Amp troubleshooting, flabby bass ( AC15c1)

[majotaur]

Hey,
Warning: A lot of text ahead, this amp took a long journey so far! I've also looked around on forums, but couldn't find many similar threads, so I apologize if this has been asked before.

I've bought an amp about a year ago, VOX ac15c1. For a few months I loved the hell out of the amp but one morning I turned it on and it just wasn't the same amp I came to love. Distortion was all flabby and everything sounded like my speaker was blown. I assumed that it was the tubes going bad and bought a new set from eurotubes( actually bought a whole set include power tubes ). The tubes came in, I installed it and nothing has changed. The amp still sounded flabby.

Disappointed, I took it to a local guitar tech, since it was still under warranty. Tech looked at the amp, agreed that it sounded bad, then in a week replaced the tubes and proclaimed that it has been fixed. I've played through the amp at the shop for a bit and it seemed better. What I have found is that it sounds OK for the first few minutes, then goes back to being all flabby.

Disappointed again, I found another tech. Took it to him and about the same story repeated itself, except this time he also replaced some cap in the pre amp section. Again, the amp sounded ok for a few minutes and it wasn't until I got home that it started to sound bad.

Finally I decided that I am tired of going to amp techs to fix my amp and it seems like my experience has been pretty sour so far, I decided to take it in my hands and fix it myself.

So far I have done the following: I've replaced all electrolytic caps in the amp with a newer ones of the same rating. This actually made a noticeable difference in the hum, the amp has virtually no ambient hum other than the one coming from pups.
This did not solve my problem of bad bass response, so I am assuming it's not a bad cap.

So, I am looking for suggestion on where the cause of my problem might be.
So far I think I've ruled out that it's:
bad transformer/power supply etc( amp tech claimed he checked all voltages, so I am assuming it's ok)
bad tubes(replaced them around 4 times total now, also tested the tubes with a tube tester and they seem ok)
bad caps( replaced these )
bad guitar pickups( i have a Pathfinder 15r and my guitar sounds better through that, than my AC15), also tested on two different guitars( One has P-rails duncan pups and the other is a telecaster )

The main symptoms are the following:
1. Amp sounds ok when not driven ( clean sound is good )
2. Amp starts to sound bad once it's driving more, distortion sounds really loose in the bass, sounds like a bad speaker
3. Amp sounds OK if I put it on Top Boost channel with treble all the way up and no bass.

I am hoping the gurus here might point me to the right direction in my project. This is my first foray into tube amp territory, but I've been planning on doing some DIY builds as well.

Also I have already messed with it ( replacing caps), so I don't feel comfortable taking it back to the tech guy, since he'll probably say that I messed with and he wouldn't want to fix the mess that I have made.

TL;DR : Bad bass response on tube amp, took to tech, they said bad tubes and that the amp sounds "fine" once the tubes are replaced. I don't like the sound, replaced all electrolytic caps, but it still sounds bad. Help!

I can also supply some sound clips if it would make it easier to identify.

[DrGonz78]

Did you try this amp with another speaker? Did you make sure that the speaker is tightened down good? You mention speaker as a potential culprit of the problem, but never mention anything about checking that one factor. Instead, the tubes have been replaced about 4 times and many different caps are now replaced.

What are the specs on the speaker you have in this amp? You have another amp there... What are the specs on that speaker? Try hooking up another speaker to hear what that sounds like. Lets just start that simple and then go forward.

[Enzo]

AC15C1 or AC15CC1?

I would also say that bouncing tech to tech is not doing any good. When the first guy "fixed" it under warranty, but it didn;t solve the problem, the first thing to do would be go back to the same guy and explain that the problem has not been solved. If the problem is evident at first, then doing something inside makes it seem better, that can fool a tech.

To any tech, you need to demonstrate the problem, not just hope he hears what you hear. You need to take your guitar in and play it through the amp while he listens, so he can know for sure what your complaint is. I have had repairs where a guy complains of sound getting crappy. The amp works for me, but he takes it home and still sounds crappy. I invite him to bring in HIS guitar and cord and we listen together. We then find his active pickups had a almost dead battery.


Yes, once you have taken the amp back and done a lot of work yourself, you can;t THEN return with it expecting a free visit. But in general, if you paid money or got warranty coverage, and it isn;lt made right, at least give them a chance to make it right. If a baseball team fired every player that struck out, they wouldn't have a team after a short while.

DrGonz is right, at least connect the amp to different speakers.

Troubleshooting is a systematic process. Throwing parts at the amp - tubes, caps, whatever - is not the way to do it. You could just as likely have a cracked solder connection on a tube socket or even a crack in the circuit board. A little thermal expansion and voila.

A signal tracer is a useful way to follow a signal through an amp. Google "signal tracer" and find a million sources of this very basic little item. In fact, to make one all you need do is make a little probe you can plpug into some other amp.

And alternatively, we can take a music source, and inject it into the circuit at each stage through the amp. We use a cap to do this to block any DC from getting into your signal source.

[majotaur]

Did you try this amp with another speaker? Did you make sure that the speaker is tightened down good? You mention speaker as a potential culprit of the problem, but never mention anything about checking that one factor. Instead, the tubes have been replaced about 4 times and many different caps are now replaced.

What are the specs on the speaker you have in this amp? You have another amp there... What are the specs on that speaker? Try hooking up another speaker to hear what that sounds like. Lets just start that simple and then go forward.

I forgot to mention that we did try this amp with another speaker and it sounded exactly the same. I have the stock Greenback in it and we tried it through a known good extension cab. We've also tried switching from 8 to 16 ohm load with similar results. This was actually one of the first things that were ruled out.

AC15C1 or AC15CC1?

I would also say that bouncing tech to tech is not doing any good. When the first guy "fixed" it under warranty, but it didn;t solve the problem, the first thing to do would be go back to the same guy and explain that the problem has not been solved. If the problem is evident at first, then doing something inside makes it seem better, that can fool a tech.

To any tech, you need to demonstrate the problem, not just hope he hears what you hear. You need to take your guitar in and play it through the amp while he listens, so he can know for sure what your complaint is. I have had repairs where a guy complains of sound getting crappy. The amp works for me, but he takes it home and still sounds crappy. I invite him to bring in HIS guitar and cord and we listen together. We then find his active pickups had a almost dead battery.


Yes, once you have taken the amp back and done a lot of work yourself, you can;t THEN return with it expecting a free visit. But in general, if you paid money or got warranty coverage, and it isn;lt made right, at least give them a chance to make it right. If a baseball team fired every player that struck out, they wouldn't have a team after a short while.

DrGonz is right, at least connect the amp to different speakers.

Troubleshooting is a systematic process. Throwing parts at the amp - tubes, caps, whatever - is not the way to do it. You could just as likely have a cracked solder connection on a tube socket or even a crack in the circuit board. A little thermal expansion and voila.

A signal tracer is a useful way to follow a signal through an amp. Google "signal tracer" and find a million sources of this very basic little item. In fact, to make one all you need do is make a little probe you can plpug into some other amp.

And alternatively, we can take a music source, and inject it into the circuit at each stage through the amp. We use a cap to do this to block any DC from getting into your signal source.

It's AC15C1. I understand that I am taking drastic measures and jumping into repair, but you also have to understand that the tech is about two hours away from me and both of us have somewhat restricted schedule, I can't go to him during a weekday and he doesn't stay open for long on weekends. I can only see him once in 2 weeks, meaning my amp will be in repairs for months! I've also paid for repairs myself so far, since every time I went it was "bad tube" and it's not covered by warranty.

At this point I've went through multiple tech trying to make it right and none of them think that there is anything wrong with an amp. In fact I went to the first tech about 3 or 4 times complaining and what he ended up saying is that it's my guitar/amp combination that sounds bad. I remember what it sounded like when I bought it and I had changed nothing since then, so I am fairly certain it's not my guitar. I've also tried with other amp and the guitar sounds fine. One thing I have not tried is another ac15c1 with my guitar, but where am I going to find another AC15c1? I feel like I am just getting a run around and my amp sits in tech shops for weeks for nothing, I might as well tinker with it myself and learn something in process.

I am not really expecting anyone to solve this for me, just looking for guidance. It seems like the problem is somewhat localized in a sense that it happens at certain conditions, so I figured someone might know the likely culprit. I will look into signal tracer.

As a side note, I forgot to mention that when I was inspecting the sockets, the V2 and V3 sockets had black powder on the side of the socket, is this normal operating thing? Google doesn't seem to know.

[Sowhat]

I have read this post a few times and something about it sound wrong. I had a bass player who's Ampeg B15 just did not slam like it used to so he brought it to me and I checked it over and I declared it "not broken"... I didn't declare it "fixed" but "not broken", of course I waived his bench charge and knew him well enough to accompany him to his rehearsal studio to see the problem in action. Sure enough, the thing sounded weak, very weak. At my shop it nearly blew the roof off the place so I was puzzled to say the least. I looked around and suddenly noticed that he had the thing up against a wall. I scooched the thing out a bit from the wall and just as if I was the second coming of Jesus Christ himself the amp came back to it's powerful booming self. The Ampeg like the Vox ports out the rear of the cabinet and with the amp pushed hard against the wall, there was no porting...has the environment changed where you check out your amp? Could you be chasing ghosts? Just a though on my part because your problem sounds so impossible, afterall you have nearly replace the whole amp part by part and none of those replacement have really made any difference.

[Jazz P Bass]

'Scooched'?
Sounds Pennsylvania Dutch.

[Sowhat]

I don't know where that came from... I must be possessed!

[Chuck H]

Since the amp has been tested in several locations, I'll guess that it's not a "scootching" issue. "I" think the problem is that once the speakers started to break in an abundance of bottom end flabbiness became more appearent. This isn't going to get better either. The amp is likely working properly, it just doesn't sound good. I would take the amp to the local mega mart music store type place and play it side by side with another of it's type. I would also run the amp through a new amps speaker cab for an apples to apples comparison. I think this test could be VERY revealing. Overdriven amp tone is VERY sensitive. Speaker break in, the age of the strings on your guitar, a different preamp tube and even a cheap, high capacitance guitar cable can all make a notable difference. If one thing isn't just righ the whole shebang goes to $h!t. Definitely compare your amp to another to actually hear the difference. Be sure to run your amp through the other amps speakers too. It the problem still remains exclusive to your amp then there may be a "fixable" problem.

[DrGonz78]

Another totally different angle than the speaker... Which I am totally focused on here... But lets look at the solder connections on the amp too. I mean things heat up and I fix so many things by re-flowing solder joints. I would say it never hurts to reflow the solder but that is not always true... However, many times I can find cold joints by flowing solder and seeing it cool to a shiny or dull look. I would definitely be looking at some of the solder joints that connect my tubes sockets and the speaker wire joints. Just a thought beyond the speaker itself and since you have done this much already you might as well.

[majotaur]

Great suggestions! I am about 90% sure that it's not speaker, since we have tested it with an external speaker and it sounded the same, but that was like 4 repairs ago! I might take it to a local music store just to be 100% sure that it's MY particular amp behaving like this.

I will look into re-flowing. I was kinda curious about big balls of solder on the tube sockets, surely there is no need to have a "dome" of solder above contacts. I am assuming it's like that because that's basically what's holding socket and the attached tube in place ( sockets are vertical pointed down away from the connection ), aside from tube holders. In any case, I am not exactly sure why the connections have to have that much solder.

I was planning on re purposing a cable as an audio probe and tracing the signal. Should be interesting.

I might post an audio sample to show the issue, but it pretty much sounds like what you'd expect from the word "flabby".

[kevin mask]

Great suggestions! I am about 90% sure that it's not speaker, since we have tested it with an external speaker and it sounded the same, but that was like 4 repairs ago! I might take it to a local music store just to be 100% sure that it's MY particular amp behaving like this.

I will look into re-flowing. I was kinda curious about big balls of solder on the tube sockets, surely there is no need to have a "dome" of solder above contacts. I am assuming it's like that because that's basically what's holding socket and the attached tube in place ( sockets are vertical pointed down away from the connection ), aside from tube holders. In any case, I am not exactly sure why the connections have to have that much solder.

I was planning on re purposing a cable as an audio probe and tracing the signal. Should be interesting.

I might post an audio sample to show the issue, but it pretty much sounds like what you'd expect from the word "flabby".

anything up? I'm curious to know if you have fixed it, I sometimes have the same issue with the AC15C1, sometimes it sounds great, sometimes it gets flabby to the point that the bass freqs leave a farty decay with exactly the same settings and guitar, this amp really seems to have his personal mood, but when it's in a good mood it delivers exactly the tone I'm after, so I wonder what can cause an amp to be so fickle? I also replaced most of the cables of my rig and the daisy chain which was kinda beat and seemed to have a shortage, all good the last gig, let's see the next one, but if we find out the real cause of this "flabby mood" I'd be much happier

[Sowhat]

Big Balls of solder??? If I actually understand what you mean by that, I have run across that type of thing with very high voltage circuitry like some horizontal oscillator circuits in video systems and old radar power systems before. In those cases that was done on purpose to eliminate any sharp edges on the solder connections that could cause an arc or act as an antenna. Maybe some previous tech did this because of the somewhat high voltage present on the tubes. It's not really necessary with a few hundred volts you find in a tube amp but in the case of higher voltage equipment with a few tens of thousands of volts it makes a significant difference.

[bob p]

reading two complaints about "farting" makes me wonder if you guys are driving your amps into blocking distortion.

[olddawg]

I would never advise anyone to do this of course (although I know it is done occasionally out of frustration with a new product and customer support issues). A person orders a new amp from MF. They then swap the innards from the new amp into the old one.....Then they return the new amp for full credit.

[Chuck H]

Huh?!? That's an interesting and unscrupulous notion. But I can't blame anyone doing it. New amps are built to appeal and not much more. If any concessions on the expense of the design are made it's only because of warranty issues. As a result it's not uncommon for inexpensive amps to last about as long as the warranty. I'm not trying to be profound in any way. This has been common practice with American corporations for decades. And, to be sure, it IS American corporations at the heart of the problem. NOT the Chinese manufacturing machine. They only make crap as directed by their clients. In the end it's sort of a "power to the people" move that circumvents the big money in favor of the working class musician. Just the sort of thing that young rockers have been doing and advocating for at least as long!!! In my unfortunate maturity I understand why it's wrong. But I just can't fault people for trying to get a little more for their money from "the man". Short story...

Though Randall Smith gets a lot of heat here for his patent frenzy in the amps design arena, Mesa, as a company, is pretty easy to deal with and seems very customer oriented. I actually don't like most Mesa amps. But that's not the issue. I did deal with them once regarding a used amp I bought that was having problems with the circuit under the "secret epoxy". The amp was about ten years old. They told me that if I paid shipping they would repair it for free! Remember, I bought the amp used!!! When the amp came back to me it had all new innards (with no secret epoxy goo). They replaced the boards entirely at no charge to me. New filer caps in the deal! They knew that their product had a failure mode and they made good on it with NO questions asked. That's an American company that manufactures here! Try getting that from LOUD tech or any other big, corporate, made in China brand.

Rant over. But remember this... When you buy a Chinese made amp commissioned by a corporation you should know that what you're paying for is a working product for the serviceable warranty period only. In fact, many of those products are under DNR (do not repair) protocol because it's cheaper to provide a new product than repair the broken one?!? But that starts to get into a whole different issue.