The tube for the input section is the double triode ECC88 or 6DJ8. Better versions are the E88CC, CCa or 6922 or even with less noise the E188CC or 7308. There are many brands NOS and from current productions available. I have good experience with Philips ECC88 NOS and JAN-Philips 6922.
This amplifier (which I call the VR2) is the follow-up of my previous HeadWize project (Tube Headphone Amplifier/Preamp with Relay-Based Switching) and has been designed with following conditions having in mind:1) Pure Class-A triode OTL design and only one tube for amplification.
2) The plate voltages should be low.
3) The output impedance should be as low as possible and the maximum output current should be as high as possible. This amplifier is a stand alone Headphone amplifier without the property of input selection as mentioned in my previous project but could be expanded if desired.
ad 1) Preferring the Class-A OTL design I decided again to implement a long tailed pair for the input section and a parallel connected cathode follower for the output section.
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ad 2) The voltages should be as low as possible. This condition was taken into account mainly because the risk of any high voltages at the output of the amplifier, if any damage occurs, should be kept as low as possible. This could be the case if the output capacitor shortens and then the full cathode voltage would be connected direct to the output, damaging the headphone or there would be the risk of touching lethal voltages! In my opinion it’s better that this voltage is 40V and not 150V or even higher.
ad 3) Low output impedance and a relatively high current for the output sections are needed to drive headphones and in connection with the condition 1) and 2) only a few tube types are suitable.
These conditions could be fulfilled very good with the 6DJ8, ECC88 tube type family. An operating point with a plate voltage of only 80V is well between the limits and the gain µ of 33 is well enough. For the parallel connected cathode follower the E288CC is well suited, delivering higher currents and an output impedance of 25 Ohm. The voltage at the output coupling capacitor could be with this tube about 40V.
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