#1
I bought a used vox Valvetronix VT120+ from guitar center which stopped working after about 6 months. Inb4 the obvious turned it on and off unplugged volume was up differemt outlets different houses etc i tried that. It doesnt seem to make any sound at all doesnt pick up cable noise no feedback or anything when on with every needle pinned - but when i turn on sound effects and put my ear right to th speaker i can faintly hear it. Possibly overheating? Any idea where I can find a schematic or info on how to change a fuse or anything? Ive been googling for hours. Sorry for grammar and such but posting from cell which keeps freezing. Nothing works
#3
What information do you need on changing a fuse? You take the old one out and put a new one in. The fuse holder unscrews in the back, and the ones on the circuit board usually just pop out. The mains fuse would not cause this issue but if there are some on the board they're worth checking.

Not many techs or resources are available for SS amps, unfortunately.
#4
edit: ninja'd twice

you have one tube. i don't remember what it does, and don't think it does anything important. as far as i can remember, but it wouldn't be bad try another tube. this may seem obvious or dumb, but if you think it is over heating, is it hot to touch? (obviously don't stick you fingers inside touching things).
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Last edited by trashedlostfdup at Feb 16, 2015,
#5
I've seen this happen with tube amps. I have a Mesa that did this and got real quiet - you could faintly hear the audio. One of the 12AX7 preamp tubes developed a leak - the getter turned white, so the tube was shot.

According to the internet, which is always right... Your amp has one tube in the power stage. Since that amp develops 120w, not sure how they're using that tube in the power stage - no way you'll get 120w out of a little 12AX7. It can't hurt to go ahead and swap it, though. You do have a spare on-hand. Right?
#6
^When I was doing some research on the VT series, a user (who I can't remember the name of but knew much more about amp circuitry then me) suggested that the 12AX7 wasn't really in the power amp, nor in the pre amp. I imagine it is somewhat in between, mostly in the pre amp all things considered.
#7
Quote by Will Lane
^When I was doing some research on the VT series, a user (who I can't remember the name of but knew much more about amp circuitry then me) suggested that the 12AX7 wasn't really in the power amp, nor in the pre amp. I imagine it is somewhat in between, mostly in the pre amp all things considered.


That may have been me.
I think what I said was that the tube has no effect on the sound of the line out/headphone jack and the tube is after that in the circuit.
There are several Op Amps before the tube and at least one after it.
A 12ax7 is commonly used as a preamp tube.

The schematic doesn't specify which section(s) are the preamp section like it does the power amp section, and there are several op amps in there.
The tube is before one of them, and after 2 or 3 of them.
The tube looks like its just being used as another opamp in the middle of the preamp section.

If the tube is dead, its not gonna make any sound.
If its just in bad shape, you may get something out of it, I can't say for sure though.

On the back of the VT (at least on the earlier ones, can't speak for the location on the newer ones) is a power level knob. Thats sets the "wattage" of the amp. According to the manual anyway. In actuality, its more like another level/volume control, circuit-wise anyway (it controls the signal level going into the power amp).
That may be bad. The one on my AD30VT went bad, but only because it got knocked over and broke.

I would fiddle with that knob while the amp is on and see if there is ANY change in the sound level or any noise created by moving the knob.
After that, a tube change might help.

If its still under warranty, I would consider letting whoever is supposed to do the warranty repair deal with it.

Edit:
Also, in the manual are directions for "resetting" the amp. Try that.

Also, everything above describing the circuit applies only to the AD15VT and AD30VT.
It may not all apply to later and/or higher wattage models, but as far as preamp sections go, I see no reason why they would differ other than extra parts for an effects loop (The AD30VT and 15 can be modified to add an effects loop BTW).
Last edited by CodeMonk at Feb 16, 2015,
#8
I honestly don't really know what I'm looking at in this thing. I'm not mechanically inept but I've never owned an amp with a tube in it before this one, always cheap solid state (I'm poor). I repair and mod all my own guitars though pick up swaps soldering pot changes I even have one of those two cherries kaos pad midi controller screens in an epiphone explorer. So I'm confident I can fix this, and really don't have any choice because I'll likely never be able to afford anything else.

Trashedlostfdup: There's a blister on my finger from this white plasticish thing thats attached to the circuit board with some sort of opaque plastic or appoxy which is why I thought over heating. It was hella hot. It has numbers on it 5W56ΠJ BARO. Don't know if thats supposed to be super hot but it was.

I got it to come on for a few minutes then it cuts out. Lights on power on but no sound with any manipulation of knobs. Turn it off and wait a while and it will work again but cuts off after a few minutes.
#9
Quote by ChampofNJ
I honestly don't really know what I'm looking at in this thing. I'm not mechanically inept but I've never owned an amp with a tube in it before this one, always cheap solid state (I'm poor). I repair and mod all my own guitars though pick up swaps soldering pot changes I even have one of those two cherries kaos pad midi controller screens in an epiphone explorer. So I'm confident I can fix this, and really don't have any choice because I'll likely never be able to afford anything else.

Trashedlostfdup: There's a blister on my finger from this white plasticish thing thats attached to the circuit board with some sort of opaque plastic or appoxy which is why I thought over heating. It was hella hot. It has numbers on it 5W56ΠJ BARO. Don't know if thats supposed to be super hot but it was.

I got it to come on for a few minutes then it cuts out. Lights on power on but no sound with any manipulation of knobs. Turn it off and wait a while and it will work again but cuts off after a few minutes.


that certainly doesn't sound good, nothing should be that hot other than tubes. you may want to look further into that.

i would get a new tube to try as the simplest and easiest way.
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Quote by andersondb7
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Quote by trashedlostfdup
nope i am "GOD of the guitar forum" i think that fits me better.


Quote by andersondb7
youre just being a jerk man.



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#10
Quote by ChampofNJ
I honestly don't really know what I'm looking at in this thing. I'm not mechanically inept but I've never owned an amp with a tube in it before this one, always cheap solid state (I'm poor). I repair and mod all my own guitars though pick up swaps soldering pot changes I even have one of those two cherries kaos pad midi controller screens in an epiphone explorer. So I'm confident I can fix this, and really don't have any choice because I'll likely never be able to afford anything else.

Trashedlostfdup: There's a blister on my finger from this white plasticish thing thats attached to the circuit board with some sort of opaque plastic or appoxy which is why I thought over heating. It was hella hot. It has numbers on it 5W56ΠJ BARO. Don't know if thats supposed to be super hot but it was.

I got it to come on for a few minutes then it cuts out. Lights on power on but no sound with any manipulation of knobs. Turn it off and wait a while and it will work again but cuts off after a few minutes.



Looks like this doesn't it?
#11
Codemonk: Not identical to that but nearly so. What is that? Is it suppose to be hot? Could it be the sole cause of the problem? It seems that once that cools back down it works fine but gradually gets hotter and hotter. Is it supposed to be tight to the circuit board or with a gap between the white numbered rectangle and the board? Since the circuit is upside down mounted to the top of the cab it almost looks melty and falling off.

Thanks all for the help, probably my fifth post on these boards, lurked for years before even making this account. I'm almost embarrassed to post this but I couldn't find any information on this series of amps really besides something in Spanish from a South American electronics forum.
#12
Thats a 5 watt resistor.
Which Is large. In most audio electronic equipment, except in the power or output sections, 1/4 watt is standard (1/8 or even 1/2 can be used often as well).
A resistor that large is either part of the power section (as in power source, like from the wall), or the power output section (like heading towards your speaker).
And no, it should not be getting hot. Warm is normal. Finger blistering hot is not common.
If it was, there would likely be a heatsink on it.

I've only got the schematic for the AD15VT, AD30VT, and AD120VT (The AD120VT, and AD60VT were the first of the valvetronix series and likely very different circuit wise from your amp).
I don't have one for your model so I can't say what its being used for.
But its not likely that that item is the problem, but something that it is connected to is the problem.

Looking over the other schematics of the AD30 and the AD15, those circuits probably only differ in minor way to yours. Different chips, memory and such are likely the only major differences.
That and the Power amp sections, whose only differences would likely be in component values rather than design differences.

On the AD30 and AD15, the only large wattage resistors are used in the power amp section.
3 Watt resistors in their cases. It would be logical to assume that a 120 watt amp would use a higher wattage resistor in the same location.

Its possible your power amp is dying.
IME, a power amp failure is the most common failure in Solid State amps.
But I can't say thats what the issue is and be 100% sure without at least a schematic and pictures.

You could try replacing the power amp.

You could get the numbers off of it and search those numbers on some place like mouser.com.
Power amps are usually low in cost, $5 or so.

But like I said, I can't say thats the issue with 100% certainty.

A troubleshooting/repair technique is to look at the easiest to repair item first.
In this case it would probably be the tube.
But with that resistor getting hot, that seems less likely, unless its hooked up to the tube.
And I only see 1 watt resistors hooked up to the tubes in the other Valvetronix schematics I have.
Last edited by CodeMonk at Feb 17, 2015,
#13
Since it snowed 8 inches last night and the nearest place with tubes is about a 2 1/2 hour drive from here I'll have to wait a few days before replacing the tube but it's first on my to do list. I guess I could order one, a google search turned up a lot of 12AX7 tubes, is there anything I should look for or are they all pretty much the same? I'm looking in to replacing the power amp now doing some research.
#14
Quote by ChampofNJ
Trashedlostfdup: There's a blister on my finger from this white plasticish thing thats attached to the circuit board with some sort of opaque plastic or appoxy which is why I thought over heating. It was hella hot. It has numbers on it 5W56ΠJ BARO. Don't know if thats supposed to be super hot but it was.

I got it to come on for a few minutes then it cuts out. Lights on power on but no sound with any manipulation of knobs. Turn it off and wait a while and it will work again but cuts off after a few minutes.


That white thing on the circuit board sounds like a power resistor. The part number also seems to confirm that. The 5W likely indicates it is capable of handling 5 watts.

Quote by Trashedlostfdup
that certainly doesn't sound good, nothing should be that hot other than tubes. you may want to look further into that.


As an electronics tech with many years of experience, I am going to refute that statement. The fact is, there are other components on a circuit board that can get hot - hot enough to burn an unsuspecting finger. Everything from power resistors to integrated circuits to transistors and even transformers - all can get quite warm. And if there isn't something warm or hot on the circuit board to get you, then you have to watch out for the voltages and current. The voltages in tube circuits can be high enough to give you a bad day. I have one tube amplifier that develops over 1600 volts. Complete the circuit on that one with your finger and an elbow and you won't be happy. Complete the circuit on that one with your finger and some other part of your body that puts your heart in the path and you're dead. If you aren't familiar with electronic circuits and repairs, then leave it to the professionals.


Quote by ChampofNJ
Since it snowed 8 inches last night and the nearest place with tubes is about a 2 1/2 hour drive from here I'll have to wait a few days before replacing the tube but it's first on my to do list. I guess I could order one, a google search turned up a lot of 12AX7 tubes, is there anything I should look for or are they all pretty much the same? I'm looking in to replacing the power amp now doing some research.


Personally, I like Tube Depot and The Tube Store. I would avoid eBay at all costs for tube purchases. Too many used tubes end up for sale as "new" on eBay.
Last edited by KG6_Steven at Feb 17, 2015,
#15
The tube in those doesn't have much of an effect on the sound, if any could be heard at all.
IMO, its more of a marketing thing.
Any decent 12ax7 will do.
I used a Ruby in mine, which is just a rebranded tube from the "manufacturer of the month".
A new tube should cost around maybe $10 (depending on your location).

You may also want to take a look at : http://valvetronix.net for all things valvetonix related.
Lots of good info there.
I'll probably be heading back there after I get another Valvetronix (Which I definitely plan to do at some point down the road).
#16
The 12AX7 is not much more than a lightbulb in the VT. But considering you have the 120W combo, I suggest a Tung Sol 12AX7 (clear and bright). You can also get 12AT7/12AU7/etc. if you want less gain and a smoother tone. But this is considering that your amp will be fixed... c:

I suggest taking it to a tech before you get a new tube, though so that you can confirm what the original problem is. You might need to pay him for servicing n' such as well.
Last edited by Will Lane at Feb 17, 2015,
#17
Quote by Will Lane
The 12AX7 is not much more than a lightbulb in the VT. But considering you have the 120W combo, I suggest a Tung Sol 12AX7 (clear and bright). You can also get 12AT7/12AU7/etc. if you want less gain and a smoother tone. But this is considering that your amp will be fixed... c:

I suggest taking it to a tech before you get a new tube
, though so that you can confirm what the original problem is. You might need to pay him for servicing n' such as well.


huh? a $10 tube, or a $50 bench fee (plus that an additional hour). i would take the crap-shoot and see if it is the tube.
____________

to steven - my apologies on my comment on temperature. i don't really poke around in warm amps, and touch components, but know you know your shit. i am still a beginner with tech work on amps . it just seemed that a blister from touching is pretty hot.
WTLT 2014 GG&A

Quote by andersondb7
alright "king of the guitar forum"


Quote by trashedlostfdup
nope i am "GOD of the guitar forum" i think that fits me better.


Quote by andersondb7
youre just being a jerk man.



****** NEW NEW NEW!
2017-07-07 2017-07-07 Update and a Chat On Noise Constraints *** NEW FRIDAY 7/7
2017-04-13 RUN AWAY from COMPUTERS!!! TCE? RANT ALERT!!!
2017-03-02 - Guitar Philosophy 1001- Be Prepared For the Situation (Thursday 2017-03-02)
2017-02-21 How to Hot-Rod the Hell of your Stratocaster for $50! (Tuesday 2017-2-21)
Resentments and Rambling from a Guitar Junkie
---> http://trashedengineering.blogspot.com/
#18
Quote by KG6_Steven
That white thing on the circuit board sounds like a power resistor. The part number also seems to confirm that. The 5W likely indicates it is capable of handling 5 watts.


As an electronics tech with many years of experience, I am going to refute that statement. The fact is, there are other components on a circuit board that can get hot - hot enough to burn an unsuspecting finger. Everything from power resistors to integrated circuits to transistors and even transformers - all can get quite warm. And if there isn't something warm or hot on the circuit board to get you, then you have to watch out for the voltages and current. The voltages in tube circuits can be high enough to give you a bad day. I have one tube amplifier that develops over 1600 volts. Complete the circuit on that one with your finger and an elbow and you won't be happy. Complete the circuit on that one with your finger and some other part of your body that puts your heart in the path and you're dead. If you aren't familiar with electronic circuits and repairs, then leave it to the professionals.


Personally, I like Tube Depot and The Tube Store. I would avoid eBay at all costs for tube purchases. Too many used tubes end up for sale as "new" on eBay.

Regardless, his description sounds like either something overheating or a filter cap going bad.
And being that its SS and not tube, I would vote for something overheating.
And the most likely thing to go out in an SS amp IME, is the power amp chip.
And as I've said before, anything in an SS amp likely to get hot enough to bun your skin, should have heat sink on it.
Thats generally the power amp chip and the heat sink for those is often also attached to the metal case.

The Vox AC30 amps are well known to be like ovens inside. They get REALLY hot.
I know, I have one. You could put a pan over one and cook your breakfast.
The Valvetronix series amps, not so much. I used to have one and have repaired a few.
Those generally don't get so hot inside unless something is wrong.

Either way OP, when getting inside ANY amp, Solid State or tube, be careful as there could be lethal voltages inside. Especially in tube amps.
There are probably youtube videos about draining the power from capacitors that around found inside amps.
Last edited by CodeMonk at Feb 17, 2015,
#20
I swapped the tube on my VT30. I've also owned the VT100 but I didn't screw around with that at all, it was helplessly muddy at drummer volume. But the VT30, I could tell wasn't meant for tube swaps. They show the tube in the vent but there are dozens of screws and you have to pull out the chasis. Given its a starter amp, I don't think the tube is meant to be replaced as maintenance like a normal tube amp. They do sound good though.

If you've been rockin' this amp for awhile, it's time for an upgrade anyways. I'm a huge fan of the Valvetronix series, I still have my VT30, but there are much better amps out there if you need a big amp.
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#21
Quote by lucky1978
I swapped the tube on my VT30. I've also owned the VT100 but I didn't screw around with that at all, it was helplessly muddy at drummer volume. But the VT30, I could tell wasn't meant for tube swaps. They show the tube in the vent but there are dozens of screws and you have to pull out the chasis. Given its a starter amp, I don't think the tube is meant to be replaced as maintenance like a normal tube amp. They do sound good though.

If you've been rockin' this amp for awhile, it's time for an upgrade anyways. I'm a huge fan of the Valvetronix series, I still have my VT30, but there are much better amps out there if you need a big amp.

Agree.
Changing the tube in the AD30VT was a big PITA.
I also had an AD50VT at one time and doing a tube swap in that one was a piece of cake.