Rafael, no I do not agree with all his comments, although at at an earlier time some were true.
For example, the comment
applies to Futterman amplifiers only. The Circlotron output circuit (we were the first to use this in a practical real-world OTL) eliminates this problem in both tube and transistor circuits, allowing one to use non-complementary pairs (and for the record, complementary transistor pairs, such as NPN and PNP are never exact matches, so the argument is really a red herring).
I also have to clarify something about this statement:
There *are* tubes that have low plate resistances. The 6AS7G is an example, as is the 6C33. So in an OTL, you will not find anyone using 6550s or EL34s! It is true that you still have to parallel tubes, but of any higher power amp that is a fact of life. Kirkus is correct about the filament issue, although the filaments often get blamed for excess heat, which they are *not* responsible for. That comes from the class of operation.
Comment #3 applies to Futtermans OTLs only. Circlotrons don't use a split-voltage supply, nor is there any need for an output coupling capacitor. Many people think that if its an OTL, it has to be set up like the old Futtermans, which do have these 'design features' but that is not true.
Comment #4... not IME; many customers of ours have commented on the fact that they can set the DC Offset of the amplifier and it will be exactly right 6 months later. The trick (and you would think this is obvious) is to be able to control the power tubes.
Comment #5 was never true- even the old Futterman amplifiers from the early 60s had slew rates far in excess of their transformer-coupled counterparts. We've measured slew rates of 600V/micro-second in the output section of our amps. This translates to extremely wide bandwidth. Our early prototypes exhibited this trait right away- they made very capable RF booster amplifiers at frequencies as high as 50MHz without oscillation. In our production amps we limit the bandwidth in the driver circuit to minimize RF issues, but the output section retains its speed.
For example, the comment
1. The transconductance characteristics of vacuum tubes operated in an OTL push-pull fashion is both inherently non-conjugate and non-complimentary - essentially similar to a the "all-NPN" solid-state amplifier designs of the early-1970s.
applies to Futterman amplifiers only. The Circlotron output circuit (we were the first to use this in a practical real-world OTL) eliminates this problem in both tube and transistor circuits, allowing one to use non-complementary pairs (and for the record, complementary transistor pairs, such as NPN and PNP are never exact matches, so the argument is really a red herring).
I also have to clarify something about this statement:
2. The plate resistance of virtually all vacuum tubes is WAY too high for efficient power transfer to a typical loudspeaker load. Paralleling a bunch of output tubes is the usual solution, and power-efficiency of OTLs is still very poor, even worse with all of those filaments to run. Now when direct-coupling to electrostatics, it's a whole different story . . .
There *are* tubes that have low plate resistances. The 6AS7G is an example, as is the 6C33. So in an OTL, you will not find anyone using 6550s or EL34s! It is true that you still have to parallel tubes, but of any higher power amp that is a fact of life. Kirkus is correct about the filament issue, although the filaments often get blamed for excess heat, which they are *not* responsible for. That comes from the class of operation.
Comment #3 applies to Futtermans OTLs only. Circlotrons don't use a split-voltage supply, nor is there any need for an output coupling capacitor. Many people think that if its an OTL, it has to be set up like the old Futtermans, which do have these 'design features' but that is not true.
Comment #4... not IME; many customers of ours have commented on the fact that they can set the DC Offset of the amplifier and it will be exactly right 6 months later. The trick (and you would think this is obvious) is to be able to control the power tubes.
Comment #5 was never true- even the old Futterman amplifiers from the early 60s had slew rates far in excess of their transformer-coupled counterparts. We've measured slew rates of 600V/micro-second in the output section of our amps. This translates to extremely wide bandwidth. Our early prototypes exhibited this trait right away- they made very capable RF booster amplifiers at frequencies as high as 50MHz without oscillation. In our production amps we limit the bandwidth in the driver circuit to minimize RF issues, but the output section retains its speed.