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04-27-2023, 05:20 PM
Hi Guys
Drew, it would be easier with a schematic of either the starting circuit or the result you want to get to.
To address the last post first: Isolating a cathode bypass cap (Ck) requires a resistance in series with it that is 10x the Rk value. Going higher than this in the form of a pot makes most of the sweep of the pot ineffective.
Keep the top of the cap tied to the top of Rk and switch or dial the negative end of the cap to ground. If using a switch, add a resistor across the switch to eliminate thumps - again, 10x Rk min for the new resistor.
Feedback around tube stages: note that each triode GAIN stage is inverting, and there is only one (general) way to apply feedback around a single stage. There is already feedback around each gain stage in the form of internal capacitance of the tube along with parasitic capacitance from wiring. There are in picoFarads, but they roll off very high frequencies. TUT Fig.4-8 shows the frequency response change made by this capacitance as it works against the grid-stop resistor.
If you add a high-value resistor from anode to grid on a triode stage, the new R works along with the internal capacitance to form another high-frequency roll-off. You could use a low-value cap instead and have predictable high-frequency filtering, good for an input stage. The feedback resistor or cap can be tied in after the plate-coupling cap to avoid DC issues. Similarly, the cap can be a shunt to ground, simply loading the triode at high frequencies.
Feedback around two triodes provides reasonably good gain control, better applied in bass amps than for guitar. Generally, guitar players want the harmonics from free-running stages working at medium to high signal levels with resp[ct to the stage's transfer curve for its given voltage environment. The absolute voltages do not have to be especially high, but everything factors in to the sound you want.
Drew, it would be easier with a schematic of either the starting circuit or the result you want to get to.
To address the last post first: Isolating a cathode bypass cap (Ck) requires a resistance in series with it that is 10x the Rk value. Going higher than this in the form of a pot makes most of the sweep of the pot ineffective.
Keep the top of the cap tied to the top of Rk and switch or dial the negative end of the cap to ground. If using a switch, add a resistor across the switch to eliminate thumps - again, 10x Rk min for the new resistor.
Feedback around tube stages: note that each triode GAIN stage is inverting, and there is only one (general) way to apply feedback around a single stage. There is already feedback around each gain stage in the form of internal capacitance of the tube along with parasitic capacitance from wiring. There are in picoFarads, but they roll off very high frequencies. TUT Fig.4-8 shows the frequency response change made by this capacitance as it works against the grid-stop resistor.
If you add a high-value resistor from anode to grid on a triode stage, the new R works along with the internal capacitance to form another high-frequency roll-off. You could use a low-value cap instead and have predictable high-frequency filtering, good for an input stage. The feedback resistor or cap can be tied in after the plate-coupling cap to avoid DC issues. Similarly, the cap can be a shunt to ground, simply loading the triode at high frequencies.
Feedback around two triodes provides reasonably good gain control, better applied in bass amps than for guitar. Generally, guitar players want the harmonics from free-running stages working at medium to high signal levels with resp[ct to the stage's transfer curve for its given voltage environment. The absolute voltages do not have to be especially high, but everything factors in to the sound you want.
