Well, one weird turn deserves another.
This amplifier output stage takes the Graeme Cohen design to upside down.
This "Twilight-Zone" amplifier takes a P-P output stage and splits it into two SE amplifier outputs. It even looks practical and economical. It may be pointless, but that remains to be seen. Ordinary P-P OTs are used ( no gaps ), so some cheap speaker 70V distributon Xfmrs could be used.
The batteries shown as B1 and B2, or floating supplies, are low voltage. They are between two points with the same nominal AC voltage, but they insert DC current to just null the tube DC currents in the primary windings ( DC current = Vbatt / Rwinding ). Since each tube will have some small opposite SE distortion components, the small inductors L1 and L2 are inserted to isolate the difference. These could be just a string of ferrite beads, and could be bifilar ( two wires thru the beads ).
If the design is stretched to amplify two -different- ( L and R ) SE signals, the L1 and L2 inductors could be something like a 1000:1000 Ohm bifilar interstage xfmer possibly. They have to handle the DC tube compensation currents, while isolating the AC signal differences.
This amplifier output stage takes the Graeme Cohen design to upside down.
This "Twilight-Zone" amplifier takes a P-P output stage and splits it into two SE amplifier outputs. It even looks practical and economical. It may be pointless, but that remains to be seen. Ordinary P-P OTs are used ( no gaps ), so some cheap speaker 70V distributon Xfmrs could be used.
The batteries shown as B1 and B2, or floating supplies, are low voltage. They are between two points with the same nominal AC voltage, but they insert DC current to just null the tube DC currents in the primary windings ( DC current = Vbatt / Rwinding ). Since each tube will have some small opposite SE distortion components, the small inductors L1 and L2 are inserted to isolate the difference. These could be just a string of ferrite beads, and could be bifilar ( two wires thru the beads ).
If the design is stretched to amplify two -different- ( L and R ) SE signals, the L1 and L2 inductors could be something like a 1000:1000 Ohm bifilar interstage xfmer possibly. They have to handle the DC tube compensation currents, while isolating the AC signal differences.
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Looks like one can go even further. Use just one P-P OT with the two tubes as normal and put the L1, L2 inductors in series with the tube plates. No batteries this time. L1, L2 get changed to very small xfmers with the same turns ratio (to secondary) as the main OT. The L1 and L2 still absorb the distortion differences between the tube outputs (class A ), but then their low turn secondary is put in series (separately) with the main OT secondary to re-form the two SE outputs again. They can be low turns for the same stage input signal, since they only carry the SE distortion component voltages. Heavier wire for the secondary side wire.
For the case of two separate ( L and R ) input signals, one is just back to the old classic econo scheme of a small OT to carry the difference signal to the main OT channel, and L and R are reconstituted at the main OT secondary by adding/subtracting the difference channel secondaries. Usually found in some stereo consoles.
The difference channel was often done as a small SE Amp, and sometimes combined with the main OT using a gapped center leg E lamination. P-P around the outside path and SE thru the center leg. Not practical for DIY though.
Two birds with one stone.
For the case of two separate ( L and R ) input signals, one is just back to the old classic econo scheme of a small OT to carry the difference signal to the main OT channel, and L and R are reconstituted at the main OT secondary by adding/subtracting the difference channel secondaries. Usually found in some stereo consoles.
The difference channel was often done as a small SE Amp, and sometimes combined with the main OT using a gapped center leg E lamination. P-P around the outside path and SE thru the center leg. Not practical for DIY though.
Two birds with one stone.
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Thanks fellow DIYers; I'm beginning to understand.
For the relatively small advantage of making something useful out of bits someone has lying around and might as well dispose of in something other than the trash can / metal recycle house, the disadvantages win.
1. The doubled up or at least higher power supply voltage value. However that comes about, it is an offsetting extra expense.
2. An inductor in the plate circuit. A good inductor. Or a SS constant current source, or large value resistor or tube current source with its own isolated heater power. If that part cant be made simply from another distribution transformer primary winding, its a significant offset. Plus that part burns a lot of energy; managing that heat offsets any hoped for "savings".
3. Mark mentioned that - in a guitar amp context - perhaps a semi-saturated iron core sounds way different than a core at zero on its BH curve. An "all for nought" possible outcome is a significant offset if building a guitar amp this way.
4. Thanks for the link to the Broskie article! Sounds like PSRR is an issue with the Parafeed and any expense in dealing with that, or implementing some feedback trick to get rid of it is an offset to any "economy" in making use of these particular parts. Although same is true using a proper SE OPT I believe.
For the relatively small advantage of making something useful out of bits someone has lying around and might as well dispose of in something other than the trash can / metal recycle house, the disadvantages win.
1. The doubled up or at least higher power supply voltage value. However that comes about, it is an offsetting extra expense.
2. An inductor in the plate circuit. A good inductor. Or a SS constant current source, or large value resistor or tube current source with its own isolated heater power. If that part cant be made simply from another distribution transformer primary winding, its a significant offset. Plus that part burns a lot of energy; managing that heat offsets any hoped for "savings".
3. Mark mentioned that - in a guitar amp context - perhaps a semi-saturated iron core sounds way different than a core at zero on its BH curve. An "all for nought" possible outcome is a significant offset if building a guitar amp this way.
4. Thanks for the link to the Broskie article! Sounds like PSRR is an issue with the Parafeed and any expense in dealing with that, or implementing some feedback trick to get rid of it is an offset to any "economy" in making use of these particular parts. Although same is true using a proper SE OPT I believe.
