Odd tube vibrations...what does it mean?

JimmyM · #1 12-11-2010, 07:04 PM

i had a bum tube in my svt, and went out to go find it and change it just now. as i was doing so, i picked up a jj 12dw7 and noticed that if i touched out of the socket pins, the whole tube would vibrate. so i tapped the pins lightly on top of the head while holding it, and i could feel them vibrating for a good second after i tapped. i had another jj 12dw7 as the phase inverter, and i took it out, and it did the same thing. i've also got ge, sylvania and amperex nos 12dw7's but none of them vibrated.

what's the deal? is that a sign they're trashed? it was a jj that was causing the problem, but since channel 2 works fine and both channels go through the phase inverter, i'm wondering if that's a sign of problems to vibrate like that, or if it's just the way they're made.

beans-on-toast · #2 12-11-2010, 07:55 PM

I'm not sure that I understand. Are you saying that everything inside the tube vibrates when you whack it?

Normally there are mica (or some other material) spacers and support rods that give the tube structure and help prevent microphonic noise. I put some arrows on the spacers. The spacers push up against the glass and pressure helps to prevent the innards from vibrating. If your spacers are not touching the glass, this could be why it is vibrating when you tap it.

It is also possible that if there is a support rod that isn't welded and just that part of the internal structure is vibrating.

External vibrations can cause the internal structure of a tube to come loose. They work but normally they are microphonic. I don't know if this is normal for the JJ 12DW7's. If your head is not subjected to a lot of vibration, you might not hear any microphonics. Try pinging the tubes and see if you hear anything.

David

Jerrold Tiers · #3 12-11-2010, 08:32 PM

They are not supposed to vibrate....

If you look at the mica pieces in the JJ B-O-T posted the picture of, you see little points on the spacers. Those points are supposed to crush against the tube inner surface and hold the insides solidly.

You may notice that the pictured JJ has some of the "points" NOT touching, certainly not deformed in any way, but others are. The one the upper arrow points to is probably not even touching the glass. That tube might possibly do the same thing as yours. Looks like JJ has some issues there.

JimmyM · #4 12-11-2010, 08:37 PM

that pic looks almost exactly like mine with the bent up spacer tine. wasn't quite that bad but they were bad enough. compared to the nos tubes in there, those jj's look piddly. especially compared to the amperex. look at this pic of a philips 12dw7:

now that's a tube! thanks a lot, david and jerrold...i think that's exactly what's going on. those tubes vibrate in the same way as when you strike a tuning fork. those jj's sure don't look like quality tubes by comparison. i'll be sticking with nos, i do believe.

Interceptor · #5 12-11-2010, 08:39 PM

Tubes are not supposed to vibrate.

But some do.

It is why some of us "geezer class" techs thump around in an amp with chop sticks.

When it comes to vintage gear, my rule is 80% of the problems are either mechanical or dried out caps.

You found a cheezy batch of tubes. Change suppliers.

Jim

okcrum · #6 12-11-2010, 08:57 PM

Back in the day, I'd occasionally see a tube like that. Mostly sweep or power tubes, though. Even the NOS can have some duds like this. I expected JJ to be better, considering how much I've heard people talk them up.

JimmyM · #7 12-11-2010, 08:59 PM

ain't the caps, thank god. all caps were changed out a year and a half ago, and the problem is only in channel 1.

well, i guess the price is right on the jj's. $10. you get what you pay for. i think i'm going to get a couple more amperexes. they looked like the most solid tubes and the price is right. i could get teles or mullards at $85 a pop, but i doubt i'll be able to tell the difference.

beans-on-toast · #8 12-11-2010, 09:40 PM

Antique Electronics has NOS Philips 12DW7's for just under $17, GE for a few bucks more. Not much more than the JJ's. Unfortunately, they won't test them for microphonics for you like your local dealer will.

UncleFluffy · #9 12-11-2010, 09:50 PM

Quote:

Originally Posted by beans-on-toast

Unfortunately, they won't test them for microphonics for you like your local dealer will.

Yes, but at that price you can buy 3, keep the best one, and sell the other two on eBay for twice what you paid.

Jim C · #10 12-11-2010, 09:54 PM

Maybe this explains my SVT tube vibration issues:
The head is on a Berg cabinet and I use an Ampeg Speaker selector switch to A/B with different amps and cabs
When the SVT is turned off, the filaments sound like they are vibrating when a different amp is driving the cab below; when the SVT is being used I don't get the sound; don't think it is just the fan masking the sound either
Could these filaments vibrate when turned off but not vibrate when the amp is being used?
Hesitate to touch this head as it sounds so right to me

JimmyM · #11 12-11-2010, 10:14 PM

jim, no idea if that's the problem, but there are plenty of other things in those amps that can vibrate besides the tubes. if it sounds good otherwise, i probably wouldn't worry too much about it.

anyway, so for the time being, i need my svt, so i have to use one of the jj's. i'll use the one that isn't causing problems in channel 1, though

so atm it's in the phase inverter slot. should i move it to the midrange selector slot? would that be a better place for it?

will33 · #12 12-11-2010, 10:35 PM

Jimmy, I know you're not one to crank the hell out of/abuse your gear but any amount of heat makes things expand and cooling contracts to some degree anyway.

Possibly over time this causes warps/uneven expanding and shrinking, etc.? Sooner if metal or other material quality/purity is a dime short of a dollar?

I don't have the technical background to back up blaming it on the tube or whatever but I do know from experience that quality in the long run can be quite different from quality out of the gate. For example, any new car will test great when it's new.....some last 5 years, some last 50 or more.

Gort · #13 12-12-2010, 03:03 AM

Quote:

Originally Posted by Interceptor

It is why some of us "geezer class" techs thump around in an amp with chop sticks.

Never thought of that, I've always used the eraser on the end of my pencil.

Mr. Foxen · #14 12-12-2010, 04:24 AM

Probably should note that just because a valve is microphonic doesn't mean it is dead, if they work fine and aren't feeding back or anything, just aim to put them in positions with less gain after them.

beans-on-toast · #15 12-12-2010, 07:45 AM

Quote:

Originally Posted by Jim C

Maybe this explains my SVT tube vibration issues:
The head is on a Berg cabinet and I use an Ampeg Speaker selector switch to A/B with different amps and cabs
When the SVT is turned off, the filaments sound like they are vibrating when a different amp is driving the cab below; when the SVT is being used I don't get the sound; don't think it is just the fan masking the sound either
Could these filaments vibrate when turned off but not vibrate when the amp is being used?
Hesitate to touch this head as it sounds so right to me

Perhaps when the Ampeg is powered up the heat expands things just enough to stop the rattle. One way to test it is to pull the tubes when they are cool and tap them.

If you have a newer head with the spring retainers on the power tubes, they can be a source of vibration noise. You can try putting a heat resistant tubing over the springs to dampen them.

beans-on-toast · #16 12-12-2010, 08:10 AM

Quote:

Originally Posted by JimmyM

anyway, so for the time being, i need my svt, so i have to use one of the jj's. i'll use the one that isn't causing problems in channel 1, though

so atm it's in the phase inverter slot. should i move it to the midrange selector slot? would that be a better place for it?

If you are short a good 12DW7, you can substitute a 12AX7 for the one in the power amp. Then put your best 12DW7 in the pre. At least you can try it and see how it sounds.

Whenever you change the PI tube it is a good idea to check the balance setting. A good reason to have a 300W dummy load and a voltmeter.

JimmyM · #17 12-12-2010, 09:56 AM

right on, david...i think i'll try that when i get back from getting a new fan. this thing finally gave up the ghost. it still works but it rattles.

Jim C · #18 12-12-2010, 10:08 AM

Quote:

Originally Posted by beans-on-toast

Perhaps when the Ampeg is powered up the heat expands things just enough to stop the rattle. One way to test it is to pull the tubes when they are cool and tap them.

If you have a newer head with the spring retainers on the power tubes, they can be a source of vibration noise. You can try putting a heat resistant tubing over the springs to dampen them.

Great idea; thanks.

seamonkey · #19 12-12-2010, 10:54 AM

Quote:

Originally Posted by UncleFluffy

Yes, but at that price you can buy 3, keep the best one, and sell the other two on eBay for twice what you paid.

Yeah that's it, pass the crap tubes onto a poor unsuspecting tube noob. The tube manufacturer and distributor already did it to you, so you should justifiable pass it on to someone else.

Tube manufacturers don't care - they can skip quality checks and still sell product.

UncleFluffy · #20 12-12-2010, 11:07 AM

Quote:

Originally Posted by seamonkey

Yeah that's it, pass the crap tubes onto a poor unsuspecting tube noob. The tube manufacturer and distributor already did it to you, so you should justifiable pass it on to someone else.

Tube manufacturers don't care - they can skip quality checks and still sell product.

I get your point, but there's a lot of distance between "best" and "crap".