OTL amps, my pick for the tube used.

Those who are fans of the Futterman H3aa should keep an eye out for a new OTL designed and built by Roger Modjeski that uses the 26DQ5 which is also a sweep tube designed for very high peak currents. If I remember correctly Roger mentioned the 26DQ5 tube is capable of 1 amp of current and in this circuit, which he designed as a tribute to Futterman, uses 6 output tubes per monoblock, 3 to pull up and 3 to pull down, giving an output current of 3 amps into any load.

I’ve always been interested in these which use the 6C33C but at $22k/pair they are out of my ballpark:

http://www.atma-sphere.com/Products/#Novacronhttp://

@bobheinatz, here is what I can say at the moment. The design is past the prototype stage and a few pilot amps will be built for listening tests.These will be monoblocks with moderate power and so far have been tested on a pair of Quad 63 with great results. I am going to have the prototypes in my system shortly on my Quad 57s. Ideally these will work quite well with 16 and 8 ohm box speakers and can include an external autoformer if you have speakers with lower or more difficult loads.

I can say while Roger studied the H3aa circuit as part of his research, this OTL will have more in common with the earlier Futterman designs which were simpler and in the process Roger was able to further reduce the parts count in half. A Pentode/Triode switch will also be available.

IIRC Roger mentioned the Futterman amplifiers have 7 adjustments with two pages of instructions to set up the amp. Roger simplified this by installing a meter with a 6 position switch so you can view the current in each tube and easily see if one is becoming weak or running too hot. There is also just one bias adjustment as the tubes are supplied computer matched. The only other adjustment is the bias for the driver stage and that is done on the same meter.