Hum/buzz only at low volume

[Beefy]

Hi everyone! I have a strange hum/buzz issue that has just popped up on my Crack with Speedball. The hum has shown up during the course of some upgrades - and I can't see how the upgrades might have changed anything, so I need a sanity check as to potential causes and solutions....

Basically, the Crack has a hum/buzz, but only in one channel, only at very low positions on the volume knob. The noise progressively disappears as the volume knob is increased. It happens whether or not there is music playing, whether or not the source is turned on, and whether or not the source is even connected. I first noticed it when switching over to a brand new input tube immediately after my upgrade spree, but have since been able to hear it in several other input tubes, albeit some are better/worse than others. So, probably not the input tube. If I switch out the power tube from my beloved 5998 to a Sovtek 6AS7G, the buzz is reduced, but still audible. So, probably not the output tube.

What I don't understand, is why the buzz only happens at very low volumes. Perhaps something noisy in the ground, that the input tube can see when the volume pot is shorted to ground - but wouldn't a noisy ground show up in both channels? Perhaps something wrong with the pot - but why would the noise still be there when the source is disconnected?

Swinging back to the upgrades I performed immediately prior to noticing this noise. I switched out the stock rectifier diodes for Cree Schottky's (using the PCB from Parts Connexion), I replaced Axon 2.2uF film bypass caps in the PSU with Mundorf 2.2uF, and I replaced Axon 91uF output caps with Mundorf 100uF caps. So the only direct signal path change is the output caps; I could perhaps understand that one of the output caps is faulty or poorly connected, but I still can't understand why the noise would only be present at low positions on the volume knob. Only other thing of note related to grounding is that the amp does have a ground loop breaker installed, in the form of a 10R 5W resistor paralleled with a 0.1uF cap between safety ground and input ground. Pot is a TKD 2CP2511 50k, padded with 50k series resistors for about -6dB pre-attenuation.

I'll probably reflow all the solder joints in the noisy channel, double check my voltages (they were fine when I first noticed this) and try to see if my multimeter can see the noise frequency or amplitude at various points along the ground and signal chain. But other than that, I'm stumped.

Thoughts?

[Paul Birkeland]

If noise crops up when changing out parts, try putting the old parts back.

[Beefy]

If noise crops up when changing out parts, try putting the old parts back.

That's absolutely on the cards. But I think before I do that, I'd really like to try and understand the nature of the problem and go through as much troubleshooting as possible. In my experience - albeit limited and atrophied! - I've never really seen a lot of hum/buzz issues at low volumes. It's a real headscratcher for me, and I'm hoping there is a good/interesting explanation rather than 'bad parts'!

[Paul Birkeland]

It's most likely that you have a loose connection that has been disturbed by installing the new parts.

[Beefy]

So I've been playing around with this on and off. I switched back the old capacitors and replaced/shortened the fly leads. I reflowed all the solder joints through the whole amp, and confirmed all my voltages are AOK. The only change I couldn't revert is the rectifier diodes because I had already thrown them out, but I did try a few different mounting orientations of the Schottky PCB board just to ensure it wasn't just a matter of AC being too close to the input wiring.

None of this made a lick of difference, but I think I found the problem anyway......

The hum seems to come from EMI or RFI between the input and output tubes. If I put my hand between the tubes, the hum goes away. I can make it worse if I route the AC power cord between the tubes as you might expect, but keeping all other sources of interference out of the way doesn't help. Shutting off all other electrical devices in the room doesn't seem to help.

Some tubes combos are quite noisy (a new Genalex Gold Lion 12AU7 causes loud hum with a 5998 and a 6AS7G), while some tube combos are much more faint (JJ ECC802S is barely audible with 5998 and 6AS7G), but all of my tube stash has the noise to some degree. I am beginning to suspect this hum has always been there, but I only noticed it when it was really loud with the new Gold Lion tube.

So yeah, not sure what my next step here is. Still confused why it is only one channel (orientation of the tubes?), and why it is worse at low volumes (still going with ground pollution). Maybe I just pick my quietest tubes and pretend the problem doesn't exist......

[Deke609]

That's fascinating (for me, anyway -- bothersome for you).

Have you tried using the amp in different locations in your home? If possible, I'd try each of the following:

(1) different location but same mains wiring line to the panel;

(2) same location but different mains wiring line via a good extension cord; and

(3) different location with different mains wiring line.

I'm curious how your hand is acting as a shield. If you wear rubber/plastic soled shoes, is there a difference with your hand touching the chassis between the tubes versus not touching the chassis? I'm wondering whether your hand/body might be acting as an electrostatic shield that drains to ground (either the metal chassis or, if bare foot, the floor).

cheers, Derek

[Paul Birkeland]

Is all the hardware tight on your amp?

[Beefy]

That's fascinating (for me, anyway -- bothersome for you)

I'm very good at creating unique problems. A blessing and a curse.

I'll try different locations/power solutions in a bit, but I can address the 'hand' questions. I'm standing barefoot on synthetic carpet, on wooden floors, on the second floor, so zero path to ground. My hand can block the noise whether I am touching the chassis or not, there just needs to be meat physically located between the tubes. Meat shields don't really attenuate magnetic fields, so I am thinking electromagnetic.

It's all very strange.

Is all the hardware tight on your amp?

Seems to be.

[Doc B.]

It really boils down to solid connections. When the volume knob is turned up some noisy ground loop is being reduced, implying that some ground connection is not 100%. Reflowing solder joints is usually the solution. Also try cleaning the tube socket contacts on the side that makes the noise. You can squirt some contact cleaner on the tube's pins and insert and remove it a few times.

[Beefy]

Thanks Doc, I'll have another 'crack' at all my solder joints and see how it goes.

[Beefy]

Funny, I just came across this thread from a year ago: https://forum.bottlehead.com/index.php?topic=12387

It didn't come up in my original search for this issue, amongst all the other hum/buzz threads, but did come up when I searched just now for 'Genalex'. The poster in that threads seemed to have an identical issue with picking up RF using a Genalex ECC82 tube. Intriguingly, my second-worst tube for this noise is an Electro Harmonix 12AU7 Gold; I am convinced the EH and the Genalex are the exact same tube with different labels, because the construction and date code font are identical.

I'm starting to increasingly think this is just a poorly shielded tube, and that through bad luck, strange circumstance and the position of the stars, it causes weird issues in some rare cases.

[Paul Joppa]

A tube shield might be in order here  :^)  Plop an empty tin can over the ECC82 and see if that helps?

[Beefy]

A tube shield might be in order here  :^)  Plop an empty tin can over the ECC82 and see if that helps?

Empty! Of course! That explains why my last amp ended up filled with tomato soup.

I actually had no cans available, but tried with a stainless steel milk frothing jug. And sure enough, covering the input tube immediately and completely cut the noise (even more effective than 'meat'). Covering the power tube did nothing. Considering stainless steel is non magnetic, I think we can definitely confirm some kind of radio interference to the input tube.

I'll grab some tube shields then. Are there any preferred models? I was going to order some tubes from The Tube Store, so I wonder whether this cheap little number would be sufficient? https://www.thetubestore.com/shield-for-9-pin-tube-socket. Looks like the base should nicely fit over my Teflon tube sockets......

[Paul Birkeland]

That shield will fit the mounting holes we provide.

[Beefy]

Shield received and installed, and it cleared up my problem 100% - my Crack is now practically dead silent across the entire volume range. The shield looks pretty dapper to boot, although I will probably spray paint it in hammered finish to match the rest of the amp.

I'm still unclear as to whether there was an underlying issue that made this hum/buzz come to the fore, but for now I'm going to go with 'no'. I think I just have an RFI issue in my amp's location, that was highlighted by a particularly sensitive tube.

Thanks for all the help everyone!