Rumbling tube preamp has had me baffled for months

[Douglie]

Was the rumble/hum ever fixed? I’m having pretty much the exact same problem. I have just completed restorations on C8, C8S and two A-116. They sound good together but I got the hum. Power amps are quiet on a different preamp. I’ve swapped tubes, etc. there is a post by Mark Brown that does the Blueglow YouTube videos on usaudiomart regarding this. He is selling the preamps. I tried to attach a link but you can find it by a search. He talks about a design problem having the audio wire in the umbilical chords. I have my audio wires separate.

Here’s a picture of the setup. Its a good sounding system and looks nice. (Would like to get rid of the hum but it can’t be heard during normal listening.) From what I can tell A-116’s we’re the first Mac power amps with a built in power supplied and the C8S was their first piece that was aimed at stereo.

 

[tcdriver]

Was the rumble/hum ever fixed? I’m having pretty much the exact same problem. I have just completed restorations on C8, C8S and two A-116. They sound good together but I got the hum. Power amps are quiet on a different preamp. ...

Based on my limited experience with my McIntosh C4 and C8, I believe the hum is due to the AC voltage present in the umbilical cord. The close proximity of the audio output jack to the line voltage present at the termination of the umbilical cord is also problematic. The way to eliminate the problem is to build a separate power supply / umbilical cord that only passes DC voltage for both the filaments and B+.

 

[Douglie]

Based on my limited experience with my McIntosh C4 and C8, I believe the hum is due to the AC voltage present in the umbilical cord. The close proximity of the audio output jack to the line voltage present at the termination of the umbilical cord is also problematic. The way to eliminate the problem is to build a separate power supply / umbilical cord that only passes DC voltage for both the filaments and B+.

Yes, the audio out jack is as close to the AC outlet plugs and umbilical chord as you could make it! I looked at moving the rectifier components to the power amps but there wasn’t a real good place to mount them. Moving the rectifiers would definitely be the best fix. Hum is not real noticeable at all but I still don’t like it…

 

[nickdinatale]

This is a very late update, but last summer I finally found the solution to my rumble. Maybe this will help someone in the future.

I had replaced close to every component in these, but there were a few that I hadn't. Of course it was one of those. There's a 1k resistor off of the cathode of V1A in the C8S schematic. In my particular C8S, it was a strange looking thing that I actually assumed was an old ceramic capacitor or something, and completely ignored replacing. I finally realized that it was actually that 1k resistor and figured I should probably just replace it to be sure. Well, that solved the problem.

 

[Dswankey]

Glad you finally got it figured out, how does amp sound now?

 

[Horstinator]

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