31-05-2021, 07:58 AM
Some good points there Tracy. I did think of using either a mosfet source follower or emitter follower on the IP but that would need a separate power supply really.I also have some LT6080 high voltage opamps that run off +/- 75v rails I could have used but in the end it was easier to finnagle an ECC82 in. The diode I mentioned goes from grid to cathode and is reverse biased in normal operation,but at power up it's forward biased keeping the grid and cathode at 0.5v or whatever, see attached. I agree LF oscillation is usually a symptom of a resonant PSU, the classic cause of motor boating,don't think that's the issue here though.
Yesterday I pulled at the suspect CF and re-biased it a tad more positive which seems to have cured the issue, I also got rid of the diode. I think the pot ( see schematic) has been affecting things somehow. I think at power up as the heater warms up and current starts to flow, the cathode has been rising in voltage but somehow the grid has not been settling at it's bias point, could it be the pot has been pulling the grid down? This has caused the valve to re-cycle through a "start up" condition, thus causing the whole amp to oscillate at LF. hope that makes sense.
Andy.
Yesterday I pulled at the suspect CF and re-biased it a tad more positive which seems to have cured the issue, I also got rid of the diode. I think the pot ( see schematic) has been affecting things somehow. I think at power up as the heater warms up and current starts to flow, the cathode has been rising in voltage but somehow the grid has not been settling at it's bias point, could it be the pot has been pulling the grid down? This has caused the valve to re-cycle through a "start up" condition, thus causing the whole amp to oscillate at LF. hope that makes sense.
Andy.
