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Magnavox 9302 - Rebuild

Yeah, it really surprised me how hot the power and rectifier tubes get. I've used Nixies and bargraph tubes before, but these don't have a heater, so they don't really produce much heat at all. I've still got a handfull of IN-13 tubes laying around; might be fun too use these for a VU meter.

I'm nowhere near damaging the paint; I was just wondering about the longevity of the transformers. Edcor doesn't publish working temperature ratings (just a flammability rating), but from what I'm reading, it sounds like 100-110F is generally the upper limit of "okay." It will be interesting to see the numbers. I certainly believe your claim that they can remove fingerprints; I've got a distinct 5U4-shaped ring on my arm today.

You mentioned you aren't currently using your 9300; just out of curiosity, what do you use normally in your sound system? I honestly can't remember what prompted me to go with the 9300.
 
I have a pair of amps that started life as Bogen MO-100a's. They were re-worked following @dcgillespie 's efforts on its bigger brother, the MO-200a. Its basically a pair of 90 watt mono amps. Not that I really need the power, but the stupid shelf thing I have currently has small openings that the Bogens happen to fit neatly in. The 9300 is a smidge too wide to fit so its either sitting on the carpet or turned sideways.
 
I looked up your Bogen amps. I never would have thought to use a PA amplifier for HiFi, but that's a great idea. I don't know what I'd do with 90 watts per channel, but I do know that my neighbors would probably hate me.

I've been running my Magnavox amp a lot lately; I work from home, so it gets several hours of run time every day. So far, it sounds fantastic. I still haven't checked the temperatures yet, but I'm not overly concerned. One issue that's somewhat irritating, though, is the hum from the power transformer. To be clear, the audio from the speakers is crystal clear, it's the mechanical 60hz hum coming from the transformer itself, as well as a harmonic from the bell housing that goes away if I place my hand on the transformer. I've been researching this over the last few days, and it looks like there are a few possibilities.
  • Pulling too much current from the transformer. I'm using an Edcor XPWR008, which meets or exceeds the specs of the stock transformer, so I'm very doubtful about this.
  • The plates that make up the core of the transformer are loose. I checked all the machine screws on the bell housing, and they're tight, so I don't think this is the problem. Plus, the transformer is brand new.
  • The transformer isn't mounted securely. This is a distinct possibility. The amp chassis wasn't in great shape when I got it, and it has a bit of Bondo and epoxy to level out the surface, so I'm not able to really torque down the mounting screws without damaging the chassis. It's mounted well enough that it won't move, but I suppose it could be better, I think I might get a rubber pad and mount it under the transformer to see if this helps.
  • DC on the power line from poorly engineered electronics and noise from loading/unloading of switching power supplies. This is something I'm very interested in. My house is old and probably has pretty bad wiring, plus I have a large number of electronic devices running, as well as several computers and servers. I'm wondering if it might be worth getting an isolation transformer and a power conditioner.
Overall, the mechanical noise from the transformer isn't *that* bad, but it's something I'd like to eliminate if possible. I can't hear it when music is playing, but it's definitely audible in the quiet space between tracks. I'm open to suggestions.
 
I've run it with a voltmeter connected to the speaker outputs just to get an idea of the power I'm actually using and its rarely more than a couple of watts. Not all PA amps have decent enough transformers to get by with hifi duty, but these do.

Might be able to use some rubber grommets or washers to isolate the transformer from the chassis and/or play with the screws that clamp it all together. Sometimes making it a little less tight is the answer, or maybe pull the bells off and put a strip of tape between the bell and the lamination stack.

I have a big isolation transformer in use on mine. It does make the power transformers quieter, though they weren't noisy enough for me to even notice they were humming until they weren't anymore. I got it cheap and figure it wasn't going to hurt. Also its got 6 outlets on the side and I needed a power strip. Its massively overkill at 2.5 kva but I paid about a dollar per pound for it, which was so cheap I couldn't pass it up.
 
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