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AI500 1 tube overglow
AI500 1 tube overglow
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Old 14th January 2019, 11:17 AM   #1
leiflende is offline leiflende
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Default AI500 1 tube overglow

I have a modded Audio Innovations 500 where the extreme left tube is started overglowing. Changing tube makes no difference. Is it likely to be a capacitor leaking problem?

Last edited by leiflende; 14th January 2019 at 11:47 AM.
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Old 14th January 2019, 11:45 AM   #2
leiflende is offline leiflende
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A sorry, many here in the Forum refereres the Audio Innovations 500 amp as AI500.

Last edited by leiflende; 14th January 2019 at 11:50 AM.
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Old 14th January 2019, 12:13 PM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Check the voltage on the grids of the EL34s, pin 5. They should be almost equal in value. The cathodes, pins 1&8, should sit at around 28volts.If the grid is not at or around zero volts DC, check the coupling capacitor.
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Old 14th January 2019, 01:01 PM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
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It tends to be faulty coupling capacitor letting dc through and upsetting grid voltage.
Or burned grid stopper which is unlikely.
Or cold soldered joint in grid circuit.
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Old 14th January 2019, 01:54 PM   #5
leiflende is offline leiflende
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Well this one, sitting directly under the tube, seems pretty burned, so much that I cant read the original value.

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Old 14th January 2019, 02:16 PM   #6
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Looks like it could do with refurbishing by the look of it.
Let me know which pin it connects to and I can work out what the value is for you.
Bottom right of your photo shows a discoloured component, I would check that one as well.


However that will be the result of a fault, not the cause.
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Last edited by JonSnell Electronic; 14th January 2019 at 02:19 PM. Reason: More info ...
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Old 14th January 2019, 02:41 PM   #7
leiflende is offline leiflende
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The resistor is over legs 3-5 of the EL34.
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Old 14th January 2019, 02:53 PM   #8
Kay Pirinha is offline Kay Pirinha  Germany
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That would mean between the plate and the control grid. Doesn't seem that likely...!
Best regards!
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Old 14th January 2019, 03:10 PM   #9
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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The pins read clockwise from underneath so if it was pins 6 & 4 that would be the screen grid supply resistor. Pin 6 has no connection in the valve so we used it as a tag to connect components to.
It is a 100R 2Watt resistor. That is definitely not a cause for red plating in a good valve! Check the control grid voltage, pin5, it should be around zero volts.
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Old 14th January 2019, 03:24 PM   #10
leiflende is offline leiflende
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Well, it is pin 3 and 5, it's easy to see since they are numbered underneath. At the other side socket, its easier to read the numbers Click the image to open in full size.
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