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12-24-2023, 07:14 PM
Happy Holidays
When working on tube guitar amplifiers I believe it is a common practice to have the rear of the amplifier facing you, as this gives you the best view of the tubes. Once you are at the point of testing with power tubes, you should preset the bias to the lowest-current end of the bias control or controls and tie one meter probe to ground, as Mr. O'Connor suggested above. Then power up and see hat the tubes are at a low or close-to-zero idle current.
Watch for any suggestion of red-plating. The metal of the anode (plate) is a dull grey and should remain so; If it begins to change colour turn off the power immediately. Pull the tube that changed colour and check that the bias voltage is still present at the tube socket. If all the tubes red-plated then something common to them all has failed. If just one tube failed it could be the tube or the circuit. In a 4-tube output stage, two tubes on the same side red-plating suggests a circuit error where a single tube red-plating suggests something wrong with the tube or the circuitry for that socket alone.
Always take your time when doing repairs or testing of high-voltage circuits, and especially when dealing with power tubes. I believe there are several cautions throughout The Ultimate Tone books.
Cheerio
When working on tube guitar amplifiers I believe it is a common practice to have the rear of the amplifier facing you, as this gives you the best view of the tubes. Once you are at the point of testing with power tubes, you should preset the bias to the lowest-current end of the bias control or controls and tie one meter probe to ground, as Mr. O'Connor suggested above. Then power up and see hat the tubes are at a low or close-to-zero idle current.
Watch for any suggestion of red-plating. The metal of the anode (plate) is a dull grey and should remain so; If it begins to change colour turn off the power immediately. Pull the tube that changed colour and check that the bias voltage is still present at the tube socket. If all the tubes red-plated then something common to them all has failed. If just one tube failed it could be the tube or the circuit. In a 4-tube output stage, two tubes on the same side red-plating suggests a circuit error where a single tube red-plating suggests something wrong with the tube or the circuitry for that socket alone.
Always take your time when doing repairs or testing of high-voltage circuits, and especially when dealing with power tubes. I believe there are several cautions throughout The Ultimate Tone books.
Cheerio
