31-10-2016, 03:56 PM
(This post was last modified: 31-10-2016, 04:11 PM by Mike Watterson.)
The SG resistor seems high, but I'd need to check.
Really the only reason for SE was reduced cost. Push Pull, (even if class A!) cancels out the first even harmonic and means any suitable dual winding 120 / 240 (110 /220) mains transformer can be used as Audio out as the DC bias currents cancel. Voltage ratio of one winding to speaker equals Square Root of Impedance ratio. PP also reduces ripple out output. Add extra Resistor and 32uF cap for preamp and output screen grid (with then no resistor) HT supply
The other half of the ECC82 can be the "gain = -1" phase inverter for the second EL84.
NFB from speaker to driver amp input is best, though you can simply leave of cathode resistor decoupler (SE or PP), and use one resistor for both EL84 if PP.
A SE transformer if not an Audio type (a gap) may even saturate at bias current.
Really the only reason for SE was reduced cost. Push Pull, (even if class A!) cancels out the first even harmonic and means any suitable dual winding 120 / 240 (110 /220) mains transformer can be used as Audio out as the DC bias currents cancel. Voltage ratio of one winding to speaker equals Square Root of Impedance ratio. PP also reduces ripple out output. Add extra Resistor and 32uF cap for preamp and output screen grid (with then no resistor) HT supply
The other half of the ECC82 can be the "gain = -1" phase inverter for the second EL84.
NFB from speaker to driver amp input is best, though you can simply leave of cathode resistor decoupler (SE or PP), and use one resistor for both EL84 if PP.
A SE transformer if not an Audio type (a gap) may even saturate at bias current.
