My El34 SET

[darkmoebius]

Hi Alexmoose,

I have a 1997 12wpc Audion Sterling ETSE EL-34 SET integrated amp(3 stage) that recently went "Pop!" and died. Upon inspection, it turns out that one of my newish Tesla EL34's had an internal failure causing it to draw current like crazy, the result was one of the 105R ultralinear resistor blew, then both channel's the 510R/7W and 100uf/100V cathode resistor/capacitors blew, literally. Burned the PCB below them.

Anyway, I dissected the amp and with a lot of advice/direction from more experienced people on AudioAsylum's Tube DIY forum was able to recreate a schematic for it. You can view the entire thread here with pictures of the amp's internals.

One big problem, it turns out, was that Audion was running the tubes and components close to 90% of their rated values. A gentle rise in my home's AC voltage was all that was probably needed to push things into the critical failure range.

I tested some points on the amp power supply PCB (unloaded) and it seems the power transformer puts out 412Vac before the full-wave rectifier and 557Vdc after it. Far too high. Also, many people advised me that the underrated 250V series-connected capacitors in the CRCRCR filter section are a recipe for disaster. Their max rating in series is 500V and the dc rectified output is probably in the 450V range. Some have said that EL-34's sound best in the 430V range.

Hopefully, this schematic can give you some ideas for future modifications. Perhaps changing the EL-34 cathode bias/bypass scheme to something similar with a higher rated voltage for the cap.

BTW, I am rebuilding the entire amp with point-to-point wiring, higher rated components, a choke loaded pi filter, and lower operating points for the tubes.