The 6C33 is a nice sounding tube. The real issue with it is 2-fold:1) its been out of production for about 20 years. The tooling was destroyed with the plant that made it so there won't be any reissues.
2) the tube was designed to use the socket for the Sylvania 3C33, which is a dual power triode with less than half the filament current. As a result, the 6C33 overloads the socket connections which can fail within the lifetime of the tube itself. The sockets that hold up the best are NOS; the Chinese socket which is the most common is not very reliable. As a result special precautions have to be taken to extend the socket life but sooner or later when the socket fails it simply has to be replaced. This may not be a trivial matter depending on how the amplifier is built. Because of the heat involved circuit boards should be avoided in the output section.
A further issue that is a common problem with OTLs is how the output tubes are biased. Because with any OTL multiple output tubes are used, the grid capacitance tends to be quite high in the output section. This in turn leads to a fair amount of grid current compared to higher voltage power tubes. The result of all this is that controlling the output tubes properly has been a traditional problem in OTL design which has resulted in the reputation that OTLs are unreliable (in a good number of cases this is well justified). This is caused by the fact that to get bass out of the amp, larger coupling capacitors have to be employed in the output section, but the bigger you make them the more coloration they have. This is countered by the fact that the bias network has to employ fairly low values in order to actually control the power tubes (re. the grid current I previously mentioned); because these two factors run counter to each other, designers often fudge the numbers a bit- and often run into issues like blocking distortion (as seen in the Joule Electra amps, which will literally shut down for a few seconds if overloaded) or bias drift, such as seen in the Counterpoint SA-4. When the amp is properly designed, you should be able to easily overdrive it all day long without the amp oscillating (regardless of the speaker load), bias going off or anything like that- and it should recover instantly from overload.
When these factors are right, the amp is simultaneously more transparent and will have greater authority.
2) the tube was designed to use the socket for the Sylvania 3C33, which is a dual power triode with less than half the filament current. As a result, the 6C33 overloads the socket connections which can fail within the lifetime of the tube itself. The sockets that hold up the best are NOS; the Chinese socket which is the most common is not very reliable. As a result special precautions have to be taken to extend the socket life but sooner or later when the socket fails it simply has to be replaced. This may not be a trivial matter depending on how the amplifier is built. Because of the heat involved circuit boards should be avoided in the output section.
A further issue that is a common problem with OTLs is how the output tubes are biased. Because with any OTL multiple output tubes are used, the grid capacitance tends to be quite high in the output section. This in turn leads to a fair amount of grid current compared to higher voltage power tubes. The result of all this is that controlling the output tubes properly has been a traditional problem in OTL design which has resulted in the reputation that OTLs are unreliable (in a good number of cases this is well justified). This is caused by the fact that to get bass out of the amp, larger coupling capacitors have to be employed in the output section, but the bigger you make them the more coloration they have. This is countered by the fact that the bias network has to employ fairly low values in order to actually control the power tubes (re. the grid current I previously mentioned); because these two factors run counter to each other, designers often fudge the numbers a bit- and often run into issues like blocking distortion (as seen in the Joule Electra amps, which will literally shut down for a few seconds if overloaded) or bias drift, such as seen in the Counterpoint SA-4. When the amp is properly designed, you should be able to easily overdrive it all day long without the amp oscillating (regardless of the speaker load), bias going off or anything like that- and it should recover instantly from overload.
When these factors are right, the amp is simultaneously more transparent and will have greater authority.