Intermittent buzz, sometimes static

[MikeSattler]

Hey guys

So I've had a crack+speedball now for about a year and a half with no issues, however lately I've been getting this really annoying buzz the migrates between channels and sometimes is in both. It's not a constant buzz, and it fluctuates in both volume and pitch, kind of like a bee flying around my head. It's really only audible when music isn't playing. However, every once in a while my right channel will sound distorted with static, and after turning the amp on and off again it usually goes away. One other thing to note, the buzz doesn't happen immediately, but usually after a minute or so when things have started to warm up.

Things I've ruled out so far:

• I've reflowed almost every joint, especially the joints on the RCA jacks, volume control, caps, tube sockets, and headphone jack.
• Voltage checks are all within spec.
• It's not the tubes, because it happens with any 12au7, 6sn7, 5998, or 6080 tube i've rolled. Oddly enough the buzz doesn't occur when I use 12bh7 tubes
• It's not interference from other electronics or dirty power. I took the amp to work and plugged it into a socket in an empty room and it's still there
• I don't think its a ground short, I checked the ground points based on Grainger's tutorial and they're all good
• It's not due to my DAC or anything else in the chain because it happens with nothing connected to the RCA jacks, but also happens when something is connected

The only modding I've done to the amp is installed a 23-step 100k attenuator. I did notice that after installing it a while back it would pick up some of the local radio station frequencies and play them at a very low level when the volume was all the way down, but it goes away immediately upon turning the volume up. I'm going to remove it tonight and reinstall the original pot to see if the attenuator is the culprit. I'd love any other suggestions you guys might have as to what to check. At this point I'm not quite sure what else to do.

[Doc B.]

It sounds like the attenuator may be the problem and that your plan of attack is sensible.

[Grainger49]

Mike,

This kind of stuff can drive a monk to curse!  I pulled my hair out a couple of years ago because the imaging disappeared.......   I had the speakers out of phase!

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi244.photobucket.com%2Falbums%2Fgg7%2FGrainger49%2FSmiles%2FSlapHead.gif&hash=e5c580c3cfdf525c26703ae0328aefb586b04de5)

I would guess, an educated guess, that this interference is something outside your Crack and something that the 121BH7s are not susceptible to.  The #1 culprit is a computer within 3' of the amp.  The #2 is a cell phone.  After that you bunch up refrigerators, light dimmers, air conditioner compressors and washing machine motors in a group.  Ok, that last group wouldn't have followed you to work, but they are problems a lot of places.

And, still, it could be a bad solder joint.  They become diodes and do screwy stuff.

As for the RF interference you can go to Grainger.com (no relation I'm sorry to say) and buy some copper sticky backed tape and put it all around the inside of the base.  Then ground it to the top plate.

[MikeSattler]

Thanks for all the tips guys. I went through the amp this weekend and reflowed every single joint but the problem is still there. Still can't figure out why the amp is dead silent when using a 12bh7 but has the noise with every other 12au7 and 6sn7 i've got. I'm thinking heat is a big factor, because it's pretty consistent in that it only only appears once the amp has warmed up over about 2 minutes or so.

I'm going to try getting some of the copper tape and line the base with it, but after that I'm out of things to try. What would be some of the reasons other than RF that its not an issue with a 12bh7? I don't mind using 12bh7's but I've got a whole lot of RCA cleartops and Amperex 12au7's that sound really nice (just not when they buzz)

[Grainger49]

You are right, you should be able to use the tube designed for this amp.  There are thousands of these out there and most of them use 12AU7s.  So something is screwy about the setup or your location.  Some locations are just RFI rich, a bad word for a bad situation.

When trying the tape be sure that all four sides are grounded to a ground lug.

[MikeSattler]

Wanted to close this out with a resolution. After re-soldering all the connections and fixing a few wires that I nicked with the wirestrippers, I think the problem was a combination of router noise, and dirty tube sockets. I went and got those little dental brushes that you're supposed to use to floss your teeth with and cleaned out the 9-pin socket with some alcohol and that helped a lot. The remaining noise I'm getting sounds like interference and it's only coming through in the cleartops so maybe those particular tubes are just sensitive.

Any recommendations on a better solvent to clean the sockets with? I was planning on ordering some deoxit as some people on other forums have recommended it.

[Doc B.]

Deoxit is good stuff. Tube pins are usually worse than the socket contacts. The trick I learned was to cut a Q-tip in half, chuck it in a Dremel and use it as a buff with some metal polish to clean tube pins. Then treat them with a little bit of Deoxit. Getting that tube pin/socket interface to be a really good connection can help a lot to reduce the pickup of EMI.