| Author | Message | ||
     joe |
I have a Music Man RD112-100. All the board says is 11-24-78. The bias seems off. One 3.9 ohm says 25mv and the other is 10mv. This seems very lopsided. Overall the bass is lacking and it's not loud enough for a 100 watt and my reverb stopped working - I opened up the reverb up (everything appeared fine), then switched the rca jacks and still nothing. | ||
| Steve Kennedy |
Reverb tanks often fail because the little transformers inside open up. The only way to tell for sure is measuring the impedance at the in and out jacks, then comparing that against the manufacturer's specs for your particular model of spring assembly (usually Accutronics, which is still in business and has a handy web page). If you CAN verify that the reverb tank is OK, then it will require some technical troubleshooting to determine what has failed (assuming it was plugged in correctly and the RCA jacks/cables are in good shape). After measuring the voltage drop across the 3.9 ohm resistor, turn off the amp and swap the output tubes in their sockets. Do the numbers stay the same or swap sides? If the measurements stay the same, then you probably have a matched set of tubes, but one of the 3.9 ohm cathode resistors (or other component)is off in its resistance value. These should be matched reasonably close (1-5%) before you judge the condition of your tubes or set bias. If the numbers swap, then the difference is caused by your tubes not being matched and you should replace them with a matched pair before you set any bias adjustment. There is a section for biasing different models in this Discussion Forum. Follow the directions for your model and type. |