Roger,
There seem to be three popular methods of OTL. Futterman, Circlotron, and Rozenblit. Could you please comment on them ( for educating us) and the popular tubes used? There probably are only about a dozen people in the country that understand OTL amps. Thank you.
steve
Other than a few very low power single ended designs, OTLs are push-pull and usually not class A though they do, as all push-pull amps, have a class A region. The class A region may be just a few watts on a 100 watt OTL. More about that later. A single ended OTL is hampered in that the output current will be only twice the standing current. As the standing current is likely to be limited to 0.2 amps per tube the peak output power is that squared times the 8 ohm load resulting in 0.32 watts per tube. More tubes, more power. As you double the number of tubes you quadruple the power, hence it is tempting to have a lot of tubes.
In all push-pull amps there is a tube or bank of tubes for the positive half cycle and the same for the negative. The Futterman amps employ a single power supply around 300 Volts with the negative end on ground. He also employs a large output coupling capacitor (1000 uF) to couple the 150 VDC at the output point to the speaker.
In the Futterman the output tubes are arranged as a "totem pole" one above the other with 150 V at their junction. The top tube can pull up to 300 volts and the bottom can pull down to ground. The amount of current is about 1.5 amps per tube. The tubes idle at about 100-200 mA per tube resulting in 15-30 watts dissipation per tube. To get more power just put more tubes in parallel. I used 4 in parallel per bank in the SA-4.
In the SA-4 and the designs I am working on now I use a split supply of plus 150 and minus 150 V (referenced to ground) and direct couple to the speaker thus eliminating the output capacitor making it a DC amplifier. To keep it centered (zero offset) I use a servo and a protection circuit should a tube short or large DC appear at the output.
The Atmasphere amps are a Circlotron configuration which is essentially a bridge with two floating power supplies of about 135 Vdc each neither referenced to ground. The output has a virtual ground at center of the speaker (if it had one). The original Electrovoice amp had this ground on the center tap of it's output transformer. The EV amps had an output transformer which Ralph eliminated by using a great number of tubes in parallel. I have worked on a MA-1 which has 12 output tubes per channel. Although Ralph feels tube matching is not important, I did match the output tubes so that they shared current equally. Otherwise some ran hot and others cool, shortening the life of the hot ones.
I note that the Atmasphere S-30 has an odd number of output tubes per channel. I have not seen a schematic of this amp and would be interested in looking at one. I suspect the odd tube may be the driver for the other parallel output tubes and Ralph may be going into positive grid voltages to get more power. Any comments?
As to Rozenblit's cicuits, the early ones I saw in Audio Amateur and Glass audio were of totem pole (Futterman) design but he was trying to improve on the driver. There were several mistakes in that article. I don't know what he is doing now. Again, a schematic or perhaps the patent number would help.
Poll