How about low voltage tubes such as ECC86, EF98, EF87?

For some strange reason on Pleiades experiments they have not worked quite as well or as expected.


They did work and but not as well as ECC82, EF183 triode connected, Nuvistor 7586 with pull up bias usually called Pleiades bias on these posts.


Many circuits with single ECC86 had been made before 2000. The ordinary way. Anode load resistor, cathode bias resistor, 6V power supply for both heaters and anode. Input and output coupling capacitors.


They do not sound as dynamic, high gain etc as Pleiades V4, V6 for example with EF183 biased with a 6MΩ resistor from anode to grid.


But they showed to way...


A headphones amplifier had been built with ECC86 after 2000. It was the first Pleiades headphone amplifier with 1 electron tube per channel. The Hi Z Sennheiser HD580 were connected instead of cathode resistors.


The circuit worked with ECC86 but output was low. It was immediately increased by inserting an old 1.5V coin battery for each channel, the plus side at the grid instead of a coupling capacitor. The sound immediately increased. This circuit must be the first example of pull up bias used on a Pleiades circuit. Or was it the Pleiades V1 electrometer microphone headamp?


In later years a similar pull up battery bias circuit on electron tubes for headphones was found on the internet but the link cannot be remembered, nor the designer.


It must be investigated why special made electron tubes for low voltage leave something to be desired. What was done wrong on Pleiades circuits? In fact the ECC86 or E88CC do not operate white as well as other more common electron tubes even with pull up resistor bias. Why?


Does the grid become too much negative as soon as the cathode is emitting electrons?


Such measurements have not been done yet.


EF183, 6SK7, 6K7,  EF37A, EF86 triode connected immediately looked more promising in terms of anode current or gain and this is the reason the EF183 and ECC82 were immediately chosen for the Pleiades V series microphone or instrument head amps.


In fact it was the old ECC86 small box constructions where E88CC, ECC83, ECC82, ECC81 were connected and the ECC82 made immediately a mark in terms of performance and low noise at low (6V) anode suplly.


Now many of these circuits are not only with the cathode connected to ground but also with a pull up resistor to create Pleiades bias from anode to grid. A 10MΩ or less pull up resistor is used possibly bringing the electron tube to the straight partbofnthe characteristic. The gain is immediacy higher and there is low noise. EF183 or ECC82 are used in those old modified shamal box constructions.


They sing.