What's your OTL tube amp experience and suggestion?

The baby on the changing table analogy is a good one. I called my Joule Electra amp a labour of love, perhaps because I had to treat it like a baby. I really liked the sound, it was wonderful within my system, but I battled buzzing and humming which would occasionally arise. That said, I never experienced any of the problems that you listed, other than feeling like I was sitting in a sauna.

These were my issues:

Grounding – For me, the IC’s had to be inserted “just right” to stop a slight buzzing. This was hard to get right. I suppose I could have got a technician to check this out, but I was always able to resolve the issue myself with patience. I never found out if it was a design issue, or just a problem with a loose ground at the rca connector. The latter was a good possibility, as I used to have heavy, unwieldy rca cables that may have loosened something inside the connector.

When a tube went bad, it was more complex to diagnose because of the several output and driver tubes. In my experience, 6c33c output tubes going bad created a hum through the speakers. There were 8 output tubes. When only one was bad, it was easy to figure out. But when 2 or more output tubes went bad at the same time, it was difficult to troubleshoot. My original tubes lasted 5-6 years. It was only the last few years that I owned the amp that I had problems. I ended up just buying a whole new set of output tubes. Same thing with my driver tubes. When they were going bad, I went nuts trying to figure out which one was causing the problem. I knew something was wrong because one of the dc offset meters built into the amp got stuck at  “-1” and I could not set the correct value (I should have been able to set it to 120). An entire set of new driver tubes (10 of them) fixed that issue. It would have been much cheaper if I could have bought only the tubes I needed, but I couldn’t figure out which one or ones were bad. I suppose a tube tester such as a Hickock tester would have been handy for the driver tubes, but as far as I know, there was not an easy way to test the big output tubes.

Also, my OTL amp was susceptible to noise from dc on the electrical line. For example, when sliding a dimmer switch to the low position, it would create an audible buzz through the speakers. It took awhile to figure this one out, as the dimmer switch was located in a hallway far from my music room and it didn’t dawn on me at first that a switch outside the music room could be causing the problem. I changed the switch to a new one, problem solved. But there are other amplifier designs that also have problems with dc, so I wouldn’t say OTL’s are inherently worse than other types of amps when it comes to dc.

Tips: Make sure grounding is good, not just with the amp but throughout your system. Keep good, spare tubes around for testing. Minimize dc as much as you can (maybe too much to ask?). And most of all, give your amp room to breathe.

Bottom line, I wouldn’t say the OTL topology is inherently problematic. And bear in mind there are different ways to design an OTL amp, with some designs better than other designs, just as with any amplifier topology. I have owned and heard other high end amps in my system and at dealerships that made more noise than my Joule ever did (Vac 70/70 in my system, for example). I have only owned one amp that ever went “poof” and stopped working, and it wasn’t the Joule. The Joule was a workhorse that never failed me over the course of 8 years, other than the fixable, relatively minor issues that I wrote about above.

Well, wait a minute, there was the one time when the fuse holder for the variac melted and there was that chemical-like burning smell … but that was because the manufacturer installed a 7.5 amp value fuse holder when it should have been 10 amps (according to the technician who fixed it). So then again, you might be onto something!