The story about the reliable low distortion OTL amplifier
Here I am going to describe something about how we designed our unique OTL amplifiers.
OTL amplifiers have always had a reputation for excellent sound, the idea to be able to remove the output transformer with its inherent imperfections in a tube amplifier make it possible to design an amplifier with very high bandwidth and stability even with relatively high level of feedback.
However, How do you really design an OTL amplifier with high reliability? Reliability is not what you spontaneously think about when you hear the acronym OTL. OTL amplifiers have often instead been synonymous with low reliability, using many tubes and very low efficiency.
Design criteria 1:
Use as few tubes as possible and don't overload them.
Most commercial OTL amplifiers have many parallel connected tubes, (sometimes 16 or even 32) in order to provide enough output power. These tubes have often been biased at maximum of their specification or even beyond that. As it is difficult to get that many tubes that are closely matched, these amplifiers use unmatched tubes which are bound to have different performance and thereby draw different current so some live a very easy life while others are very overloaded from the start. When one of these overloaded tubes inevitably fails, the other tubes will also be overloaded and this start a chain reaction often taking out several or all tubes at one time.
To use matched tubes in a an OTL using 16 or more is not a practical solution, it is a monumental task to match 16 tubes close enough.
A much better way is therefore to minimise the number of output tubes, To find a pair or a quartet of tubes that are matched is of course much easier than to find 16!
Design criteria 2:
Use the circuit with lowest output impedance
solution for being able to use few tubes is both to find the circuit
that can give the lowest possible output impedance and to find the
most potent output tube available.
The OTL circuit giving the lowest output impedance is the so called inverted Futterman configuration which gives an output impedance which is less than half of the commonly used Circlotron.
The most potent output tube available today at a reasonable price is the Russian 6C33C. This tube is able to withstand continuous anode currents of 600mA with peaks of more than 2.5A while giving excellent service life. Our PA12 OTL have only 2 tubes of type 6C33C to provide 25W of output power at less than 1% distortion, the tubes are biased for an idle dissipation of 30W which is only 50% of the allowed maximum thereby giving excellent reliability.
The first prototype OTL we designed with this tube is still equipped with the original 6C33C tubes which are still working well despite being used since 2004.