Weird Symtoms

[wedgeSG]

I've been using Valve Juniors for a while now but really haven't been around tubes that much

in the grand scheme of things time wise. Tonight during an extended session, (wife and son are

on vacation and I have the whole place to myself!!!!), I got this strange, low-sounding, whoosh of

feedback that wouldn't change up or down in volume and stayed constant until I shut the amp down.

I checked all the connections, everything's fine, turned it back on, powers-up OK, although it did seem

to take just a little bit longer than usuall - but not by much if at all. (I might have just been so freaked

out you know.) First few minutes it seemed to exhibit some oscillations of sound during sustained

notes, not the motorboating that modders refer to, this sounded like a Leslie effect. After a few more

minutes of playing at the same level of volume and output it was much less noticable. So the questions

are what the #$%&*!@ happened/is happening? Is this a symptom of tubes going out or something

much worse? Finally, and this is really a newb kinda question, how do ya know and what should be a

tip off that tubes need to be addressed? I've changed tubes for tone, I've replaced tubes that were

DOA, but I've never actually had one die on me before..... please help shed some light on what I've

experienced since I'm totally perplexed by all of this. (To help further describe the noise/feedback I

had: think of a single-tone foghorn stuck in an on position)

 

Wedgie

[Dave]

Sounds like the feedback that occurs when you leave the amp on and put the guitar on a stand at a gig. After a few minutes, the strings oscillate with the ambient noise, the amp reproduces the sound, and the strings begin to oscillate more and it gets louder and louder. That can happen if you are right next to the amp with the amp cranked. Does it go away when you damp the strings with your hand? If not, it may be a microphonic tube. Tap on them one a time with the eraser end of a pencil. If you hear noise when you tap, that's the microphonic one. Since it didn't respond to the volume control it may be a power output tube or even a preamp tube could do it since the volume control is before the grid of the preamp.

[m-theory]

With a tube amp, you can quite often attribute bizarre oddities to tube failures. Usually, they'll just gradually fade away. In fact, the wear is so gradual that, unless the player is VERY familiar with the amp and plays it VERY frequently, he probably won't even notice it. Sometimes, however, odd symptoms can appear. If you've checked and verified the guitar, the cable(s), and anything else in the signal chain, your next step is to swap tubes, one at a time.

[wedgeSG]

thanks for the imput guys... have you ever heard anything like what I tried to describe? Everything

was playing fine and then WHOOOOOOOOOOOOMMMMMMMMMMMMMMMMM.... couldn't make it

shut-up. Knobs, hand mute, everything short of yanking the cord outta the jack until I just killed

the power at the on/off amp switch. Noise came on full and never changed. Shaken up by all of this-

and checking connections; I suppose it had maybe 10 minutes before I turned it back on fully expecting

smoke to rise like a Pope had been chosen when I did. Seems OK now - but like you mentioned, maybe

I've gotten so accquainted with it, I ain't missin' what used to be there. Which tube would you suggest

swapping out first? One thing I forgot to mention was that I did have a pedal in the chain.... a tubed

E-H English Muff'n. It also checks out fine, and the gain was less than 1/4, voulme less than 1/2, when

the tornado roar occured. Cables are nearly new Monster Performer 500's that are never abused and

always stored loosely coiled. Anything else that might be a culprit I've overlooked?

 

Wedgie

[m-theory]

have you ever heard anything like what I tried to describe?

No

 

Seems OK now

Then don't sweat it.

 

Which tube would you suggest swapping out first?

Output, but you've only got two to deal with, and they live right beside each other, so it's not as if you're talking about major amounts of effort to swap either.

 

Anything else that might be a culprit I've overlooked?

Sure, it could be a filter cap shorting out, or a flaky connection in the chassis somewhere. Hard to say offhand.

 

Here's what I'd do, if it were mine:

 

1) Have both tubes on hand in case it happens again

2) As soon as it happens again, unplug and swap the output tube

3) If it continues, unplug and swap the preamp tube

4) If it continues, you've got something going on inside the chassis.

 

At this point, with it not acting up currently, I'd just get the tubes and have them nearby while I continued to play the amp as normal. I'm sure it'll act up again at some point. Until it does, an erratic problem such as this can be difficult to pinpoint.

[wedgeSG]

Good info, makes sense, thanks!!

 

Wedgie