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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

AI500 1 tube overglow

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Well this one, sitting directly under the tube, seems pretty burned, so much that I cant read the original value.

stopper.jpg
 
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.
 
I dont know how you would count that otherwise than clockwise from after fitting slit of socket/tube. I am just reading the numbers pregnated on the pictured downside of the socket. As you can see on picture they are numbered 3 and 5, while 4 is the middel (bottom) one not connected with resistance. So you mean its a 100 ohms and not 1 k.ohms resistans?
 
It blows the 2,5A main fuse quite fast with that tube 4 inserted, so I have to measure fastly.

Please, what does FIR processing mean ?

Yeah the resistor burn is a consequens, when the tube is burned off. I cant really mesure voltages fast enough with tube inserted, cause it happens so fast. Just fired to more tubes and 2 resistors. So now I have to locate the coupling capasitor. What leg is it connected to?

Looks like leg and 1 and 8 (the joint ones) have to much voltage compared to the other chanel tube.
 

PRR

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Joined 2003
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Well, it is pin 3 and 5, it's easy to see since they are numbered underneath....

I have not followed the discussion to know what pins are in dispute.

However the molded numbers are sometimes confusing. Start from the fitting slot/tab. It is between pins 8 and 1. The molded "1" is near socket 1 but looks closer to socket 2. They should mold the numbers very close to the sockets. They didn't.
 

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To finish the tread: It was, as I suspected from the start, the 100uF capacitor at the picture/tubes that was not funtioning and caused the resistance, tubes and fuse to burn off. After replacing one capacitor 100uF/100V at each channel and the burned out restanses and the 2 Tubes at both channels, it's playing nicely again. Thanks to everybody, and especially Jon for the help. As I explained I am a high (Kilo-)Voltage electrician and not an electronic and tube amp qualified. Am Happy!!
 
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