ECC82 directly driving headphones

At your risk. Take all safety precautions. Also always use a suitable fuse in series with a battery for safety and fire hazard protection.


The Pleiades Electra 3 schematic was used which is very similar to the Pleiades V6 schematic. The Pleiades V6 is intended as a low noise mic booster amplifier. The Pleiades Electra is intended as power amplifier. They are virtually identical circuits. Both have a pull up bias resistor from anode to grid to make operation possible at low plate voltage. See Pleiades Bias.


Today an old, (before 2000) unused Pleiades amplifier aluminum box was modified to include two 10MΩ anode to grid resistors. The intention is to direct drive magnetic tape recording heads with an ECC83.


Other modifications... Cathodes are connected to ground. Grids are connected to 2.7nF for DC isolation. The above mentioned 10MΩ from anode to grid, making the grid less negative, ie pull it up out of cutoff.


Why is the open grid to cutoff? Because the cathode is emitting electrons to form an electron cloud between cathode and grid which repels electrons. The cathode is positive with respect to grid since protons are left behind inside the cathode. Therefore the grid is negative with respect to cathode.


The headphones are the load themselves, with DC passing through them.


Signal path:


The song: Is it OK if I call you mine? - Gore, from CD - Sony CD Wlkman line out - Pleiades V6 - Sennheiser HD580


Before connecting anything to the anodes, current was measured between anode and Vb.


At Vb=10.5V and ECC83, Ia was approx. 20μA. The headphones were connected and a very faint sound but of good but distorted sound quality (a lot of distortion could be heard).


Then Vb was changed to 24V, marked improvement, 36V even better, 48V even better. At 48V anode
current is of the order of 350μA. Great quality sound with amazing treble. This may be used to direct drive tape recording heads. Magnetic heads need to be driven by a high output impedance and want voltage headroom for HF. (Constant current drive with respect to frequency since a head is an inductor whose impedance increases with frequency). It is amazing how the ECC83 with an output impedance of the order of 50kΩ could drive 300Ω headphones with power loss of course but a clear sound.


Then the ECC82 was plugged in. At Vb=10.5V, Ia is already 400μA. So the amplifier operates even at 12V.


At 24V and 36V the sound improves together with sensitivity and max power. At 48V where current is 3mA it may already be too much.


A triode connected EF183 ekectron tube gives 1.5mA at Vb=12V. And 2.5mA with Rag of 1.5MΩ.


Would E88CC operate well with Vb=12V or Vb=24V? Ia with Rag=10MΩ and Vb=12V is 300μA, at Vheater=5V.


How would ECC82, ECC83, E88CC perform with Rag=1.5MΩ?