OTLs and Beyond

I experimented with a push-pull triode-connected 1625 amplifier, and learned much from that. I moved on to using a 6L6GB instead of the 1625. It was during this project that I accidentally made what proved to be an important discovery when it comes to building better tube amplifiers — the coupling capacitor size is more crucial than one would think. At the time, I was unable to find an explanation. I just knew that using certain larger valued capacitors resulted in an incredible, full sound. The downside of all this was that low frequency oscillations were the result….

Discouraged, but not defeated, I purchased a book on tube amplifiers for inspiration. The author of the book sold OTL amplifier kits. I purchased one and built it. The sound was really good, but something was lacking…. I upgraded the capacitors in it, which coupled the power stage to the speaker and mitigated the problem.

I decided to build an OTL using no output capacitor, going with a single-ended design. This would prove to be a wise choice as far as the sound was concerned. I discovered a better implementation of negative feedback, which helped the sound out (all OTLs must have negative feedback due to their design). Regrettably, the project, while having better sound than the kit I had bought, was too unreliable, and thus became scrap.

OTLs and BeyondFinally, I decided to go with something simpler, use an output transformer, and hope for the best. As it turned out, this sounded even better than the OTL still and all! In retrospect, I think the reason for that was largely the proper sizing of the one coupling capacitor in it, as well as the use of no negative feedback.

What became of this amplifier? It is now being used by my mother to replace the original 6F6 amplifier. This was my first truly successful amplifier (though it didn’t quite meet my design goals).