redshark92
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Post by redshark92 on Sept 25, 2018 9:24:23 GMT -5
I've got a Vox AC15C1, which I bought used a few months back. I've started noticing some excessive noise while playing, the usual hiss/hum kind of sound but it seems louder than it should be. It's hard for me to know for sure, because while it's possible it was there before and I didn't notice it, I think it's relatively recent. ----------------------------------------- Here are some details that may help: I have tried different power sources (basically just different outlets in my house), different power cords and different guitar cables. None of these have made a difference. I have two guitars. First is a fairly standard Stratocaster. It's a MIJ standard, the guts are largely from an MIM standard and the pick-ups are CS Fat 50s. The other is a Blacktop Tele. Basically a MIM Tele with humbucking pick-ups. These are fairly high output pups, the overall volume with the Tele around 3 is the same as with the Strat around 8-9. This sound is evident with either guitar, though it is slightly (but noticeably) worse with the Strat. On the Strat, I notice NO difference when switching between the humbucking positions (2 and 4) versus the single pick-up positions.
If I turn down either guitar's volume all the way the sound is still present but much quieter. If I unplug everything from the amp and turn it on there is no extraneous sound even at high volume settings. This particular amp has two channels, Standard and Top Boost (Bright). There is a single Master Volume and a separate one for each Channel. The noise is evident either way I set it but is worse with Master Low/Channel High than the reverse. ----------------------------------------- Looking for ideas. Most of the above would make me think it's more to do with the guitars, and I do plan on putting my Strat through the QTB mod at some point, because it's always been a little noisy but in other amps where I've gotten excessive noise off the Strat it was at least somewhat mitigated when switching pick-up positions. OTOH, as I noted earlier, I am not 100% certain, but I think this is relatively new, e.g., that it wasn't doing it before and if that's true its probably something to do with the amp. Unfortunately it's hard for me to know that for certain. In that case I'm wondering if maybe there's a tube starting to go bad. I did buy this used and while I can make a rough guess at its age I can't get very specific and I also don't know how hard it was used by its previous owner(s). I was planning on retubing it anyway (this amp is self-biasing), but I may push that up on the schedule if it seems like that's a possible source of this issue. Any other thoughts?
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Post by blademaster2 on Sept 25, 2018 12:13:30 GMT -5
This might be silly to say, but I once thought that my AC15 was excessively noisy until I saw that I had the volume turned up on the unused input channel and I was playing clean from the other channel so the Master was up on full. The noise was 'hissy' (white) rather than hum or other indications of a guitar picking up on something like an antenna. It was a 'Doh' moment for me.
In your case you said it happens less when everything is unplugged from the amp, however the noise usually is not white coming from a guitar (I do not know if the AC15 has a shorted input that is switched when a cable is plugged in - my AMPEG V4 does). It also appears that you have no electronics such as stomp boxes or a preamplifier, but if you did they can create white noise, too.
If nothing simple like the above makes any sense, then it becomes more challenging. I have never traced my amplifier issues to tubes themselves since I play relatively infrequently, but perhaps they may be the culprit. In very sensitive circuits the type of resistors can influence noise (carbon resistors versus wire wound or metal film resistors) but that would require changing them (in the input stages of the amplifier circuit where gain is highest) and I doubt that they are likely to change and create noise like you are hearing but I understand that it is possible.
Sorry for the randomness of the above thoughts.
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Post by thetragichero on Sept 25, 2018 12:35:24 GMT -5
my first thought was to try brand new tubes. one idea would be to swap the preamp tubes (the little ones) around and see if at least the noise gets *different*
my second thought was that it probably has a shorting input jack (+1 blademaster) and that's why there's no noise with nothing plugged in
other thoughts: are you playing in front of computer monitors/tvs/powered speakers/ceiling fans/fluorescent lights/etc?
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redshark92
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Post by redshark92 on Sept 25, 2018 13:11:55 GMT -5
This might be silly to say, but I once thought that my AC15 was excessively noisy until I saw that I had the volume turned up on the unused input channel and I was playing clean from the other channel so the Master was up on full. The noise was 'hissy' (white) rather than hum or other indications of a guitar picking up on something like an antenna. It was a 'Doh' moment for me. Interesting. I'll mess around with this and see what happens. Usually I have the volumes on both set around where I use it (usually ranging from around 9 o'clock to 12 o'clock on the Channel Volumes) no matter which of them I'm actually using at any given time. I should have mentioned this. I usually do have a few boxes plugged in. Right now a DigiTech Multi-Effects Pedal and a couple of different Overdrives. Removing these from the chain was the first thing I tried when I started noticing this and removing them makes no difference. Not at all. Thanks!
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redshark92
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Post by redshark92 on Sept 25, 2018 13:17:16 GMT -5
my first thought was to try brand new tubes. one idea would be to swap the preamp tubes (the little ones) around and see if at least the noise gets *different* Hmm, I might try that. This seems unlikely. Each of the two channels has it's own independent input jack and the noise is consistently present on either channel. Thought of this, too. The room where I'm usually playing does have fluorescent lights but I've tried turning them all off and it makes no difference. The other stuff isn't consistently present where I'm usually playing.
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redshark92
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Post by redshark92 on Sept 25, 2018 15:19:55 GMT -5
Was looking at another thread in this forum: guitarnuts2.proboards.com/thread/7399/amp-humsAnd filter caps are brought up as a potential source of the problem in the amplifier discussed in that thread. Given what I've described, is that a possibility I should look into?
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Post by thetragichero on Sept 25, 2018 16:37:28 GMT -5
new tubes or swapping around what you have is a relatively cheap exercise that you can do from your own home. no bench fee with that if it turns out that the preamp tubes didn't fix the problem, then you can spend the money on an amp tech. plus then you'll have backup tubes (i have boxes of them, keep a couple in little toolbox i would take on tour, etc)
edit: as a point in favor of trying the simple things first: if you peek over at my thread on modifying my fender excelsior, i ended up with some buzzing when i played at low volume. i frantically tried to diagnose what could've went wrong in the circuit, saying many eff words. it turns out that the speaker i put in there was bad. moral of the story is that maybe we should listen to what that Occam feller was telling us
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Post by newey on Sept 25, 2018 20:59:01 GMT -5
You probably can get specific. It will have a serial number, and (I'm guessing here) there is probably a VOX support page or fanboy site somewhere a'web that can translate the serial number into a date of manufacture. If it were some off-brand or rarer make, I'd guess "no chance" but Vox has made AC15s for a long time, they're popular, and so likely there are some resources available.
Not that any of that helps with your noise issue, sorry. Sumgai is the one likely to have the skinny on this issue, but he's on walkabout (Well, more like RV-about . . .).
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Post by blademaster2 on Sept 26, 2018 10:32:25 GMT -5
Was looking at another thread in this forum: guitarnuts2.proboards.com/thread/7399/amp-humsAnd filter caps are brought up as a potential source of the problem in the amplifier discussed in that thread. Given what I've described, is that a possibility I should look into? With this notion it might be worth assessing more closely what the noise sounds like that you are hearing from the amp. Buzzing and humming sounds indicate something like power supply noise and might implicate the power supply filter capacitors, but hissing (closer to white noise) would be less likely from the supply and comes from amplification of low-level thermal noise (shot noise, for example) in a component that is usually at the front end of the amplifier in the preamp.
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redshark92
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Post by redshark92 on Sept 26, 2018 18:37:01 GMT -5
I'll follow up on the rest of the replies later, but I want to start here, because I have more info that may be helpful: With this notion it might be worth assessing more closely what the noise sounds like that you are hearing from the amp. Buzzing and humming sounds indicate something like power supply noise and might implicate the power supply filter capacitors, but hissing (closer to white noise) would be less likely from the supply and comes from amplification of low-level thermal noise (shot noise, for example) in a component that is usually at the front end of the amplifier in the preamp. Just plugged in again. I'd describe this as more of a hissing than a hum. In addition, I realized I have a noise gate (as part of my multi-effects pedal) so I thought I'd do an experiment. Note that this amp does not have an effects loop so this is Guitar -> Effects -> Amp Input This was carried out with my Strat. Plugged in everything and got the usual noise. With the noise gate turned completely off I get the hissing noise AND the normal 60-cycle hum from the Single Coil pickups. Bringing the noise gate up from nothing to it's max setting I can pinpoint where the 60-cycle hum drops out completely (around a third of the way up) but the main hissing noise is never affected even when the noise gate reaches it's maximum setting.
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redshark92
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Post by redshark92 on Sept 27, 2018 7:42:21 GMT -5
For now, I'm going to go ahead with a retube (which I was going to do anyway) and completing the QTB mod on my Strat and I may do something similar with my Tele once I get a feel for it.
I'll compare results after each of these to see if I get an improvement. As it is it is loud enough to be noticeable when not playing (especially as the volume goes up) but it's not to the point of affecting the sound while playing so I can live with it for now while I wait for the tubes and stuff needed for the QTB mod.
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pj
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Post by pj on Oct 4, 2018 8:15:47 GMT -5
How are you getting on with the problem?
The fact that the amp makes no unusual noise with no input would imply to me that even if it has shorting jacks to ground and quiet the unused input, then the first stage/valve isnt producing unusual amounts of noise as it would generate it anyway.
Assuming your Blacktop tele has shielded humbuckers i would expect it to be less sensitive to the noise youre hearing. But, this will be the same noise that is potentially being picked up elsewhere.
Shot noise was mentioned earlier and this is a potential issue though it is unlikely to be from the plate resistors if theres no noise with no input. The grid resistors possibly, but as they are much smaller and the potential across them much less I wouldnt put too much emphasis there. You were saying that with the channel volumes maxed and the master down the noise is worse but I would expect that to be the case as you are raising the noise floor in much the same way a compressor or overdrive does, making the signal to noise ratio worse. If it were a fault in the amp I would expect the opposite to be the case as you maximise the latter stages potential to amplify the internal noise.
What I would be curious of is your mention of your noise gate test. How were you powering the noise gate? Ive seen some power supplies go nasty in various ways and though high frequency hiss is less common, it is certainly possible. I would like to see if the same thing happened with the noise gate powered by a battery and no other effects or power supplies connected at the front. The variations on the modern switching type power supplies have all kinds of symptoms so I wouldn't rule that side of things out. Just to be clear make sure the potentially offending power supply is also unplugged from the mains, it could well be radiating the noise just as much as its injecting it into the pedals.
Also the first port of call with these kind of things is cables and though I see you mentioned and tried various things, at the risk of getting a slap I would come back to the simple and obvious. If the amp makes no noise with no input, try just plugging different cables in, no combinations, nothing else.... is one worse than another? Nothing else connected, introduce elements step by step. Yes, this is a noisy scenario not having something on the end of the lead but it does give you a method of comparison to compare the cables like for like, were the whole inner conductor is a potential antenna. Maybe you have a partial break or some ingress of moisture that is reduce the shielding capability of a cable.
Recently I discovered a nasty noise was the result of my daughters mobile phone charger having bit the dust but remaining plugged in where she abandoned it a stones throw from my home amp.
Have you tried different mains connector leads? Do you always use the same extension leads? Anything were an earth can break loose can upset the shielding cart.
Sorry for going over the menial. We are all fallible to missing things simple or otherwise and sometimes these faults are the most annoying and hard to find and the sooner you revisit the early steps the better, before you disappear down the rabbit hole of amp tweaks and costs, tube rolling etc to find the fault remains.
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Post by newey on Oct 4, 2018 18:40:14 GMT -5
No need to apologize. Occam's Razor dictates exactly that procedure.
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redshark92
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Post by redshark92 on Oct 8, 2018 9:21:45 GMT -5
How are you getting on with the problem? New tubes in. Made another observation based on a previous comment here. More on both in a minute. What is "Shot noise" exactly? Good details here, thanks. This Noise Gate is part of a Multi-Effects pedal and only runs on AC power so I can't do a battery check with it. FYI, I've done the same test now with my Solid-State amp (Fender Princeton Chorus). That one picks up the SC hum worse than the Vox does, but with the Noise Gate engaged it is completely silent. I've tried different instrument cables, different power cables and have plugged int to various outlets in my house that are on different circuits. Not at all, understood and appreciated! OK, few additional bits: This might be silly to say, but I once thought that my AC15 was excessively noisy until I saw that I had the volume turned up on the unused input channel and I was playing clean from the other channel so the Master was up on full. The noise was 'hissy' (white) rather than hum or other indications of a guitar picking up on something like an antenna. It was a 'Doh' moment for me. Messed around with this some. There is definitely some volume bleed from one channel into the other, because when I plug into one channel, turn it completely down and then dime the other channel I do get volume from the guitar (though not as much as I would if actually plugged into that channel). On the tubes, I did switch out the tubes with this set: www.tubeboutique.com/kits/ac15c1-full-re-tube-kit. One variation is that they were currently out of stock on the 12AX7HG so they sent two of the 12AX7LM. They'll be sending me an HG in a few weeks when they are back in stock and I'm currently undecided about whether I'll switch out the LM for the HG at that point. I can say that the tubes that were installed were still the stock ones, which are a fairly basic set and I noticed a definite improvement in the tone after making the switch. OK, after all of that, the changes I've made so far (controlling the opposite channel volume, the retube and the noise-gate, with the QTB mod on the Strat still to come) the hiss is still present but seems to have been reduced somewhat. I am relatively new to exploring tube amps* but ISTR that some amount of hiss/noise is normal and some amps are just noisier than others, so I'm wondering if maybe the other fixes have just left me with that. It is still louder than I'd prefer, but not really at the level of getting in the way of the tone of the amp/guitar, noticeable when not playing (and more so as the volume goes up) but not really noticeable while the amp is putting out sound. *I had a Fender Twin for a while and I don't recall it being especially noisy, but OTOH I also rarely pushed it at all. On the re-tube, though I do have one more question, possibly related to all of this and possibly not. flic.kr/p/N5iUpvThese are the two EL84s I pulled out of the amp. After doing so I was looking at them again and noticed that one of them (the uppermost one in this photo) has what looks like slight burn mark along the interior diameter. The other one is completely lacking this. Is this a possible concern at all?
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Post by newey on Oct 8, 2018 10:20:30 GMT -5
You mentioned having tried the amp on different household circuits. To eliminate what pj suggested about the noise gate power supply, try putting that on a different circuit than the amp, if you have not already done so. Use a long extension cord if you need to. Although, if the power supply is putting out RF noise, this procedure probably won't help.
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Post by thetragichero on Oct 8, 2018 11:17:57 GMT -5
burn mark on inside or outside of power tube?
while I do enjoy moving around preamp tubes for different sounds, any letters following the 12ax7 generally just make for good ad copy and are meaningless never heard of a PM brand tube. will have to do some research online
edit
makes sense
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redshark92
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Post by redshark92 on Oct 8, 2018 12:54:37 GMT -5
burn mark on inside or outside of power tube? Did you check the photo I linked? Looks like the inside to me. Good to know. Like I said, I'm fairly new to this end of things. Had heard of similar designs with slightly different qualities or specs and assumed it was along the same lines. I first found them because they had a full set of what I needed as a single package and I saw mostly positive responses from other people who'd bought from them. makes sense[/quote] I wasn't too worried about trying to find a new PM-labelled tube, specifically. They filled the order with an "LM" in place of the "HG" because the latter was back-ordered and they're sending it, when available, at no extra charge. When it arrives, I may switch it out and see if it makes any difference or I may just tuck it away and save it as a back-up.
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Post by thetragichero on Oct 8, 2018 13:10:51 GMT -5
hey you don't seem to have bought into some special mojo so you're at a much better than I was when I entered the tube amp game. pretty much all commercially-available tubes come from one of two factories so this is something that all of the companies do
can you provide us with more photos of that tube, at different angles?
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redshark92
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Post by redshark92 on Oct 8, 2018 13:36:20 GMT -5
hey you don't seem to have bought into some special mojo so you're at a much better than I was when I entered the tube amp game. pretty much all commercially-available tubes come from one of two factories so this is something that all of the companies do Yeah, that makes sense, I do tend to be skeptical by nature which helps. I have seen lots of recommendations for NOS tubes and I did some looking into those. Most of the ones I found that were recommended were very expensive so I quickly thought better of going that route. Any better? In both cases the unaffected tube on the left, the affected one on the right. flic.kr/p/2asjx5xflic.kr/p/2aJZGn5
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