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Original Message

RE: OTL amps in contrast

Posted by Volker on May 5, 2021 at 11:18:52:
"I once visited Harvey Rosenberg in his home and he said: "We will gladly publish the circuitry. Anyone that tries to build one will find out that it is a self-destructing booby-trap. It took us more that a year to make it work without blowing up."
This is myth from Mr. Rosenberg.
Years ago i scratch-built a pair of H3AAa using the original schematics found on the net, only difference was PL519s output tubes instead of 6LF6.
The amps worked stable from start on without any tendency of oscillation, instability or "blow up". Stability tests were done with R/C combinations connected as load and square wave test signals.
The only critical things to keep in mind are:
-You have to put ferrite beads on the anode and G2 connections of each output tube to prevent local HF oscillation (typical for this tubes and not related to the Futterman circuit). I used small toroid cores with 2 cable windings.
-you have to use true star grounding. I used the short connection between input rca and speaker cold connector as central ground point.
-the connection between the input tube anode and phase splitter tube grid has to as short as possible (due to high output impedance of the "starvation" input circuit.
Summary: The original Futterman circuit is stable and there is no "secret" or "magical" component. The only secret is that Mr. Futterman coming from tube test equipment design did know how to build things right.