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In the Forum: Melquiades Amplifier
In the Thread: Melquiades For Dummies™ - step by step.
Post Subject: 6c33c heater wiringPosted by anthony on: 6/20/2023
A small word of advice regarding wiring the 6c33c heaters.
My DSET's have now have several thousand hours of use and I love them to death. I've not heard any amplifier able to do what they do. The first amp has over 10,000 hours now and has never failed. The second amp has fewer hours (still 10,000 hours or so) and has failed twice and I now know the very simple cause and offer it as advice to those still contemplating building this amplifier or any amplifier using 6c33c tubes.
The first failure I attributed to a tube that failed in a way to short circuit the 6c33c heaters which caused the transformer to short circuit, push crazy amps into the wiring, melt some insulation, release a little bit of the magic smoke and because I was out of the room when it started eventually damaging the heater transformer. This was an incorrect diagnosis. I replaced the wiring and transformer considering it a freak event and happily listened to that amplifier for about another 6,000 hours.
A couple of days ago the same thing happened six hours into the days playlist. Mmmm...a freak should not happen twice...there is obviously a design or implementation issue. After much thinking I got out the FLIR thermal camera and poked it around inside the DSET that has never failed and the underside to the 6c33c socket, including the adjacent section of wiring was running at about 100 degrees celcius, which is very warm, certainly warmer than I expected. The heaters have been wired with an 18AWG solid core with ultra thin mPPE insulation. I chose ultra-thin so that the two conductors would be very close together when twisted thus aiding AC field cancellations. The datasheet rates this insulation at 105 degrees celcius which is roughly the temperature underneath the 6c33c sockets. Bingo!
What has been happening, in my estimation at least, is that at some point the insulation eventually runs warm enough to begin to weaken or melt and when that melt reaches the twisted section of wiring a short circuit is created and the whole thing runs away from there. Considering the sheer amount of time these amplifiers have been turned on, I may be fortunate that it has only happened twice now, but the solution is very simple: the amplifier filaments to all 6c33c tubes is to be re-wired using wire with teflon insulation which has double the temperature rating (200C) of the original wire I used. Builders should be careful to route all wiring so that is is not close to the 6c33c tubes and also consider a fuse on the secondaries of any heater transformers just in case.Rerurn to Romy the Cat's Site