E88CC with Pleiades bias directly diving headphones

At your risk. Protect your ears. A suitable fuse should always be used in series with a battery for fire hazard protection.


Signal path:


Sony CD Walkman line out - Pleiades Electra 3, E88CC Mullard - Sennheiser HD580


It sounds spectacular.


Cathodes connected to ground. 10MΩ resistors from anode to grid to reduce grid negative space potential. 2.7nF coupling capacitors to grid so as not by upset DC potential of grid. Vheater=5V (so PCC88 could be used with 6.3V?).


Headphones directly connected to anodes. Vb connected to headphones ground (an insulated chassis headphone socket is used).


At Vb=12V (actually 10.4V as battery had discharged) there is some distortion as anode current is 300μA.


At Vb=24V E88CC nails it. At Vb=36V it may already be too dangerous for human hearing. Even 24V may be too much, it is not known.


The sound is big and amazing. Great bass, mid and treble. This is what electron tubes are for.


Would E88CC operate well at 12V with a lower anode to grid resitor? How would the sound compare to EF183 triode connected? How would ECC86 sound? How would vari μ ECC189 sound?


ECC86 almost nails it with just Vb=12V (10.4V), Big sound. Vh=4.7V. Rag=10MΩ. At such conditions Ia=1mA, Vg is pulled up to approx -300mV. Further cathode under heating sounds nice too, gentle peak distortion seems decreasing as sensitivity is decreased too, so rails are less often approached when music is playing. At Vb=24V, Ia=4mA, (anode dissipation 24Vx4mA=96mW so theoretically 48mW max output if there is impedance matching) much louder, possibly hearing unsafe.


ECC86 was tried with lower Rag resistors ie approx 1MΩ. At Vb=5V, it is amazing how much distortion is reduced by changing from 10MΩ to 1MΩ. At Vb=12V, Ia becomes 2-2.5 mA. Great sound but there is some lack of bass. Possibly due to reduced input impedance so the Cc capacitors have to be increased.


ECC82, 12AU7 could operate at Vb=12V, Vh=12V or less and Rag=10MΩ. Decreasing Rag increased Ia but bass again reduced, Cc must be increased? At Vb=24V, Rag=10MΩ the sound is very nice.


E88CC could operate too at Vb=10.5V with Rag of the order of 1MΩ. Similar perhaps problem with bass.


Gut feeling is that the best sound so far on Pleiades Electra driving headphones is E88CC at Vb=24V, Rag=10MΩ. Very big sound. Protect your hearing by listening at low SPLs.


Next the ECC83 will be plugged in as the objective of these recent Pleiades experiments is direct driving a tape recording head with the usual needed high output impedance for constant current drive of the head coil, ie increasing treble with frequency. See nearby posts.