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RE: My 3 years to the goal of the non-compromise SET and still learning..

Thank you!

I'm using my spare time during week-ends to repair them. Fortunately I have another SET playing music for me right now. Not my property, but a friend's. I took it home with me to do some mods to it and currently, I'm "burning it in".

The reason of my accident was the foolish mistake of putting the binding posts near the output transformer. Due to some loosening of one of them, its edges seemed to have dug up in the transformer's insulation and to the primary. I wasn't aware of this.
When the time had come for me to take some measurements last week, my goal was to measure frequency response and power output. I had connected a resistor load and my oscilloscope's probe across the amplifier's output terminals. A function generator was also running with its RCA terminated cable ready to be plugged in the input of the monoblock. The measurements were successful with the first channel. I got 12W RMS at 1kHz and 25Hz-28kHz FR at -3dB, HF limited by the interstage transformer.

Happy from my results, I moved the equipment to the other channel and plugged the load and probe to its output. At the moment I plugged the RCA cable in the input, the amp went BZZZ, the input tube arced heavily in purple and while I rushed for the power switch, I heard a bang and later there was a smell of a burnt component!
The aftermath was "welded" grid to cathode input tube, blown series grid resistor (Amtrans, it hurts), blown function generator and insulation breakdown of the output transformer.





Luckily I managed to repair the function generator last week. The power supply regulators and the final output stage were blown. It took me a few days to find the faults, measure, buy new parts and replace them.
The output transformer is also okay right now. In order to fix it, I carefully cut the outer insulation to reach the burnt spot of the primary. Visually there was a winding turn with charred insulation at the top that resulted from the arcing.





I know the person who wounded my OPT, so I was familiar with the materials he used to manufacture it. I put a tiny droplet of solvent, waiting for it to soften the glue and using a very thin plastic card, I inserted it between the charred turn and the healthy ones, separating them with a micron distance. Afterwards I sprayed some PCB insulating lacquer on the windings and finally sealed it back using the same glue. This procedure was made to make sure I won't get a primary winding shorted turn, which would result in a lesser useful transformer inductance, downgrading its performance.

The resistor and the tube are, of course, beyond repair. Thankfully, I had some spare EL802 tubes left that I managed to match using our uTracer at work. I'll be replacing the resistors with Vishay metal films, which IMHO are inferior the Amtrans, but still very good for the money.

My explanation for the accident is that the current found itself a nice path, from the scope probe to the OPT primary, then from the main power supply filter capacitor holding 390V to amplifier ground, then to cathode of the input tube where it easily broke through, arcing to the grid, then from the grid resistor to the RCA input and from the low output impedance of the function generator finally going to the shared neutral between it and the scope. I guess the current was somewhere between 0,8A at its peak, considering the resistances of the primary, the arc inside the tube and the grid resistor. It was plenty enough to weld the tube and blow the resistor.

Now, the light inside the tunnel - I take each defeat as a new start. Now I've begun repairing my monoblocks and upgrading them a bit. I made oiled solid birch plywood binding posts ready to be installed, but unfortunately I'll have to wait until Monday in order to buy myself long brass screws for them.










I'm finally putting plate cap on the output tubes and I'm covering the enameled OPT wires with thick cotton sleeve.
About the Telefunken EY83s, for now I prefer letting them be, standing plugged in their ugly looking adapters until I get the opportunity to find some EY500 of the same brand.






I sincerely hope the sharing of my mistake will be useful as a safety tip and warning to other fellow audio enthusiasts.

Best regards,
Alexander.

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