#1
Last April i bought an Ampeg GVT52-112, it's a guitar amp combo.
Solid state rectifier, three 12AX7 preamp tubes, two 6L6GC power tubes.

A couple of weeks ago, it started making some noise while playing.
The noise only came up after using the amp for a while, and it wasn't constant. But once it started, it wouldn't go away until the next time i used the amp.
It was some sort of "crackling", present on both channels (clean & drive).


Just now, i was playing for about 10min, then the noise started.
Not 5min later, i heard some sort of "pop", the guitar signal stopped, and a loud hum started.
I turned the amp to standby, the loud hum started to decrease, i turned off the amp.

Now, when i turn the amp on, all i get is that loud hum. Even with nothing in the input jack, and with the gain, volume and reverb knobs turned all the way down.


Does anyone have an idea about what's wrong with my amp?
Is it an easy fix?
I have a gig next friday, and i can't shake the feeling that i'm completely screwed...
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Last edited by Linkerman at Aug 27, 2013,
#2
Since you can still turn your amp on, you most likely didn't blow a fuse. You might have blown your phase inverter, or possibly your V1.

Do you have any spare 12AX7s? If not, for shame, you always should. Swap some stuff around in the preamp.
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#3
Quote by Offworld92
Since you can still turn your amp on, you most likely didn't blow a fuse. You might have blown your phase inverter, or possibly your V1.

Do you have any spare 12AX7s? If not, for shame, you always should. Swap some stuff around in the preamp.

It's my first tube amp, and even though i've read a bit about the technical aspect of it, i'm still a complete newbie.

I don't have any spare tubes, and the amp is running on the stock tubes, the ones came with it from the store.

How do i go about "swapping some stuff around in the preamp"?
I already read some stuff about it, but i'd appreciate if you could give me step-by-step instructions... Since i don't know if the amp has to be on, off or in standby; or if i have to wear gloves handling the tubes (i remember something about the natural oils from the skin remaining on the glass and making it crack when the amp is running hot), for example...
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#5
That sounds almost exactly like what happened with my Valveking years ago. Luckily there was no permanent damage. Swap V1 and V2 of your preamps first just to see what happens. Then maybe V2 and V3.

Watch this - it might help.




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Last edited by 311ZOSOVHJH at Aug 27, 2013,
#6
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#7
Quote by 311ZOSOVHJH
That sounds almost exactly what happened with my Valveking. Luckily there was no permanent damage. Swap V1 and V2 of your preamps first just to see what happens. Then maybe V2 and V3.

Watch this - it might help.

video

Thank you!

Right now i can't do it since everyone in the house went to sleep in the meantime and i don't want to make noise, but i'll do it first thing in the morning.


So, with the amp turned on (but without anything plugged in), tap on the preamp tubes with the rubber end of a pencil.
If the tapping comes sounds through the speaker, that tube is microphonic and has to be replaced.

Then, with the amp turned off, swap V1 with V2. Turn the amp on, see (hear) what happens. Afterwards repeat the process for swapping V2 with V3.
I just have two questions about this: 1.) how do i know which one is V1, V2 and V3?, and 2.) what changes should i look for from swapping the tubes around?
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Last edited by Linkerman at Aug 27, 2013,
#8
I doubt you will notice much difference in the tone of the tubes, we are just trying to see if we can make some educated guesses about what is going on. You can also try running the amp with a preamp tube missing. If the crackling doesn't go away with moving some tubes around you can almost bet it not something to do with the preamp (although it could be a socket problem). Just saying. It is an easy test.

A microphonic tube may 'sound' different in different slots.

Typically, your V1 will the closest to the guitar insert jack and V3 will be closest to the power tubes. Your signal goes from the guitar through the preamp tubes and then onto the power tubes (simply put).
#9
Quote by 311ZOSOVHJH
I doubt you will notice much difference in the tone of the tubes, we are just trying to see if we can make some educated guesses about what is going on. You can also try running the amp with a preamp tube missing. If the crackling doesn't go away with moving some tubes around you can almost bet it not something to do with the preamp (although it could be a socket problem). Just saying. It is an easy test.

The microphonic/pencil test is super easy, so i'll start with that.

But if that test yields no results, and since you say i can run the amp with a preamp tube missing, then wouldn't it be easier to just try the amp without one preamp tube at a time? Before swapping tubes around, i mean.
Like, remove V1, power the amp on, see if the loud hum goes away. If not, re-insert V1 and remove V2, power the amp on, see if the loud hum goes away. If not, re-insert V2 and remove V3, power the amp on, see if the loud hum goes away.

Quote by 311ZOSOVHJH
A microphonic tube may 'sound' different in different slots.

Typically, your V1 will the closest to the guitar insert jack and V3 will be closest to the power tubes.

Got it.

Quote by 311ZOSOVHJH
Your signal goes from the guitar through the preamp tubes and then onto the power tubes (simply put).

That much i know, i already studied the way the sound is shaped by each gain stage, the difference between pre and power tube distortion, and my amp's block diagram.
As far as the rest goes, i'm pretty much ignorant.
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#10
^ I didn't mean to make you feel stupid, what I meant was that if your amp is laid out fairly logically you can usually determine which tube is V1 and which tube is the phase inverter, (the last preamp tube - in your case V3) just by looking at the lay out. Sometimes, like the Marshall MA or a 5150, it is not that easy.

Yes, you can try pulling a tube vs swapping.

In the mean time, get an extra preamp tube on hand for times like this and get some Dioxit contact cleaner as well for the tube pins.
Last edited by 311ZOSOVHJH at Aug 27, 2013,
#11
Quote by 311ZOSOVHJH
^ I didn't mean to make you feel stupid, what I meant was that if your amp is laid out fairly logically you can usually determine which tube is V1 and which tube is the phase inverter, (the last preamp tube - in your case V3) just by looking at the lay out. Sometimes, like the Marshall MA or a 5150, it is not that easy.

Don't get me wrong, i said that so you can have a notion of what i already know -- which is next to nothing -- to get the best advice from you and don't try anything that would blow up the amp or one of my hands. I know you weren't insulting me.

Oh, and to be absolutely clear, when i said "Got it.", those two things i did not know.
I said "Got it." meaning "I understood.".

Quote by 311ZOSOVHJH
Yes, you can try pulling a tube vs swapping.

Okay, i'll try pulling the tubes before swapping them around.

Quote by 311ZOSOVHJH
In the mean time, get an extra preamp tube on hand for times like this and get some Dioxit contact cleaner as well for the tube pins.

Around here i don't think i'll find much selection on tubes, but what brands should i look for?

Also, i've never seen Dioxit contact cleaner. Would WD-40 do the trick?
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#13
STATUS UPDATE

I just did the microphonic tube/pencil rubber tip test. No results.
The tapping noise didn't sound through the speaker, just the loud hum and slight crackling (which remained unnafected, btw).


Then i pulled one preamp tube at a time and turned the amp on, no change whatsoever.
Is it still worth swapping the preamp tubes around?


But when i opened the back of the amp to do the microphonic tube test, i noticed that one of the power tubes is glowing blue.


Here's a video demonstrating the issue and the blue glow:

http://youtu.be/V_dJgdFZQ28

Amp is in standby, then is turned on (2nd power tube glows blue).
Loud hum and slight crackling comes up.
Amp is turned to standby again (2nd power tube stops glowing blue).
Hum and crackling volume starts to decrease until stopping.
Amp is turned off after noise stops.


The noise remains unaffected by turning the gain/volume/master/reverb knobs.
It also remains unaffected by plugging a cable in the input jack.
All i get from the amp is what you can hear on that video.

Any ideas on what the problem is?
Could it be the blue glow on the power tube?
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#14
Based on the fact my power valves don't glow blue, I'd say you need a new matched pair of power valves. A single might not be rated the same so you should buy a matched pair and keep the one that doesn't glow blue as a spare. Did you tap on the power valves with a pencil as well, or just the pre-amp's?
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#15
Quote by Lavatain
Based on the fact my power valves don't glow blue, I'd say you need a new matched pair of power valves. A single might not be rated the same so you should buy a matched pair and keep the one that doesn't glow blue as a spare.

I just read a few things about it (like this), and the blue glow can be perfectly normal. How do i know for sure?

If the problem lies somewhere else, i'd rather not spend the money on new tubes right now, i'm a bit strapped for cash at the moment.
The amp has a 3-year warranty, so i just want to make sure it's not a tube problem before having Thomann collect the amp for repairs. Tubes are not covered by warranty, so it'd be a pretty expensive tube change, considering the shipping costs from Portugal to Germany and back to Portugal.

Quote by Lavatain
Did you tap on the power valves with a pencil as well, or just the pre-amp's?

I just tapped the preamp tubes. Should i tap the power amp tubes as well?
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#16
Quote by Linkerman
I just read a few things about it (like this), and the blue glow can be perfectly normal. How do i know for sure?

Then I apologise, as I just wasn't aware of blue glow.
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#17
Hey Linkerman,

Something is up and it is not good.

Unfortunately, I think you need to exercise your warranty.

Normally, a blue glow in a power tube is completely normal, especially on Stand-by, but you only have it in one tube and it appears overly bright blue to me. What I would do is swap those two power tubes and see if the blueness follows with the tube. I don't think this will matter much as I don't think that is your problem.

Was the crackling I heard at :15 what you hear all the time? Man that sucks.

Send it in.

I was really hoping it was just a preamp tube problem.
Last edited by 311ZOSOVHJH at Aug 28, 2013,
#18
I just went to do what Lavatain suggested (tap the power amp tubes), and the right power tube, the one with the blue glow, started redplating (i think it's what people call it):






You probably can't tell from the pictures, it looks like a violet glow all around, but what's happening is that the blue glow around the top and bottom "discs" remains, and the center gray "block" is glowing red.

Could it be the source of the problem?
What causes it?
What is the solution?

Quote by 311ZOSOVHJH
Hey Linkerman,

Something is up and it is not good.

Unfortunately, I think you need to exercise your warranty.

Normally, a blue glow in a power tube is completely normal, especially on Stand-by, but you only have it one tube and it appears overly bright blue to me. What I would do is swap those two power tubes and see if the blueness follows with the tube. I don't think this will matter much as I don't think that is your problem.

After this new information i just posted (about the center "block" of the tube glowing red),
  • is it still worth swapping these two power tubes, or is the problem found?
  • if you found the problem, should i: exercise the warranty; simply change the two power tubes; or is it a simple repair that won't be expensive, so i could take the amp to a nearby store instead of shipping the amp to a different country to exercise the warranty?



Quote by 311ZOSOVHJH
Was the crackling I heard at :15 what you hear all the time? Man that sucks.

As i said in the OP, that crackling came up a few weeks ago.
It wasn't constant, and it only appeared after playing the amp for a while.
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#20
Quote by 311ZOSOVHJH
yes that is red plating and yes send it in

So it isn't even worth swapping the two power tubes to see if the issue follows the tube?


And as i asked before, should i:
  • exercise the warranty (the amp will have to go from here, Portugal, to Thomann in Germany and back to Portugal);
  • or is it a simple repair that won't be expensive, so i could take the amp to a nearby store instead and pay for the repairs out my own pocket?
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#21
Quote by Linkerman
So it isn't even worth swapping the two power tubes to see if the issue follows the tube?


And as i asked before, should i:
  • exercise the warranty (the amp will have to go from here, Portugal, to Thomann in Germany and back to Portugal);
  • or is it a simple repair that won't be expensive, so i could take the amp to a nearby store instead and pay for the repairs out my own pocket?

If your 3 year warranty covers valve problems and shipping costs then yeah send it back to them. It might be that you get a 6 month warranty instead of the full 3 years for valve problems, or something like that. If you don't get that cover then take it in to a local shop if you know a good one as it MAY need biasing, or at least they can check the bias without you forking out on tools to check biases.
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#22
Send it in, whip out the good ol' acoustic.

EDIT: Since the amp has stock tubes, it should be covered even if it is a valve problem technically. My argument would be that something in the amp caused the tube to un-bias and red plate.

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Last edited by Funk Monk at Aug 28, 2013,
#23
Quote by Lavatain
If your 3 year warranty covers valve problems and shipping costs then yeah send it back to them. It might be that you get a 6 month warranty instead of the full 3 years for valve problems, or something like that.

Just sent Thomann an e-mail; i guess it's the better option, instead of paying for the repairs from my own pocket...
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#24
It could be the power tubes but it could also be something in the bias circuit that's blown. Sending it back is the wisest option.
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#25
from what i have seen on warranties on amps, (I have seen), which usually the tubes are only replaceable by warranty for 30-60 days (most common) i think splawn may be 90, but i don't know.

be sure to read your warranty before you ship it out.
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#26
Quote by Cathbard
It could be the power tubes but it could also be something in the bias circuit that's blown. Sending it back is the wisest option.


+1

Quote by trashedlostfdup
from what i have seen on warranties on amps, (I have seen), which usually the tubes are only replaceable by warranty for 30-60 days (most common) i think splawn may be 90, but i don't know.

be sure to read your warranty before you ship it out.


My argument would be, as Cathbard is saying, it could be something in the bias circuit that's causing this whole problem. The tube is merely a victim, lol.
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#27
I ended up taking it to a local shop so that their tech can take a look at the amp and diagnose the problem.

If it's a tube problem or some kind of issue on the amp that's fairly cheap to repair, i'll have him do it and pay for it myself.

If it's a fault on the amp that's expensive to repair, i'll send it to Thomann and exercise my warranty.
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