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A good customer came in on saturday afternoon with his JTM100 clone after one of his KT66s had evacuated and took out his HT fuse. He already had a new quad of TAD KT88s with him, and as long as the tube was the only cassualty and didn't cause any damage to anything else after it failed, I figured I could probably do a quick tube swap, bias, bench test, and cleaning and turn it around while he waited for it. So, I put the head up on my bench and removed the chassis so I could inspect it for any damage and replace the internal HT fuse. Screen resistors were all okay; Also everything else in the power supply looked normal while I used my variac to gradually increase the mains voltage across the primary and bring it up full power.
****I do want to pause here and make the point of how important it is to use a quality moving vain current meter in conjunction with variac. Why an alaloge meter, and not a highly and much more accurate digital display meter? Because, it order to protect the device under test during critical stages of the repair, I'm not looking for a precision measuremennt, I'm looking for a problem, and an analog moving vain meter will indicate a problem, and do it very quickly,
For our purposes, the response time is nearly instantaneous, and by observing movent and position of the needle, we deduce the severity of the problem and it can often provide clues on the nature of the trouble.
now, back to this repair:... after all the tubes were installed, biased, and tested under load, the subject of power output came up and I told him we could determine the output power of his amp with his new quad of tubes very quickly. So I ran a 1k sine wave into the amp, and used the scope to observe the waveform and measure the RMS voltage at the speaker output. I loaded the output with 8Ω, and brought the signal at the output to point where the peaks of the waveform began to just to point just before they began to flatten out, and observed an a measurement of 24.5VRMS, so I did the calculation out loud saying "24.3^2/8 = 75..... so it's 75W into 8Ω, and he paused for a second or two and asked "did you just say 8Ω? the seletor switch is in the 16Ω position." So I looked around the back and double checked and he was correct. I had misread the position of the switch, turned the switch to the 8Ω position, and re-measured the loltage (there are only settings for 8Ω and 16Ω in this model. They use a output ransformer part number 1202-84)
The weird thing was that when matching the 8Ω load secondary labele 8Ω, I measured around 20VRMS, droping the output power from 75W to 50W. When running the 16Ω output setting into a 16Ω load, I calculated 50W of output power using that setting as well.
What do you guys make of this. There was no noticable difference in waveform distortion, or indication of output tube stress or runaway when testing with the load and output lables matched or not.
****I do want to pause here and make the point of how important it is to use a quality moving vain current meter in conjunction with variac. Why an alaloge meter, and not a highly and much more accurate digital display meter? Because, it order to protect the device under test during critical stages of the repair, I'm not looking for a precision measuremennt, I'm looking for a problem, and an analog moving vain meter will indicate a problem, and do it very quickly,
For our purposes, the response time is nearly instantaneous, and by observing movent and position of the needle, we deduce the severity of the problem and it can often provide clues on the nature of the trouble.
now, back to this repair:... after all the tubes were installed, biased, and tested under load, the subject of power output came up and I told him we could determine the output power of his amp with his new quad of tubes very quickly. So I ran a 1k sine wave into the amp, and used the scope to observe the waveform and measure the RMS voltage at the speaker output. I loaded the output with 8Ω, and brought the signal at the output to point where the peaks of the waveform began to just to point just before they began to flatten out, and observed an a measurement of 24.5VRMS, so I did the calculation out loud saying "24.3^2/8 = 75..... so it's 75W into 8Ω, and he paused for a second or two and asked "did you just say 8Ω? the seletor switch is in the 16Ω position." So I looked around the back and double checked and he was correct. I had misread the position of the switch, turned the switch to the 8Ω position, and re-measured the loltage (there are only settings for 8Ω and 16Ω in this model. They use a output ransformer part number 1202-84)
The weird thing was that when matching the 8Ω load secondary labele 8Ω, I measured around 20VRMS, droping the output power from 75W to 50W. When running the 16Ω output setting into a 16Ω load, I calculated 50W of output power using that setting as well.
What do you guys make of this. There was no noticable difference in waveform distortion, or indication of output tube stress or runaway when testing with the load and output lables matched or not.
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