Power Transformer and hum?

a 3W single ended EL84 amp for home, work & school

Power Transformer and hum?

Postby AmadeusMozart » Fri Dec 20, 2013 2:05 am

I've designed and build a SE 6L6-GC amp with James iron (mains, choke, output) and although potted I could hear the mains transformer. Just bought a switched mode SE EL84 but the darn thing runs far too hot - after a few hours the thing is so hot that I can not touch it. Iron is OK up to a limit and in first instance want to re-use it. Have had a number of Chinese amplifiers before and those transformers always hum.

Had a couple of PP 4 x KT88 (ea) mono blocks from Korea and you could not hear the transformers. Needed a weightlifter to move them around and it was like having a heater in the room. For background music in my office the EL84 is good enough.

Back to subject: what are the builder's experiences with hum - which transformer would be best? DO I need to go to Sowther or VVT?

Thanks
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Re: Power Transformer and hum?

Postby Geek » Fri Dec 20, 2013 6:18 am

There's a lot of causes of transformer hum from the laminations not being properly dipped, to end bells not tight enough to plain bad design.

The Chinese *can* make a good transformer, but it's a crapshoot and you tend to get what you pay for.

I have been mounting transformers on rubber grommets to mechanically isolate them from the chassis and that works like a champ in most cases. You can try that and just run a wire from the mounting screw to the chassis as a safety/electrostatic ground. Use a lock washer, like an outside star type on the screw head that contacts the transformer mounting tang so it bites into the metal for good electrical contact.

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Re: Power Transformer and hum?

Postby AmadeusMozart » Fri Dec 20, 2013 1:45 pm

Did not mention that the chassis was sturdy (non resonating) aluminium and hence I cannot see that mounting the transformer on grommets is going to make a difference. However we moved into temporary accommodation when we were building our home and during that period I sold the amplifier with the intend to make a new one once we moved into the new one. Unfortunately health deteriorated faster than anticipated and I now only have limited capability with my right hand. So I am trying to take a shortcut with getting another one. Just bought an APPJ miniwatt from Hong Kong and it uses CCS in the finals which seems to be a fad these days but in my book is a great no-no. It makes the amplifier clip like solid state. I intended to take that CCS out and replace it with a simple resistor. Unfortunately it appears that the CCS was barely keeping the right channel from oscillating and once the resistors were in off it took. So it is a doorstop now since it is SMD and besides it was running so hot that I could not touch the darn thing after running it half a day. I want something that is reliable and will stand the test of time.

My dilemma is now - do I build the budgie and re-use the OPT from the APPJ or do I buy an expensive amplifier made somewhere else than in China? I am partial to EL84 SE While this amplifier was running it was doing fine, could be better but good enough to give me a taste that it was the way to go. I've been pondering over the Almarro A205A or the Tube finale F-138 models. ( http://www.audiophile.com/products/alma ... -amplifier http://www.tubedeforce.com/sale.htm)

Attached some pics of my 6L6-GC that I made about 4 years ago. Chassis 3mm high tensile aluminium held together with 3mm stainless screws and JB Weld. Schematics are on audiokarma under same name and I am driving some metronomes with FF105WK (my preferred speaker - I also have the FF125WK metronome but my wife has taken passion of those) and those details are on DIYaudio (same name again). Cheers!
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