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12-30-2024, 01:50 AM
Hi guys
As TUT2 reported, low-frequency oscillations can be set off by high-frequency oscillations, but either is the result of too much gain between two in-phase signal points. Some of the low-frequency gain should be sacrificed for stability, by reducing some plate/grid coupling cap values: First to attack are to the 6L6 grids, reduce to 22nF each; remove the cap to ground off the splitter grid; remove the bypass across the feedback resistor; remove the plate bypass across the 470k for V2B.
Is the 150k-2W tied V2B cathode still in place? I would remove it.
The feedback loop could be opened, or altered depending on how other modifications go.
Since there are no grid-stops except for the 1k for each 6L6, you need to add a few. Use at least 68k for V1A and V1B although I would use 100k or higher. Same for V2A. Personally, I do not like grid-bias preamp stages as they limit versatility and voicing options, so I would add cathode resistors for V1A/B. This allows adding bypass caps if desired to focus the gain.
A simple fix that might tame the oscillation is to remove one or both bright caps from the output of each input stage (the 150pF across 470k feeding the respective Volume-1/2 wipers).
Since you referred to using both inputs ala plexi methods, you might want to reconfigure the mixing to just copy the plexi? On the other hand, how it is now is fairly simple although the Master Volume could be repositioned after the EQ or between the PA front-end and splitter.
Merlin's article is pretty interesting. Never had any of those problems BUT I only use self-biased concertina splitters with proper grid-stops and provide local filtering for each stage right at the circuit. Menno stressed the importance of having a high-value filter for the concertina since on signal is referenced to the supply.
Having multiple gain stages and/or cascaded stages all running from the same supply node can be problematic or not. It depends on how things are laid out. It is always preferred to have a local filter cap for each stage even if it is effectively a common supply node.This filter cap and its physical wiring are more important than resistive isolation of the supply nodes for each stage; the cap proximity is what gives "power" to decoupling by providing the return path for the signal current of the related stage - it is the decoupling.
As TUT2 reported, low-frequency oscillations can be set off by high-frequency oscillations, but either is the result of too much gain between two in-phase signal points. Some of the low-frequency gain should be sacrificed for stability, by reducing some plate/grid coupling cap values: First to attack are to the 6L6 grids, reduce to 22nF each; remove the cap to ground off the splitter grid; remove the bypass across the feedback resistor; remove the plate bypass across the 470k for V2B.
Is the 150k-2W tied V2B cathode still in place? I would remove it.
The feedback loop could be opened, or altered depending on how other modifications go.
Since there are no grid-stops except for the 1k for each 6L6, you need to add a few. Use at least 68k for V1A and V1B although I would use 100k or higher. Same for V2A. Personally, I do not like grid-bias preamp stages as they limit versatility and voicing options, so I would add cathode resistors for V1A/B. This allows adding bypass caps if desired to focus the gain.
A simple fix that might tame the oscillation is to remove one or both bright caps from the output of each input stage (the 150pF across 470k feeding the respective Volume-1/2 wipers).
Since you referred to using both inputs ala plexi methods, you might want to reconfigure the mixing to just copy the plexi? On the other hand, how it is now is fairly simple although the Master Volume could be repositioned after the EQ or between the PA front-end and splitter.
Merlin's article is pretty interesting. Never had any of those problems BUT I only use self-biased concertina splitters with proper grid-stops and provide local filtering for each stage right at the circuit. Menno stressed the importance of having a high-value filter for the concertina since on signal is referenced to the supply.
Having multiple gain stages and/or cascaded stages all running from the same supply node can be problematic or not. It depends on how things are laid out. It is always preferred to have a local filter cap for each stage even if it is effectively a common supply node.This filter cap and its physical wiring are more important than resistive isolation of the supply nodes for each stage; the cap proximity is what gives "power" to decoupling by providing the return path for the signal current of the related stage - it is the decoupling.
