Quote:
Originally Posted by benman94
Well I didn't mention it because RCA never officially sanctioned the 6AS7 as an OTL amp output tube. Still, it is an interesting application.
How stable is your OTL? I've toyed with the idea of building one, a variation of the Dickie and Macovski circuit from 1954, but I really don't like the idea of running the 6AS7s fixed bias, nor do I like the idea of having B+ at the speaker terminals...
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One of the RCA tube manuals does give a 6AS7 OTL schematic/parts list...
I went with a reverse Futterman circuit with some negative feedback in the driver stage. With proper adjustment, I was able to achieve almost no DC voltage across the speaker terminals (someday I may rework the bais adjustment circuit to require less finesse). After a year of heavy service, I rechecked the bias balance and tube emission and there did not seem to be a change. My amp is a 2/4 channel with the rear outputs AC drive path and feedback switched over to parallel with the front when in stereo mode. I was very conservative with DC quiescent current in the outputs...I expect output tube life to be close to heater life. My output rails are only +/-60V so there is not much more risk to the speakers in the event of a rail short than some transistor amps.
I built that amp during a summer break in college when buying the outputs at $5 a pop, the chassis sheet metal, wood, and pots (most everything else was from my junk box) was a great stretch of my budget. The power supply evolved with the amp...Eventually utilizing SS regulators for nearly all B+ and B- rails. I could not afford the right power trans so I engineered around the junk box transformers I had....That box has every valid internal mounting point utilized to it's utmost....Probably weighs as much as an RCA 630.