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previous: Stan Bailey Hello,I built an amp that h... -- 6/27/2000 10:53 PM View Thread

Re: 60 Cycle Beast from Hell ...Please help!

6/28/2000 10:33 AM
Ray Ivers
Re: 60 Cycle Beast from Hell ...Please help!
Since 60 cycles has been mentioned, and you have a 'scope, it's evidently not 120 Hz (oh, OK, cycle; I kind of like that better, anyway) hum/buzz - which pretty much rules out the B+ supply. When you disconnected the heater supply, did you disconnect both wires at once? Just one won't do it - the hum will stay as the tubes cool off.  
I think it's a grounding problem related to the first gain stage. I recently had a chance to look inside two Two-Rock amps. The early one had pretty much a textbook double star ground; every signal ground starred to right next to the input jack, every power supply ground to the green wire of the power cord near the power entry point. This amp hummed (60 cycle). The later model used a 'ground buss'; a single, thick, solid copper wire running from the input jack ground to (I believe) the speaker jack ground, with individual stage/filter cap grounds connected in order from input to output. The main power supply grounds (PT C.T., first filter cap) were still starred to the power entry point. This amp was much quieter. I'm not saying this change in grounding technique was solely responsible for the hum reduction, but my 'project amp' chassis I've been using for years uses this buss system too, and it's always been quiet, even with four cascaded gain stages (ah, the metal years).  
I would connect the FWB and first filter cap grounds to a different star point near the power cord first, and then start looking at each stage's grounds starting with stage 1. R.G.'s Tube Amp Debugging Page may also be invaluable to you for hunting this thing down.  
Ray Ivers  
R.A.G.E Electronics

Stan Bailey Thanks guys,I'll try sepera... -- 6/28/2000 5:39 PM