I had an old GE hot - chassis amplifier with a nasty hum. The hum was loud and sounded like 60 cycle AC hum to me. Same sound with volume up or down, the volume control did not affect it. Tube complement is a 50DC4 half-wave rectifier, and two 35EH5 pentodes. Good for about 1.2 watts per channel. No power switch, no fuse, no power transformer. House mains straight to the amp. Widow-maker, is what. Fortunately, I have a high quality high capacity Isolation transformer that I use to test these kinds of amps. However, its still not safe on the regular AC line once Im done testing it. To address the hum, I replaced the old electrolytic caps. It was one of those nasty old cardboard tube multi-stage caps, 100 uf / 50 uf / 30 uf. Replaced with 100/47/47 separate caps. Nope, still got hum. Replaced rectifier tube (50DC4). Nope. Replaced .047 Sprague black beauty capacitor, although almost never go bad. Nope. Gave up. Built DC power supply out of Triad N-68X isolation transformer (no more hot chassis), a 6A bridge rectifier, 3 100 uf caps, 2 100 Ohm 10 watt resistors. Now I have overkill. 124 V DC presented to the tubes. Heaters and everything are on DC. No more hum. Sure glad that rectifier is rated at 6 amps. Now, of course the Triad transformer is getting warm; Im drawing right at its rated output at the moment. But at least its safe. Listening to Esquivel. Once I build an enclosure for it (tomorrow?), it will be good to go. Will add fuse and power switch of course, but its looking good. Now that Ive been listening to it for awhile, I do notice that Ive got some hum after all; but its related to the volume control, so I suspect its the RCA inputs. Better cables should fix that; if not, a ground loop eliminator perhaps. Easy stuff. This will be a nice little desktop amp when Im done with it.
Posted on: Sat, 22 Nov 2014 20:10:18 +0000
Trending Topics
Recently Viewed Topics
© 2015