ho HUM... hum is driving me nuts

[vaughandy]

brainstorming session! Anyone have ideas what I can try next to reduce hum in my rebuilt Ace tone 601?

Things I've done / checked:

wire routing: use a big wooden stick to move wires around while the amp was running. no effect

filament grounding: pot between 6.3V secondary leads connected to ground (hum adjust nob). doesn't make much difference

ground isolater: plugged in through on of those adapter where they take off the safety ground plug. very very minor, if any difference. given that finding, I'm guessing something like the ebtech Hum X won't make a difference

transformer: I had to use one additional 15V transformer to supply voltage for biasing since I had to replace the original transformer and could only get a new one with two secondaries (6.3v and 180V). anyway, I had this 15V transformer mounted on the otherside of the amp, below the power tubes. I moved it up top and it's pretty far away from all the tubes and signal path. didn't really make a difference.

safety ground: safety ground wire is attached underneath one of the transformer screws to the chassis.

Other things to know: hum is there regardless of whether a cable or guitar are plugged in. I don't have a master volume control so can't test that. the volume pots for each of the two channels don't make much impact. even with all the pots on the entire amp turned to zero, no hum difference.

I'm running out of things to try!! Ideas??? I'll enclose some schematics (my apologies for it being in parts rather than one large file).

[vaughandy]

few things I forgot:

swapped power tubes with another set. no difference.

If I take out the preamp tubes completely, most of the hum is gone, but of course this would make sense as most of the signal isn't being amplified. The hum doesn't become really bad until I put the third preamp tube back in. I suspect this doesn't have anything to do per se with that part of the signal path, but rather that that's just the threshold gain stage where we really push that hum into the annoying volume range. Or not?

[jfutral]

One thing that my recent foray into amp building taught me through the very wise guidance on this board, do not under-estimate the issue of proper grounding! I did a lot of things that made minute changes to the noise and hum I was getting, but nothing made the most dramatic change than grounding points and grounding systems. The second most change was a 100r resistors on each heater lead to ground. (Someone suggested floating them on the cathodes of my cathode biased power tubes, but that didn't really make a difference for my layout.) Merlin's article is often referred to here, I would second that suggestion. It took me a couple of reads and many hours staring at my layout and schematic to figure out how to apply his notes, but it did wonders for me. My last amp is by far the quietest I have ever built.

Or you could just buy some frequency specific ear plugs and go about your merry way.

Oh, speaking of frequency specific, does it sound like 60hz hum or more like 120hz? And is there any clicking/static noises mixed in there? Does volume affect the hum sound at all, i.e., does it get louder when you crank it up?

Joe

[JHow]

I always double-check the input jack(s). Especially the shorting switch hookup. Just my own personal most-favorite thing to forget.

[vaughandy]

awesome, thanks for that article! I'll read through that. Definitely going to go through all my ground connections with a fine tooth comb and make sure it's grounded properly everywhere it's supposed to be.

I'm pretty sure it's 60hz hum, but i'm not 100% positive. i'll have to compare it to a reference from youtube or something.

hmm, the resistors on the heater leads is an interesting idea. 100r? i assume it's 100 ohm resistors...

Oh, and volume subtly affects the hum, but I can't just use the channel volumes to turn the hum up. it's more constant than that.

[jfutral]

Proper grounding is the new black.

All the same, thinking about your pulling of pre-amp tubes and staring at your schematic (I keep doing that a lot lately), can you check if by "3rd pre-amp tube" this is what the schematic refers to as V3? If so, half of that tube is the second stage of channel 2 and the other half is the recovery (or the other side, i can't remember exactly and not the point anyway) stage of the reverb.

I know in my latest build, the reverb gave me a lot of grief for the better part of a day or two. First thing to check is that the reverb tank is plugged back in. If it is, try switching the plugs. Either by luck or by coincidence, it _seemed_ like there was a definite "in" and "out" orientation to the tank for me.(Hey, this was my first attempt at adding reverb!) Or there was increased solar activity on the days I was having problems. Either could have been true (or both!).

If that doesn't change anything, definitely go over your grounding points. Well, do that anyway. There always seems to be some noise that can be eliminated and the Grounding gods will be appeased and pleased.

Joe

[vaughandy]

yeah, V3 is what I was referring to as the 3rd preamp tube. The reverb is all connected and working properly. I have noticed a bit of hiss (not hum) when I crank the reverb, but that's easy to live with compared to the hum.

Man looking at the way the signal is routed... kinda crazy the way they designed some of this thing right? you'd think you'd want to use seperate tubes for preamp gain stages and reverb / tremolo stuff. dunno.

[vaughandy]

oh, and I've never unhooked the reverb tank, so I can't imagine there's anything wrong with the orientation there. besides, they're soldered connections, not RCA, so it's not likely a previous owner just happened to unplug & reverse them or something.

[jfutral]

And check those solder joints in general. You never know. I thought I had done a pretty good job of avoiding cold solder joints. No sooner than I put the amp back together once, a channel stopped working. Checked everything only to find the a B+ point had the 100k plate resistor flappin' in the breeze. Tried to warn Ethel, but it was too late. (Sorry for the old Ray Stevens song reference). It looked and mostly acted fine, until it wiggled just right to let me know it wasn't tight. Um. I am speaking about electronics.

Yes, the 100r is 100 ohm resistors on the heaters to provide a virtual center tap. But I forgot you have that pot. Does the same thing, mostly. So forget that idea.

As for the soldered connection instead of RCAs and the previous owner. Remember, no sooner than someone makes something idiot proof, someone else makes a better idiot.

Joe

[jfutral]

I'm trying to figure out that little circuit in the upper left corner. Is that some sort of lead boost?

Joe

[vaughandy]

yeah, weird right!?! That part of the circuit is gone, but I believe it was this bizaare "tuning control". it's on the lower right hand side of the amp here

1969 Ace Tone Model 601 50w Tube Head | Flickr - Photo Sharing!

i kind of wish it was still there. i'd love to know what it did.

on the other hand, it'll be a nice place to install my master volume when I get around to that.

[52 Bill]

That little transistor circuit is an oscillator, probably set to sound an E note to tune your guitar to. A lot of amps had them back in the '60s.

[vaughandy]

oh man, that sounds rad as hell! wish it was still there

[vaughandy]

Not having done a lot of this before, it appears I got the hum frequency pretty wrong. After recording it and looking at the spectrum analyzer, it looks like I'm having much more audible problems at 120 hz, and even overtones higher than that. I've enclosed a small sound file of it. Does this give any clues as to further things I could try? I went through and check all the ground points yesterday. At least on the power supply it seems like i'm in good shape. I haven't looked as closesly at the signal path grounding...

(ps, the recording is a loop, so the amp isn't actually making that mic-placement noise)hum from ace tone 601.m4a

[jfutral]

That is exactly how my reverb issue sounded. Not that this is your problem. No doubt the causes are probably similar, though. If the hum pretty much goes away with that V3 tube, then it is either in the reverb or channel 2. I don't think you have heater/AC issues, but I could be wrong. 120hz is more indicative of DC problems.

Just for the heck of it, and maybe someone else more knowledgeable than I am, can say if this is a good idea or not, disconnect the reverb tank. My noise would disappear when I unplugged the recovery stage side of the tank. I re-routed my grounds several times before getting it quiet.

I've seen a post describe how to short out stages to help pin point where the problem might be located, but I am not clear how that is done. I think the only thing left to do is narrow down if it is reverb or channel 2 so you hopefully don't waste too much time on the wrong side of that tube.

Joe