I am a member of diyaudio for a few years now. This is my first post: Thanks a lot for this great forum. It is a joy to know such things exist.
My problem here - I had played around a little with a breadboarded test build of a Marantz 7 phono. It sounded great (even compared to a diy Xono). I decided to put it in box with all the bling and blong needed to be a reliably working pre. The power supply is a tube rectified CLC going into 2 mosfet gyrators for 2 high voltages: 245V and 280V. The original circuit has changed a little bit: There is no cap from grid to cathode of the first gain stage. The feedback cap from the second stage to cathode first stage is only 10 pF. The RIAA network is a little different (I worked it out with B2Spice). The output stage runs a little hotter with 22k/560 Ohm resistors.
After I had finished I stumbled upon a strange thing. The voltages at the output stage cathode where not as supposed to be: ca 22 volts instead of about 50. With the feedback-loop disconnected everything was fine. There is a picture of a scope trace (measured before the output cap9) with the feedback engaged.
I tried several things. other output caps (new vs old, different values), gridstoppers at the cathode followers grid, bypassing the mosfet-gyrator for the first 2 gain stages, receiving voltage from the output stage gyrator what else? loading the output stage with other resistances, changing the series resistance values at the output. But no luck. Nothing works. Nothing found in the forums or on the web. I need help. Do you have any suggestions, please???
My problem here - I had played around a little with a breadboarded test build of a Marantz 7 phono. It sounded great (even compared to a diy Xono). I decided to put it in box with all the bling and blong needed to be a reliably working pre. The power supply is a tube rectified CLC going into 2 mosfet gyrators for 2 high voltages: 245V and 280V. The original circuit has changed a little bit: There is no cap from grid to cathode of the first gain stage. The feedback cap from the second stage to cathode first stage is only 10 pF. The RIAA network is a little different (I worked it out with B2Spice). The output stage runs a little hotter with 22k/560 Ohm resistors.
After I had finished I stumbled upon a strange thing. The voltages at the output stage cathode where not as supposed to be: ca 22 volts instead of about 50. With the feedback-loop disconnected everything was fine. There is a picture of a scope trace (measured before the output cap9) with the feedback engaged.
I tried several things. other output caps (new vs old, different values), gridstoppers at the cathode followers grid, bypassing the mosfet-gyrator for the first 2 gain stages, receiving voltage from the output stage gyrator what else? loading the output stage with other resistances, changing the series resistance values at the output. But no luck. Nothing works. Nothing found in the forums or on the web. I need help. Do you have any suggestions, please???