tube rattle

[BlueSky]

I've had my 2-channel H combo for a few days now and I'm very happy with the amp. Unfortunately, my biggest worry about combo amps has been confirmed and today I noticed some tube rattling. I play it reasonably loud (Volume 2 o'clock and Master at about 10 o'clock). The rattle is quite audible on both channels, lead and clean. The typical ringing rattle that we all hate to hear.
Both power tubes look ok, although one of them has a slight blue glow on the outside. However, I've read in the past the blue glow is not an issue. Once I've identified the culprit (tapping method) and replace the tube(s), what can I do to avoid tube rattle in the future? I realise a combo amp is not ideal for tubes and more prone to cause that problem. The build quality of the cab seems top notch and there's a separation between the chassis and the speaker area. Plus the tubes are facing upwards. PRS did everything right to avoid the rattle and yet it happened with my amp.
Are there specific tube brands that are sturdier?

 

[László]

One possibility to be aware of -- and this might not be the case for you but it's worth testing -- is that sometimes we perceive things like rattles in light bulbs and light fixtures, furnace ducts and metal vent louvers, and other things vibrating noisily in a room as tube rattles. They can fool you and make you think they're coming straight from the amp.

I was driven literally crazy about ten years ago with what I thought was absolutely, positively tube rattle in a Mesa, only to move the amp to another spot in the room where I could hear more clearly that the rattle was coming from a light fixture that was on the ceiling near where the amp was located. When I fixed that, "tube rattle" went away.

So you might want to just move the amp around the room or in a different room and see what happens just to check.

I've had strange "blown woofer" sounds in a studio monitor that turned out to be the platform on my speaker stand vibrating (fixed by tightening bolts, and discovered only after I'd sent a speaker back to Genelec for repair, having them call and say they tested it in an anechoic chamber and there was nothing wrong with it), and ceiling tiles in basement rooms buzzing when recording a bass amp, and both of these problems took days of wasted time to solve, they each absolutely, positively appeared to come from the speakers. And I'm no audio newbie...this really fooled me.

As to tubes that exhibit less rattle, it's mainly luck of the draw, but NOS military grade tubes are often less prone to tube rattle, because they often (though not always) have thicker glass, etc. So that might be something to look at.

There are also special rings and gizmos that are made to fit around tubes that some folks swear by. I've never tried them, but it might be worth looking at them. These are often used by audiophiles:

http://www.positive-feedback.com/Issue7/halo.htm

They aren't expensive, and might be worth a try.

As you know, all tubes exhibit microphonics to some degree. So in theory these might work by damping the vibrations inside the amp. Truth is, that even a separate head that's not sitting on a cab will have a bit of microphonic noise just from the SPLs, though obviously it will be greatly reduced by distance.

 

[BlueSky]

Thanks for your quick response L. The rattle or rather ringing noise definitely comes from the tubes. Especially once the tubes have warmed up. Not really an issue when playing in a band context but annoying when playing on your own, especially on the clean channel.
I might try the tube rings http://herbiesaudiolab.net/guitar.htm, although I find it hard to see how they can stop a tube from vibrating... What type of 6L6 is in the "H"? Standard 6L6 or 6L6GC? I haven't opened the amp yet and it's not here at home.

 

[aristotle]

Tube rattle can drive you nuts. I go through phases where it bugs me and I take it on to solve the problem for some particular amp. I have a signal generator that I use and sweep the frequency until I get the thing rattling (usually equivalent to a G note on the low e string for some reason...) With the rattle happening continuously, I can usually isolate the problem. Sometimes it's a power tube, sometimes a pre-amp tube, sometimes it's in the cab itself, sometimes it's a reverb tank. There seems to be no correlation between the character of the rattle and the source of the rattle surprisingly....at least to me. Just takes trial and error.

Les is right though, it really can be something else rattling in the environment. In any case, with a combo amp, the first thing I usually do is remove the amp from the cab and separate it by 10 feet or so. That pretty much tells me if it's the cab or something in the amplifier itself. If the rattle goes away (and it usually does) then it's in the amp, and probably a tube, and with the head out and exposed while being continuously driven it's pretty easy to isolate.

I've become a bit more laid back about it lately though. Speaking of environmental rattle, my MDT head rattles like pennies in a tin can when it's sitting off to the side and I'm playing another amp. Doesn't rattle a bit when played though. Very odd...

 

[BlueSky]

Well, I gigged the amp last night and there was no rattle -- at least none that was audible. Good news. So it must be the poor acoustics in the rehearsal room that made the tubes ring. It's very boomy in there.
The amp sounded awesome by the way.

 

[Boogie]

Les and Aristotle make very good points. Psychoacoustics can be a PITA. Plus, you can cause other items in the vicinity to vibrate in weird ways. And though my PRS amp is a head, it sits in top of the cab and is subjected to the same abuse as my combos. So, for me, the form factor has little impact on this topic. I've also noticed that when trying to identify a microphonic preamp tube (can power tubes go microphonic? I've never heard one. ), the tap method doesn't always work. For example, new Tung Sols tend to have a resonant frequency that only shows up at higher volumes. And that's only in my Boogie. Strange things, these tubes.

Good luck sorting it out, but I wouldn't get too esoteric...you'll go crazy.

 

[László]

Les and Aristotle make very good points. Psychoacoustics can be a PITA.

Here's the crazy part of my Genelec studio monitor story that proves your point, Boog --

When I thought the monitor went bad I was in the middle of a really important project for a car company. I couldn't be without monitors, so I bought another pair of monitors so I wouldn't let the client down. This shows how fooled I was by the noise I heard - I could literally pinpoint the physical point on the monitor that I heard the buzzing come from, and this includes going over to the monitor, and feeling for the vibration!

I was completely fooled!

Psychoacoustics are truly amazing problems.

When I realized the problem was in fact the freakin' speaker stands I felt like the biggest idiot in the history of the world, and was out well over 3 grand for the new monitors, on top of everything. Fortunately, my son really needed a better pair of monitors, so I sent him the Genelecs, and at least got to feel good about my error.

3 grand plus, only to discover that the solution was an Allen wrench to tighten a couple of bolts on my speaker stands!!

 

[BlueSky]

I don't think it was a matter of psychoacoustics in my case. I could quite clearly hear the tubes ring. The acoustics in the rehearsal room is so bad (very bassy and boomy) that the soundwaves reflected from the walls and hitting the back of the amp caused the tubes to ring.
Thanks for your advice and encouragement.

 

[László]

I don't think it was a matter of psychoacoustics in my case. I could quite clearly hear the tubes ring. .

Yeah, I just wanted to get the whole story of the "bad speaker" off my chest.