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OTL2014  develop at begin of 2014


The main characteristics are:


This design is inspired to these design:

The purpose is always to drive the final valves with two drivers with equal reference ground to configure both final valves as cathode follower.

If we compare this circuit with the Futterman here the advantage is to have a driver that amplifies but here the disadvantage is the necessity to require a large value capacitor on the signal path (see C7 = 47uF).

In this case, I did not built this amplifier from scratch, I modified an existing one with some limits and problems.

It use the 6336A output tubes but also the 6c33c-b coudl be used without problems.


The final circuit used also in the LTspice simulation.

The following version is less perfect but it obtain the same result.

It use 8 x 2200uF 450V capacitors and there are 4 x 10uF 630V Solen MKP to bypass the output and the power supply.


Each channel have a separated power supply.

The main secondary should be 230-240V and 2A min.

The following is the negative power supply at -230V, the secondary should be 170-180V and 200mA min.

Adjust R1 to get the target voltage -230V because depend by secondary resistance.

The -50V for the bottom output tube bias are obtained from -230V with 2 x 2W resistors and one trimmer.



Follows some measurement of distortion decay on 8ohm load, 16W with 1.1% thd.

Follows the frequency response at full power on 8ohm load, dual channel with sound card compensation, there is some noise received from cables.




The output capacitor protect the loudspeakers but to eliminate a little bump on switch-on and switch-off use a loudspeaker protection with delay.