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RE: Modified Futterman OTL amplifier

81.216.133.244

In general Spice simulation of distortion is difficult and with 6C33C it is even more difficult. The main problem is to find a good enough model that resembles the real characteristics of the tube, the 6C33C exhibit some unusual behavior with a kink in the Ia characteristics at low currents and this is difficult to simulate in a model. I have used all available models for spice simulation of the 6C33C that I have found and no one give results that are accurate for distortion, the best I have found so far is from here http://www.excem.fr/download/usergui5.pdf

It is difficult to see from the schematics that you have posted but as far as I can see you have not included any balance compensation in the driver stage. As I wrote earlier a SEPP is unbalanced and need a special driver arrangement for compensation, the most well known is the Futterman. In the Futterman with split load phase inverter the cathode resistor is returned to the output rather than ground and this will compensate for the unbalance, but I can' see that you are using that circuit, (sorry but the resolution of the scematics is not good enough to see clearly).

In my own OTL where I use the Futterman connection the 2nd order product are 55dB down before feedback, I haven't removed the Futtterman connection and measured but it would certainly be much worse.

Regards Hans

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