Please, please, help!

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I have an ARC ST70 C3. I decided to get the 300 milliampere Magnequest power transformer that also has two 5 amp heater strings... so that I could run Svetlana 6550s instead of 6L6s. Anyway, when I hooked it up prior to putting in the Mquest power tranny, I noticed a hum; I'm using early 1950s James B Lansing 12 duplexes (felt surrounds!) @ about 101-103 db/1 watt, so most amps hum a bit, but not this much. I started the xformer swap anyway. I discovered that it had fried its' power tranny before, as all the leads were soldered together w/ shrink tubing. So, I hooked up the Magnequest and now it REALLY hums. Also, when you tap on the 6550s (selected from VTS) they are microphonic, as are all the tubes I tried (bad feedback cap?) It has WAY lots of gain... is it possible that the feedback is out and the radical gain and other Zero NFB qualities are causing the hum from hell? I have checked every ground... the AC ripple is incredibly low; 50 mv at the plate supply, negligible at the screen, bias, and front end caps. It hums louder with no interconnects atached; shorting the input does nothing. The hum changes in character slightly (pitch, volume) when the bias is adjusted. The hum does not get louder as the input signal is increased. As the amp warms up, the hum decreases greatly. I keep thinking power cap, but the ripple is SO low.... any thoughts? Thanks for reading this! Thanks for any help! Please!

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Topic - Please, please, help! - g 21:44:00 12/11/03 (0)

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