atmasphere
Active Member
There are three basic output topologies for tube amps: Single Ended (SE), Push-Pull (PP), and "Output Transformer-Less" (OTL). OTL amps do not use an output transformer, are fairly exotic due to the number and size of tubes requires, and other factors. They are worthy of mentioning, but beyond the scope of a document aimed at a novice. .
Not sure I agree with this but of course I'm known as a proponent of OTLs. So maybe I can offer some information:
OTLs can be SE or P-P. They can be class A, A2, AB1, AB2 or B. They can be made with or without global negative feedback. They may occur in triode, tetrode or pentode embodiments (although I don't actually know of any tetrode examples right now).
OTLs can have adjustable bias, self bias or autobias.
So they can cover a similar range of topologies as transformer-coupled amps, only there is no transformer. To do this typically an OTL will have lower plate voltages (usually not over 175Volts) and higher currents. This means that the power tubes have to have capabilities of linearity at lower voltages and higher currents. There are 3 tubes currently (pardon the pun) in common use: the 6AS7, the 6C33 and the PL509 or variants thereof.
Quite often OTLs have a higher output impedance and so often need higher impedance speakers but this is not set in stone. The OTLs I use drive 8 ohms with ease, and I know of one OTL that makes its maximum power into 3 ohms. If enough feedback is applied, OTLs can behave like a Voltage Source, like any other amp. However, OTLs tend to have a lot of bandwidth so feedback has to be applied carefully. Too much and the amp might become unstable. There was enough of this in the old days that many people are convinced that OTLs are inherently unstable or unreliable, and neither is true- they are just as stable and reliable as any other tube amp if built correctly.
The advantage of OTLs is that without a transformer, they have more bandwidth and less distortion and so are more transparent. The OTLs I play have 20 Hz squarewave tilt that is unmeasurable. No other tube amp topology can make that claim. This means that OTLs can be considerably more transparent than conventional amps, something that is brought out in reviews on a very consistent basis. Contrary to popular myth, the larger number of power tubes does not seem to contribute to distortion, in fact it appears that as you add power tubes the distortion actually goes down. So then it becomes a matter of managing the tubes, but IME this is not particularly difficult- that amps I use don't even use matched tubes and have automatic bias.
Another often-cited concern is speaker safety. OTLs can be built to be as safe with speakers as any decent transistor amplifier. IOW they are not any more dangerous to a speaker than a transistor amplifier, assuming once again that it is built correctly, but you would have to assume that about a transistor amp too. The reason I chose to compare to a transistor amp is that OTLs use either an output coupling cap or they are direct-coupled. So it makes more sense to compare them to transistors in this regard.
In short, OTLs should be on table with all other tube amp types as an option for the newbe that is considering a tube amp. They are about the most tube you can get