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As the title states, i converted an Eico model 324 sign gen into a jcm800 pre. I replaced the 12au7 with a 12ax7.
attached is the stock schematic.
My issue is a motorboating noise when i turn it on. Key things i changed are the power supply rectification and the heater wiring.
First of all i replaced the single selenium rectifier with a bridge rectifier. i put in various value filter caps with larger dropping resistors between the two tubes. made no difference. i thought maybe the stages were becoming decoupled....but no. it has 2x47uf @ 300Vdc caps installed with a 10K dropping resistor currently. i then started looking at the stock schematic. would swapping it to a bridge rectifier give me these issue? I was thinking about returning it to the stock single diode half wave rectifier circuit and see how that fairs. im getting 180vdc after the bridge and the schematic says it should be 110vdc....thats a huge increase....
Secondly i rewired the heaters and installed 100ohm balance resistors for a ground reference like in a normal tube amp. The stock unit just grounds pin 9 as the ground reference....
I put this project down a few months back as i didnt have time to tinker but i have a friend who wants to use it for some recordings soon so i wanted to get it up and running.
any suggestions are greatly appreciated.
attached is the stock schematic.
My issue is a motorboating noise when i turn it on. Key things i changed are the power supply rectification and the heater wiring.
First of all i replaced the single selenium rectifier with a bridge rectifier. i put in various value filter caps with larger dropping resistors between the two tubes. made no difference. i thought maybe the stages were becoming decoupled....but no. it has 2x47uf @ 300Vdc caps installed with a 10K dropping resistor currently. i then started looking at the stock schematic. would swapping it to a bridge rectifier give me these issue? I was thinking about returning it to the stock single diode half wave rectifier circuit and see how that fairs. im getting 180vdc after the bridge and the schematic says it should be 110vdc....thats a huge increase....
Secondly i rewired the heaters and installed 100ohm balance resistors for a ground reference like in a normal tube amp. The stock unit just grounds pin 9 as the ground reference....
I put this project down a few months back as i didnt have time to tinker but i have a friend who wants to use it for some recordings soon so i wanted to get it up and running.
any suggestions are greatly appreciated.
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