Author Topic: Advice on tube amp repair needed!  (Read 4250 times)

ZeusMalt

Advice on tube amp repair needed!
« on: September 24, 2012, 09:22:44 AM »
I have a ibanez valbee and it's not working anymore. Can someone diagnose whats wrong with it?

I can get a really faint sound out of it when maxing the volume and little gain. Turning the gain up from zero crackles a bit untill silence at about 2. Like i said really faint and lousy sound.
Plugging the guitar straight to effectloops return gives normal sound but obviously bypasses all the knobs and preamp section. As i already tried with another preamp tube the problem is somewhere between input and powertube... does someone actually know what is wrong? Better than "check the components.." All the components look ok from the outside. Tested couple of caps too and the gainknob seems to be working ok. Help?


crane

Re: Advice on tube amp repair needed!
« Reply #1 on: September 24, 2012, 09:55:50 AM »
Do you think that there is only one valbee amp problem and that's it? Someone will tell you "connect the red wire to the green one" and it will be fine?
Debugging an amp isn't much different from debugging a fuzz / distortion pedal except the high voltage. Judging from your post I wouldn't recommend you trying to fix anything but to take it to someone who actually knows what he's doing.
AFAIK Valbee has a built in tube screamer - this complicates things a little.
Anyway - any decisions should be based on logic and observations. Having repaired a few I would say that a sistematic approach checking each stage works most of the times. Since you have the PA working - you can use the return jack as and audio probe and find out which stage has problems.

R.G.

Re: Advice on tube amp repair needed!
« Reply #2 on: September 24, 2012, 10:09:31 AM »
I have a ibanez valbee and it's not working anymore. Can someone diagnose whats wrong with it?
Maybe you can. Go look at http://www.geofex.com, in the upper left hand corner for "The Tube Amp Debugging Page". http://geofex.com/ampdbug/ampdebug.htm

Quote
I can get a really faint sound out of it when maxing the volume and little gain. Turning the gain up from zero crackles a bit untill silence at about 2. Like i said really faint and lousy sound. Plugging the guitar straight to effectloops return gives normal sound but obviously bypasses all the knobs and preamp section. As i already tried with another preamp tube the problem is somewhere between input and powertube... does someone actually know what is wrong? Better than "check the components.." All the components look ok from the outside. Tested couple of caps too and the gainknob seems to be working ok. Help?
It is very difficult to do a remote diagnosis from what you have given us here. They can only be guesses.

- for tube amps, the first step in debugging is *always* to eliminate a bad tube by swapping in known-good spare tubes from the set of spares you always have ready,
- a classical but little-known problem is dirty contacts or faulty jacks in the effects loop
- issues with specific spots on controls are often that the control wiper or resistive track is worn out below that spot and needs replaced
- sometimes a bad coupling cap will let DC through and a control will mis-bias the next stage

All this applies to the classical all-tubes amplifier. I don't know the Ibanez Valbee. Can you post a schematic for it? If not, your chances of solving this by yourself are dim if it's not just a bad tube or dirty jacks.

By the way - you can get **killed** by the voltages inside that thing. Have you thought about that?

R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

ZeusMalt

Re: Advice on tube amp repair needed!
« Reply #3 on: September 24, 2012, 10:47:58 AM »
Crane you just HAD to answer didn't you... I wrote for a reason that "check the components" isn't  helping. The point was if here is someone with a lot of experience or has had the exact same problem. Someone who could tell me more specificly what component to look at. Im not asking to do magic tricks.
I have never repaired an amp before but i do study electricity and have made several stompboxes. Even though im not on the electronics side i do have some kind of scent of what im doing. And im not getting myself electrocuted...at least on purpose. 230V doesnt feel nice.

Crane how exactly you would use the pa side and returnjack as a signalprobe when the whole thing is in bits and pieces?

Someone got that schema?

The problem  isnt the tubes as i think i already mentioned the pa side working and already tested with another pretube.

Edit. I would just take it to a professional if it would be an actual AMP... but in this case it would be cheaper to get a new practiceamp than take it for repair.
« Last Edit: September 24, 2012, 10:56:52 AM by ZeusMalt »

wavley

Re: Advice on tube amp repair needed!
« Reply #4 on: September 24, 2012, 01:10:13 PM »
Try cleaning the jacks for the effects loop.

Actually, plug a cable from the send to the return and see if it's normal, those jacks getting screwed up is pretty common with A LOT of amps. Pinkjimiphoton brought this up in another thread recently and I'm surprised he didn't beat me to the explanation.

The other thing that I constantly see with amps with PCB mounted tube sockets is bad solder joints, I would re-flow all of the tube sockets and large caps.  Mounting a heat source to a PCB with no mechanical reinforcement in a high vibration environment isn't exactly the best idea amp designers have come up with, you'll see this happen in nearly every Fender Hot Rod amp eventually, when I was at the shop guys that played a lot (like house band or touring band a lot) had to have their hot rods re-soldered once a year like clockwork.

edit: I see that R.G. already beat me to the effects loop jacks
« Last Edit: September 24, 2012, 01:13:54 PM by wavley »
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ZeusMalt

Re: Advice on tube amp repair needed!
« Reply #5 on: September 24, 2012, 02:50:58 PM »
Still the same...

Plugging send-return makes no change.

Sound is week, hissy... until it faids/crackles away when rising gain. So.. something somewhere doesnt like something that happens.

I wish i find the schema. I guess no other way than try to find the faulty component from the haysack...

crane

Re: Advice on tube amp repair needed!
« Reply #6 on: September 24, 2012, 03:59:00 PM »
Crane you just HAD to answer didn't you... I wrote for a reason that "check the components" isn't  helping. The point was if here is someone with a lot of experience or has had the exact same problem. Someone who could tell me more specificly what component to look at. Im not asking to do magic tricks.
I have never repaired an amp before but i do study electricity and have made several stompboxes. Even though im not on the electronics side i do have some kind of scent of what im doing. And im not getting myself electrocuted...at least on purpose. 230V doesnt feel nice.

Crane how exactly you would use the pa side and returnjack as a signalprobe when the whole thing is in bits and pieces?

Someone got that schema?

The problem  isnt the tubes as i think i already mentioned the pa side working and already tested with another pretube.

Edit. I would just take it to a professional if it would be an actual AMP... but in this case it would be cheaper to get a new practiceamp than take it for repair.
I'm not telling you to "check components". I'm telling to follow it with a signal probe stage by stage. If  the PA is too disassambled to be used - use another amp/scope/whatever. Be carefull to use high voltage DC blocking cap. This almost always works for me and I've repaired quite a few tube and ss amps.

runmikeyrun

Re: Advice on tube amp repair needed!
« Reply #7 on: September 24, 2012, 04:42:03 PM »
use an audio probe, start from the input jack and work your way through the preamp.  Easiest way to find the problem in a case like this.
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frankpaush

Re: Advice on tube amp repair needed!
« Reply #8 on: April 06, 2017, 08:31:48 AM »
...digging a graveyard thread, I know, but since I have to face the EXACT same problem (no signal after gain over 2, noise before, tubes checked, effect insert checked) with a Valbee I just received (second hand trade ...) I would like to know: what has been the solution? Anyone?

GibsonGM

Re: Advice on tube amp repair needed!
« Reply #9 on: April 06, 2017, 09:00:22 AM »
Hi Frank, welcome to the forum.  Like they said above - can you link to a schematic?  Without a schematic, all anyone can do is guess at what would be wrong.  Most amps have just a few configurations, if you will...we need to know what basic config. it is to suggest what to look at.

Your best bet to see what happened is to PM the original poster, see if it got resolved.

In general, a tube preamp is going to have some triode gain stages. These are usually pretty universal (you can look that up online, "12AX7 gain stage" or something).    There will be 3 'legs' on each triode section (12AX7 is a dual triode, so there are 2 sections per bottle).   

Mainly, you'd be checking the signal path with an audio probe using a HIGH VOLTAGE CAP to see where the audio stops, and measuring the voltages at each triode pin to see if any are off, as well as the power supply ("B+").    Both of these actions can kill you as high voltage will be present.  The voltage measurements are easiest to start off. 

Based on what seemed to be going on for the OP, I'd suspect a bad pot or something wrong w/bias voltage supply, but that can be misleading - leaky cap can be messing up bias as control is turned, etc.  Hence the need for voltages. 
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