carlsefni
New member
I was recently getting my 2016 EL34 Shiva serviced because the original impedance selector switch broke (causing blown fuses, etc.). After my amp tech guy replaced the switch (and the new one seems fine), however, he found a couple of other strange issues.
Firstly, he ran a power-on test with a current limiter, and as he added voltage, he found that it sort of started to oscillate, as if there were a consumption peak, cut, peak, cut, peak, etc. He thought it might be a problem with the power supply, and as I already had one of the recommended Mean Well power supplies on hand as a reserve, he installed that — but although the replacement power supply seems fine, the curious oscillation behavior remains.
Secondly, after further general tests, he found a bias imbalance in the installed output tubes. He changed those, and the bias issue now seems resolved, but further testing shows the maximum dissipation to be only 13.5 mA for one of the power tubes, and 13.25 mA for the other, even at the maximum settings permitted by the bias control. Reckoning that the dissipation should be at least 25 to 30 mA, the current values are only about half that. My technician thinks that, next to the pot that calibrates the bias, there should be an additional resistor that allows configuration of the amount of voltage reaching that pot. However, we do not know whether the amp was modified (removing such a resistor?) at some point earlier in its history (before I purchased) or whether we are simply on the wrong track with this issue.
Does anyone have any thoughts on or experience with either of these kinds of issues?
Firstly, he ran a power-on test with a current limiter, and as he added voltage, he found that it sort of started to oscillate, as if there were a consumption peak, cut, peak, cut, peak, etc. He thought it might be a problem with the power supply, and as I already had one of the recommended Mean Well power supplies on hand as a reserve, he installed that — but although the replacement power supply seems fine, the curious oscillation behavior remains.
Secondly, after further general tests, he found a bias imbalance in the installed output tubes. He changed those, and the bias issue now seems resolved, but further testing shows the maximum dissipation to be only 13.5 mA for one of the power tubes, and 13.25 mA for the other, even at the maximum settings permitted by the bias control. Reckoning that the dissipation should be at least 25 to 30 mA, the current values are only about half that. My technician thinks that, next to the pot that calibrates the bias, there should be an additional resistor that allows configuration of the amount of voltage reaching that pot. However, we do not know whether the amp was modified (removing such a resistor?) at some point earlier in its history (before I purchased) or whether we are simply on the wrong track with this issue.
Does anyone have any thoughts on or experience with either of these kinds of issues?
