Curious Hum… help?

[biolite]

So I built a Two Stroke head over the summer and it’s been great. However, I get this constant hum when the tone control isn’t max’d out. Has anyone ever experienced this or know how to fix it? It seems like a 60hz type of hum…

[Andy]

When everything is wired and working right, the Two Stroke is a pretty quiet little amp so it sounds like what you have is not “normal”. Of course, there will be *some* noise but if its going away when the tone is maxed, then there is likely some issue.

Can you describe a bit more about what is happening? Is the hum always the same volume or does it increase with the volume increasing? When the tone is maxed and it goes away, does everything else sound normal? Has it always been there or did it just start? Stuff like that.

[Robin]

Did you use a brass ground plate per the book instructions, a buss wire, other grounding scheme? The tone control should from the lug to the pot case to ground OR from the lug to ground, but not both. And the pots should ground toward the preamp side of the amp and away from the power section, including the filter caps.

[biolite]

Unfortunately I’m on travel for a couple weeks and won’t be able to troubleshoot until I return. The hum is a constant noise that does increase with volume. The amp has always had this hum since I built it and I just finally decided to do something about it. I can really only hear the hum when not playing through it. Everything does sound normal when the tone is maxxed out. It doesn’t seem like the hum is caused by anything in the environment as I have turned off computers, lights, etc. In the vicinity.

I did use a brass grounding plate and grounded the lug of the tone pot to itself (pot chassis). I believe I just left the pot to ground itself to the amp chassis as it was bolted on. I believe I also ran the input shield into the same blob of solder that is on the tone pot, if that could affect it.

[Robin]

That sounds correct. Can you post some photos when you get back?

[biolite]

[attachment=48]IMG_20110530_171522.jpg[/attachment]
Alright, so I got back from travel a while ago, but let this fall by the wayside as things have been busy. Sorry. Attached is a picture of the inside of the amp from when I built it. Thinking about this here at work it came to me that I shielded the input connection to that tone pot, so when the tone pot is sending frequencies to ground, it’s also putting them over the shielded portion of the input. Do you think this could cause it? It seems that it would make sense that when the tone is full up, it’s not shedding any freqs to ground or into that shield connection. I may try to re-do the input connection and use a better shield. Thoughts? Input connection is on the top left of the picture with the small clear covered wire going to the tone pot.

[Robin]

Any shielded cable in the amp, from the input jack to pin 2 of V1 or any where else, should only have the shielding connected to ground at one end. The other end of the shielding should be trimmed short and covered so it cannot contact anything. Connecting both ends will create a ground loop that will induce hum.

Also, the “twisted pair” of the heater circuit needs to be tightly twisted the entire length, including between the sockets. And the heater circuit wiring needs to be away from the other leads.
Check out this link: http://www.freewebs.com/valvewizard/heater.html
Read the section on “layout/ lead dress”.
Single-ended circuits have a reputation of being noisy but the 2Stroke can be very quiet. When demo-ing my new 2Stroke head, one of the common comments is how quiet it is.

If that was my amp I’d:
1: rewire the 6.3v twisted pair
2: shorten the leads throughout the amp as much as possible
3: confirm that the shielded cable(s) are only grounded at one end.
and then see where I stood.

[Author]

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