tube preheat if there is filament voltage but no b+ to the other pins?

I’m hoping that is the case because in the TST tester the filament voltage is available whenever a tube is in the test sockets, even though the b+ is only available when the emission test is selected.


Yes, the tube will heat but not as quickly as with all elements active. The length of time for a problem to show up in normal use is the starting point but a few minutes longer may be required to let the tube heat up thoroughly in the tester. There's no harm in leaving a tube in the Seco for extended periods of time. I once had to let one cook in a tester for over 30 minutes to get a valid test.

You didn't ask this question but it's generally not a good idea to keep the test button on an emission tester pressed for a long period of time because most of the test current goes through the control grid. I once did an experiment by keeping all the other elements at ground during an emission test. The meter showed about 95% of the level shown will all elements used.

This kind of tester will detect very low levels of grid current. Just because a tube fails in the Seco doesn't always mean it should be thrown out. I had a tube that would cause a set to slowly fade out that was OK in another part of the circuit that had a low resistance grid return.

RRM

p.s. I'd like to have one of these gizmos but eBay folks seem to think that since they test tubes they should be priced as most other tube testers are (insanely).


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 Post subject: Re: Seco grid leak tester circuit
PostPosted: Aug Wed 17, 2022 9:16 pm 
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Joined: Jan Thu 07, 2021 9:41 am
Posts: 235
Location: Melbourne, Victoria, Australia
Retired Radio Man wrote:
.
You didn't ask this question but it's generally not a good idea to keep the test button on an emission tester pressed for a long period of time because most of the test current goes through the control grid. I once did an experiment by keeping all the other elements at ground during an emission test. The meter showed about 95% of the level shown will all elements used.

I carried out a number of mods to this TST tester. One was to include a momentary switch for the emission test, Something that was included on later models but not on this one.

I also added a pin eliminator function, something also included in later models. I realised the need for this when testing an old EL84 tube which had internal connections not included in more modern tubes.

Another mod was to add a Life Test function which adjusts the filament voltage by either 5% or 10%. I used a 3 ohm pot with markings on the dial that reflect ohms law using the filament voltage and current of the heater. This is also used with a momentary switch.


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 Post subject: Re: Seco grid leak tester circuit
PostPosted: Aug Wed 17, 2022 9:50 pm 
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Joined: Jun Sat 09, 2007 7:14 am
Posts: 5806
Location: Melbourne, Florida
TST?

RRM


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 Post subject: Re: Seco grid leak tester circuit
PostPosted: Aug Wed 17, 2022 10:29 pm 
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Joined: Jan Thu 07, 2021 9:41 am
Posts: 235
Location: Melbourne, Victoria, Australia
Retired Radio Man wrote:
TST?

RRM

TST Supertester, vintage 1940’s made by University Radio Equipment.
This photo shows the magic eye for the grid leak test fitted in the top right corner. All my controls for the mods are located on the side of the case to avoid changing the original look.

Side panel is a work in progress.

https://www.radiomuseum.org/r/universau ... r_tst.html


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 Post subject: Re: Seco grid leak tester circuit
PostPosted: Aug Wed 17, 2022 10:47 pm 
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Joined: Jun Sat 09, 2007 7:14 am
Posts: 5806
Location: Melbourne, Florida
Thanks. I thought it must be something made in the land down under.

Keep up the fun work.

RRM

Just noticed you're in the other Melbourne. Once while checking in for my return from Germany, some of my equipment almost went there. Catastrophe was only prevented at the last minute when I happened to see MEL on the bag tag instead of MLB.

RRM


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