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12-29-2024, 02:41 AM
Hi Guys
Oscillation is generally caused by poor layout and lead dress. The original Bogen circuit lacks grid-stops, screen-stops and sufficient filtering, then incorporates "bandaids" to stabilise the circuit - low-value caps across plate resistors and other roll-offs to tame the circuit. The engineering ideal does not require any of these, but reality says they are needed.
The grid-stop values you have added may not be sufficient for the specific conditions within this chassis. There is no need to reduce the grid-leak value when adding a grid-stop, particularly with preamp tubes.
"Cathodyne voltage doubling distortion"?
Microphonic tubes can be tamed using an O-ring or a simple elastic band placed in line with the internal mica supports. This will at least last ling enough for you to determine if the tube status is significant to the oscillation problem.
The Bogen schematic shows some signals wired to the terminal strip which may become problematic for the guitar amp application. These extra paths may allow signals to interact that should not, and which may lead to oscillation.
Yes, the OT will physically vibrate with the signal and you can hear it when using a bench load. This is caused by magnetic leakage due to loose windings and imperfect magnetic shielding of the core - something that is always imperfect with non-toroidals.
Doing "as good of a Galactic ground as the terminal strips allow" does not sound like an actual Galactic Ground was achieved. A major part of this is how the filter caps tie in to the circuit. It may be impractical to achieve a straight-line signal path in some situations and this may require circuit amendments if gain has been increased from stock. Note that in the case of older amplifiers repurposed for instrument use, the way you use the amp is out of spec for the original circuit inasmuch as the intent was to run the amp clean, well below clipping. Running stages beyond their linear range may cause oscillations even without cascading things that were not previously cascaded.
Oscillation is generally caused by poor layout and lead dress. The original Bogen circuit lacks grid-stops, screen-stops and sufficient filtering, then incorporates "bandaids" to stabilise the circuit - low-value caps across plate resistors and other roll-offs to tame the circuit. The engineering ideal does not require any of these, but reality says they are needed.
The grid-stop values you have added may not be sufficient for the specific conditions within this chassis. There is no need to reduce the grid-leak value when adding a grid-stop, particularly with preamp tubes.
"Cathodyne voltage doubling distortion"?
Microphonic tubes can be tamed using an O-ring or a simple elastic band placed in line with the internal mica supports. This will at least last ling enough for you to determine if the tube status is significant to the oscillation problem.
The Bogen schematic shows some signals wired to the terminal strip which may become problematic for the guitar amp application. These extra paths may allow signals to interact that should not, and which may lead to oscillation.
Yes, the OT will physically vibrate with the signal and you can hear it when using a bench load. This is caused by magnetic leakage due to loose windings and imperfect magnetic shielding of the core - something that is always imperfect with non-toroidals.
Doing "as good of a Galactic ground as the terminal strips allow" does not sound like an actual Galactic Ground was achieved. A major part of this is how the filter caps tie in to the circuit. It may be impractical to achieve a straight-line signal path in some situations and this may require circuit amendments if gain has been increased from stock. Note that in the case of older amplifiers repurposed for instrument use, the way you use the amp is out of spec for the original circuit inasmuch as the intent was to run the amp clean, well below clipping. Running stages beyond their linear range may cause oscillations even without cascading things that were not previously cascaded.
