Hello friends I'm back after a long break. Moved to Georgia about 3 years ago and have just got around to unpacking Amp and electronic "stuff". Don't have the work space I had in MI. No garage or "craft room" for myself. In fact I had my tools stored in my son-in-laws garage and on a late rainy night 6 months ago or so someone decided they wanted all my tools and the daughter's new mountain bike. 30/40 years worth of tools gone. Around $2,500/3,000 as close as we can figure. Mostly machine shop, woodworking tools and lots of socket sets. Didn't get my electronic stuff as I had a couple of machinist chests (1 mahogany box I built in 1965 high school shop class) covered up in the corner. Will revive the old show my your work space thread when I get a picture of what I have now .
OK That all said here is what I have. Magnatone Deluxe 450 with AC hum, Volume set low or high, guitar plugged in or not. The volume control on input 2 seems to vary the loudness but input ! volume doesn't. Thinking PS caps but would like to hear what else it could be. Any thoughts???
Got out the express chassis and taped it off to lay out for drilling mounting holes. The boards I already had populated before I left MI so I'm ahead of the game there. Same thing for the Rocket which I'll go to after the Express. I should lay that chassis out and get them both drilled while I'm at....think I'll do that.
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Thanks, will check back later.
Gary
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gahult wrote: Magnatone Deluxe 450 with AC hum, Volume set low or high, guitar plugged in or not. The volume control on input 2 seems to vary the loudness but input ! volume doesn't. Thinking PS caps but would like to hear what else it could be. Any thoughts???
There are a semi-infinite number of places hum could come from. I'm not familiar with the Magnatone D450. Is its output fixed bias? If so, could be hum riding in with the bias.
Hum not changing a lot with volume settings indicates the hum is coming from somewhere after the volume controls mix.
There's always heater AC current leaking in somewhere, and the First Rule of Debugging Tube Amps: swap for known good tubes first.
It's more than high time to replace all the electrolytic caps in the amp before any judgement calls are made on the amp performance or sound!
That being said I would have to say that your issue of hum on channel two is due to a leaky .01 coupling cap off of the plate that feeds the volume control and the way to check that is to look for dc voltage coming in on the volume controls center terminal.
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I'm in agreement with that - if you haven't recently (last ten years) replaced all the electrolytic caps, do so. There are a number of ways this fixes many things.
Also, as a guide on fixing hum issues, there are actually two main things that sound like "hum". Well, OK, three. I'll get to #3.
AC power line hum is a bass hum at ... 60Hz. It's either power line pickup from heater or AC power wires in the amp, or a broken/open ground wire, or a ground loop. You also tend to get 60Hz hum from a sick building electrical network. Problems with the building grounding can give you hum, too.
If it's an octave higher at 120Hz, it's related to ripple from the power supply. This can be from the first power filters not doing a good job, or ripple leaking in from the power supply distribution network, or in rare cases, magnetic pickup from the power supply choke, where the 120Hz ripple is reduced. Some very few low powered amps are half wave rectified, so this distinction is moot; their ripple is 60hz, too.
A special case of ripple hum is the hum of a not-well-enough filtered bias supply. This causes 60Hz hum, all right, but it's actually ripple because bias supplies are commonly only half-wave rectified.
The third category is "buzzy hum". This happens when the amp picks up squarks and blats of noise that sound buzzy, but does it at power line frequency or at twice-power-line frequency. These tend to come from either noisy rectifiers in the amp itself or from blats of noise from fluorescent lights breaking over once each direction when they make light. It's also possible that bursts of RF noise get propagated at 1x or 2x the line frequency from other equipment in the same building, outside the amp.
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