I can hear the ocean

November 5, 2009 6:48 AM

You are not my amp tech, but maybe you can help me diagnose the source of a strange noise on a brand new guitar amp.

Yesterday, I got a tiny tube amp in the mail. After letting it come up to room temperature, I plugged it into a cab (yes, with the right impedance) and played away. No problems at all; made some really nice sounds, by the way.

The next time I turned it on (an hour or two later), there was an awful static noise--sort of like rushing water, or waves crashing--even with the volume all the way down and no guitar attached. Playing through it produced pops and crackles and unpredictable volume changes, and the static continued.

It's got 2 12AX7 tubes (and a funky setup--V1 and one triode of V2 are the preamp, and the other triode on V2 is the power tube). Best I can tell (which is not saying much), they both look normal, glow a healthy orange, aren't microphonic.

Now, the head is brand new, so there's a really good chance it's going back to the vendor as defective, but out of curiosity (and future diagnostic needs), what could the problem be?

posted by uncleozzy to technology (6 answers total)

Tubes are the most likely culprit. Since they are both 12AX7 tubes, try swapping them around, if you haven't. Sometimes tubes that are microphonic or cause other undesirable noises in some circuits will work fine in other positions. If you've got other 12AX7s to try do that as well (or borrow some from a friend's amp).

Current production tubes have a higher failure rate than the ones made in the US and western Europe back in the day. Some vendors rigorously test tubes, some don't, so simply having (presumably) new tubes doesn't mean they are "good".

If swapping the tubes doesn't fix it then it's time to contact the vendor.
posted by 6550 at 7:06 AM on November 5, 2009

Yeah, I haven't tried swapping the tubes yet (you've got to remove the whole chassis to really get to them), but I'll give it a shot tonight. I've also got a Fender amp with a few 12AX7s in the preamp section, so maybe I'll swap those in.

It's just sort of odd that a) the preamp gain doesn't affect the static; and b) it played just fine for awhile. Oh well, if I have to RMA it, I will, but it's such a hassle that I'd rather see what I can do first.

Thanks for the insight.
posted by uncleozzy at 8:17 AM on November 5, 2009

If the noise related to adjusting the knobs, there could just be dust/dirt in them. Try spraying some Deoxit in them. (Available at Guitar Center or online) (Anyone with electrical/electronic musical gear should have a can anyways). Just a thought, as it may be more practical than having to RMA it. A can goes for about 15 bucks.
posted by peewinkle at 10:13 AM on November 5, 2009

*is related to
posted by peewinkle at 10:14 AM on November 5, 2009

It's just sort of odd that a) the preamp gain doesn't affect the static; and b) it played just fine for awhile.

If you look at the block diagram in the owner's manual you'll see that the volume control is after the first preamp stage (V1A). Since the volume doesn't affect the noise that means the noise is coming from somewhere after that stage (V1B, V2A, or V2B). Sometimes new tubes will play fine for a bit, and then begin to fail, so the fact that it worked for awhile isn't necessarily indicative of a non-tube problem, although it could be. Another potential failure is components besides a tube (resistors, capacitors, potentiometers, transformers) although I think all but the pot (prone to mechanical failure) would be somewhat unlikely here in a new amp. So peewinkle's Deoxit advice is good generally, but unlikely to help since the volume control doesn't affect the noise.

Cold solder joints can also be problematic, and crop up over time, sometimes creating intermittent failures, including noises. Even the best manufacturers miss cold solder joints occasionally. A loose or poorly routed wire can also cause noise, and in the case of a loose one may crop up only when moved/bumped into a certain position.

I do remember reading a post about improving the wiring routing in a Blackheart amp, though I think this was due to hum and maybe hiss. Those wouldn't be issues that would have cropped up like the static noise you describe.

In any case, let me know what happens when you try different tubes.
posted by 6550 at 10:46 AM on November 5, 2009

Aha, yep, it's the tubes. Swapping V1 and V2 didn't do any good (although I didn't keep track of which was which after I took them out altogether, so I don't know what position the bad one was in to begin with). In any case, I swapped in the Sovteks from the Fender, and viola, no more static.

Thanks again. Guess I'll see if Crate will send me a replacement tube, or maybe I'll just get some JJs and put the Sovteks back in the Fender.
posted by uncleozzy at 3:14 PM on November 5, 2009