30-05-2021, 06:32 AM
(This post was last modified: 30-05-2021, 06:35 AM by Diabolical Artificer.)
Thanks for both your suggestions, think I've sussed it though. I pulled the OP stage, I knew the cathode follower drivers and LTP phase splitter were good,but still the instability was there. Prior too the CF's & LTP is a common cathode gain stage, preceded by a Baxandall tone stage that uses an ECC82 & 1/2 an ECC83, see attached. The cathode follower on the IP was oscillating at very low frequency, causing everything after it to also oscillate, This caused the bias to the CF drivers to go unstable, causing the OP stage to pull high current, thus further de-stabilising HT & screen grid supplys, at least I think that is what happened.
Why the cathode follower right at the front went unstable I'm not 100% sure. Looking at it yesterday I noticed I'd wired a pot wiper straight to the control grid, that must lower the grid leak resistor or pull the grid to ground, when in actual fact the grid should be at about 80v. I had a diode connected from grid to cathode to keep V low on power up/whilst heaters warm up, maybe this was switching on/off,hence the clicking?
Schematic to follow. Andy.
Why the cathode follower right at the front went unstable I'm not 100% sure. Looking at it yesterday I noticed I'd wired a pot wiper straight to the control grid, that must lower the grid leak resistor or pull the grid to ground, when in actual fact the grid should be at about 80v. I had a diode connected from grid to cathode to keep V low on power up/whilst heaters warm up, maybe this was switching on/off,hence the clicking?
Schematic to follow. Andy.
