Noise Issues with new VOX AC15C1 - is it a Lemon?

[TVinYpsi]

UPDATE - WED. June 30

Okay ... so, I've wiped the egg off my face, and can now let the few readers who've shown probably more interest and helpfulness than I deserved, that after another trip to the service center and more testing at home, I have concluded that the buzzing noise apparently emanating from the reverb unit into the power section was induced by the room I was playing in. Not, mind you the particular outlet I was plugged into, because different outlets in that room that I know to be connected to different circuits, and into different breakers, each induced the buzzing. Removing the amplifier from that room however, and plugging into grounded outlets elsewhere in the house, reduced the buzzing to an almost inaudible, and certainly acceptable, level.

I'll leave the rest of the post as is, along with comments to capture any wisdom here.

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ORIGINAL POST

 

Would appreciate your opinions on whether this new VOX AC15C1 is more noisy than a tube amp ought to be.

 

Please view the recording on YouTube: https://youtu.be/KR_GnBS_d8I

 

I bought a new VOX AC15C1 recently, and was disappointed to discover that the reverb was inoperative right out of the box. Leaving out some of the story, I'll just mention that I took the amplifier to the nearest authorized service center. They diagnosed a bad reverb tank, and stated that the repair part had an 8 - 10 week wait. [i guess is was on a slow boat from China.]

Complaining mightily to the exclusive U.S. distributor, KorgUSA, then working with them through the service center, I was able to get a replacement.

Unfortunately, this tale hasn't yet come to a happy ending.

 

Trying out the replacement unit, (purportedly also a brand new amplifier), I find that it emits an unacceptable amount of noise, even while no guitar is connected, and I've verified that it is plugged into a properly grounded outlet.

 

I would appreciate your viewing the recording I posted to YouTube and offering your opinion: is it just me, or does this amp make too much noise? https://youtu.be/KR_GnBS_d8I

 

[t_e_l_e]

have you any idea how loud this amp is, when the master is on full?

have you ever heard from signal to noise ratio?

 

if you plug in a guitar with humbuckers and play and the hiss is noticeable while playing or even louder than the signal you have a real issue.

without the comparison how loud the guitar would be in your video compared to the noise, this noise seems to me normal...

 

hope your ears will not bleed when you play your ac15 with all volumes maxed

[TVinYpsi]

Thank you for your comment. Just to be clear, I think you're saying that your tube amp reverb makes just as much noise as this one, at half master volume (regardless of the volume setting of the input preamps) without anything plugged in, and hearing that buzz in your recordings would not trouble you. Do I understand you correctly? So this output would pass for 'clean' as far as you're concerned?

[WRGKMC]

You have to realize you're dealing with a primitive technology. Most vintage tube amp designs do make some background hiss. The Vox AC15C1 is cathode biased so its going to have more hiss then other amps and the EL84 tubes tend to be biased hot but its done to get a specific driven tone.

 

You cant get both a silent background and have hot biased sound in that amp design. You can use different tubes and bias the amp cooler to reduce it but then you loose the gain the amp produces.

 

The reverb section uses two additional amplification stages using a 12AX7 tube. One half the tube drives the spring transmitter element, the other half amplifies the receiver element at the other end of the spring. It would be highly unlikely anyone would crank the reverb and the amp gain up to max so that noise isn't heard.

 

What you hear with all the gain knobs up is normal and other then redesigning the amp, you'll get the same thing with most of them. The tube type and biasing make a much bigger difference on those amps. New tubes do make a huge difference and when they get old the background noise goes way up. I suspect this is the problem you had with your first and second amps. The tubes in those combos are good for about 6 months before they need replacement, unlike other amps that may use tubes that last much longer. They are run hot which decreases their life dramatically.

 

This is just another price you pay for getting that specific tone. I used to work at a repair shop where they had dozens of the vintage amps that melted down and took out major components because people didn't know they had a short tube life and tried to make them run as long as they did other amps. Some of those components have been improved in the newer models but the tube lifespan cant be improved without dramatically changing the amps tone. You might want to try some JJ power tubes and Tungsol preamp tubes. This is a popular combination on those amps for best tone vs lifespan. You can buy quieter tubes but it comes at huge price of tone loss.

 

The key is how does it sound run 50% or lower. Its got hot gain stages so its got allot of presence even when turned down. Its something unique to analog circuits. I wont go into a long explanation but you've likely noticed the effect when you turn a guitar pickup volume down. If it doesn't have a bleeder cap the volume and presence dips down and you loose some high end in the process.

 

Vox amps use hot stages so it maintains the presence for a jangly tone when turned down. It comes at a price however and that is allot of tube hiss when fully cranked. The amps aren't designed to be run full out so its a non issue for most users. If you wanted a 15w amp with low hiss cranked then you'd be better off with a different amp design.

 

50% should be your go to volume and if you're cranking it above 75% then you probably have an amp that's too small and would be better off with an AC30 run at lower volumes.

 

I've also found the stock speakers produce allot of highs which magnify any noise hiss. A common upgrade is to install a Bad Cat Celestion. This balaces the frequency response and will quiet the background noise. It also increases the bass response which the original speaker lacks

 

Recording or live a noise gate can be used to silence any hiss the amp produces. That would be done in any recording and the hiss would be heard when you aren't playing and the notes would be well above the noise floor when you are playing. If you use additional gain pedals then using a hush pedal before the amp would be a good choice. The additional gain staging the pedals produce can make the problem much worse then it is now.

 

These are all things you have to consider when you choose an amp and though it seems noisy to you its just normal for those amps.

If you look at these reviews you'll see the first one tells you it has little clean headroom (range between the noise floor and saturation). http://www.musiciansfriend.com/amplifiers-effects/vox-custom-ac15c1-15w-1x12-tube-guitar-combo-amp/423619

 

 

[TVinYpsi]

Thanks for you comment. I appreciate your thoughtful and thorough response, and the fact that both you and t_e_l_e have been long-time HC members. No doubt you both have much more experience with tube amps that I do. However, I feel that you've both focused on a part of my comment (don't know if you've actually watched my video) that I could have just skipped, turning up the preamp gain.

My concern is primarily with the buzz created through the power section -- both preamp gain controls at zero -- with master volume at half, and turning the reverb depth up. A microphone in front of the amp is going to pick up this buzz.

So, all else aside, are you saying the reverb tanks and circuitry in these amps are known to be this noisy and I should just live with it?

[t_e_l_e]

i doubt you will record the amp, without anything plugged in, master on full and preamp on zero, cause noise or no noise, this will be a very boring recording

 

plug your guitar in and play it through the amp and record it.

 

 

i'm pretty sure you will not play it with the master more than half, unless your deaf or you are playing with a full band

 

and the advantage from having a master volume control, for recording you can turn it down and have less hiss

[TVinYpsi]

I had been advised that Harmony Central would be a good place to get some helpful feedback. But it seems that, of only two commenters so far, you'd rather just have some sarcastic fun at my expense than actually address the question. Maybe some folks think you're clever and funny, but I think you're just wasting your time and mine, and hurting the Harmony Central reputation.

[WRGKMC]

Tele is a good guy. I don't think he's being intentionally Sarcastic, just openly frank. This is a slow forum so you may not get machine gun responses to your post but most who do respond do try and give you their best advice.

 

I did listen to the You Tube videos but I didn't hear the hum you mentioned. This is likely because the 60hz hum is low frequency and the speakers I used were just computer monitors incapable of producing those waves. I do believe you have it though.

 

Some questions. Have you tried the amp on a different outlet? Sometimes you have something like a light dimmer or other AC noise on the same line that can cause some hum. Next, does the hum remain with the volume turned down or does it change in volume when you turn the volume up. A deep bass hum that's unaffected by the volume is usually related to tube bias or power caps. I'd lean towards the power tubes because you also have the hiss. If the tubes aren't a matched pair this can cause an unbalanced output and may also be the cause of hum. If its still there with a new matching pair of tubes, then I suggest getting the bias and power caps checked. Having beefier power caps doesn't hurt either. They are very inexpensive and easy to replace.

 

If the hum is only present when you turn up the channel volume then its related to the preamp. You can try new preamp tubes. Those too are inexpensive. If its still there then there can be a couple of causes. Untidy wiring can be one. Wires are normally places against the chassis, especially the heater and signal wires. The chassis acts as a shield to prevent hum. If wires are suspended within the chassis they can act like antennas to pick up hum. What I used to use was a tape head demagnetizer which emits AC waves and go around inside and find the weak areas picking up hum and improve the shielding. Sometimes it may be as simple as moving a signal wire away from the filament wires. Other times I'd add a shielded signal wire that has a grounded sleeve. This is stuff better left to a qualified tech.

 

There can be other things like improving the chassis shield where the head mounts to the cab, but I really cant give you specifics based on what I'm hearing. Hum and hiss can have hundreds of causes and my guesses based on probability are only that.

 

Again, The tubes are your active amplifying components that do most of the work. There's a good chance they will remedy the problem or at least minimize it. If they don't do the job, there's very little you can do besides take it to a tech who can do the actual signal tracing, localize the problem and come up with the best fix. That fix can be different and unique to other amps that have the same symptoms.

[Phil O'Keefe]

The amount of hiss in the video when you cranked the channel and master volume knobs didn't seem all that unusual to me... but the reverb buzz seemed a bit out of place. I don't have an AC15C1, but I do have the model right before it - the AC15CC1, so I'll fire that up and try similar tests and see what happens and report back.

[Phil O'Keefe]

Okay, I'm getting similar levels of hiss with my volume and master volume knobs cranked, and a little bit of added hum with the reverb knob cranked, but not nearly as much hum as you're getting based on what I heard in that video, and it's only really audible when I have both the main volume and master volume knobs dimed, then crank the 'verb knob.

 

I hate to say it, but you might want to consider exchanging it for a third one.

[Phil O'Keefe]

Oh, one last comment - I've never been all that wild about the reverb on my AC15CC1 - it's always seemed rather lackluster compared to the reverbs on my Fender amps, and I tend not to use it very much. That was a fairly commonly made observation about those amps from quite a few players back when they were current... I'm not sure if the current AC15 uses the same reverb circuit or not though.

 

I'm primarily a studio / recording guy, and I have much better reverb options available to me when recording, and when I use the AC15 live, I usually use it as part of a two amp / stereo setup alongside a Fender Princeton Reverb II, and I let the Fender handle the reverb.

[WRGKMC]

What kind of wires do they use to the reverb tank Phil?

 

The old fenders used some decent braded wire and RCA jacks on the tank and head and didn't get hum. If the shielded wire is junk (especially the return wire) I can see that being replaced with something better. The exposed wires on the stock tanks look fairly long and is likely the cause of hum. You need some extension there by the inductors but the stock tanks look like the leads could be cut in half and still provide plenty of flex for assembly movement.

 

Replacing the reverb with a Accutronics 8EB2C1B Reverb Tank will make all the difference in the world. Well worth the $25 it costs to buy one.

 

[WRGKMC]

Here's the stock and upgrade versions. You can see all the exposed wires on the stock that are susceptible to hum. I believe these tanks are in a bag. The bottoms are open. Using a metal plate or copper foil to fully enclose the tank should eliminate the hum even on a stock tank.

 

[Phil O'Keefe]

What kind of wires do they use to the reverb tank Phil?

 

The reverb tank cables actually look like they're pretty decent quality - shielded OFC copper - I don't think they're likely to be the issue or contributing significantly to the hum.

 

I did notice some hum with my amp, but it's definitely less than what he demonstrated in his video. I shot a quick and blurry iPhone video for comparison - I'll post it as soon as the file finishes uploading and processing.

 

Replacing the reverb with a Accutronics 8EB2C1B Reverb Tank will make all the difference in the world. Well worth the $25 it costs to buy one.

 

Let's see if I can remember all the codes correctly - no wait, I wrote them down in an article so I wouldn't have to.

 

Is that a drop-in / direct replacement for the AC15cc1? Same I/O impedance, etc?

[Phil O'Keefe]

Well that was quick. Okay, here's the original video of the AC15c1 from Tom's OP, embedded to make A/B comparisons easier...

 

[video=youtube;KR_GnBS_d8Ip]https://www.youtube.com/watch?v=KR_GnBS_d8Ip

 

 

And here's my iPhone 5 video of my AC15cc1 for comparison...

 

[video=youtube;dIKmfS76dtw]

 

 

The hiss on mine with everything (channel volume / master volume / reverb) turned up all the way is about as loud as the buzz / hum from the reverb is. The buzz / hum in Tom's video seems louder to me, but hopefully this will give Tom something to compare his amp to.

 

Remember though - they're not the exact same amp - mine is an earlier cc1 model, and there could possibly be several differences in the circuit compared to the current c1 models.

 

My amp has seen several years of use. It's all stock, with the exception of the tubes (I put in NOS tubes not long after I bought it new, but I don't recall exactly which ones I put in there, and I don't want to have to disassemble it again to check... Changing tubes in an AC15cc1 is a PITB) and it has a Weber Blue Dog AlNiCo speaker in there instead of the Wharfedale that came with it, but there are no circuit mods - other than tubes and speaker, it's stock.

[TVinYpsi]

Thanks for taking time to check out your own VOX ... that's exactly the kind of information I was hoping for.

I have little experience with tube amps, but I'm certainly aware that guitar amps don't behave like a McIntosh 230 audio amp; I'm not expecting silence. But I really felt that the amount of buzz coming out of the reverb has to be abnormal. Some objective validation of that opinion will boost my confidence in my dealings with KorgUSA.

[TVinYpsi]

Thank you so much for going the extra mile to provide a comparison video! Very helpful!

One other thing that might be noteworthy, the microphone I was using is integrated into the webcam. It's not of high quality or sensitivity, and was positioned about 4-feet from the amp for my video. Clearly, for your iPhone video, the microphone was inches away from the front of your amp.

 

Your video leaves no question in my mind that my new AC15C1 has an excessively noisy reverb feature. As WRGKMC points out, there are numerous possible causes for the noise. But putting aside for the moment all the intriguing suggestions for improving the quality of the sound, it should never be necessary to purchase and install different components just to get proper base functionality out of a new product -- tube amp or anything else.

[TVinYpsi]

On the MetroAmp.com forum, in a post from 2012, inkomodo.dragon described a mod that cured his reverb-induced hissing, but it had the downside of disabling the reverb feature entirely. For those of us who want to play with reverb, he recommended a replacement similar to what you've described, and pointed out (as one can find on the Accutronicsreverb.com site, that the '2' in the middle of the part number indicates a medium decay, while the stock reverb tank in the AC15C1 is a long delay tank. [if the reverb level is too low, he suggests, try changing R60 with a 28k.]

In the AC15C1, the stock Belton BS3EB3C1B tank is connected using RCA jacks. It's screwed, open side down, to the bottom of the cabinet.

With 21-inches of open width in the base of the cabinet, it does seem that it should be possible to fit a Type 4 Accutronics unit (at just under 17-inches long) in there (within identical impedances, obviously). Accutronics claims that's the design that give the Fender Twin Reverb its characteristic sound.

[TVinYpsi]

Are you certain that's a Type 8 Accutronics? Their website says the type 8 is 9.75-inches long, which is about how long the stock unit in the AC15C1 measures. But the two units you show appear to be very different lengths ... or is that just geometric distortion induced by the low angle of view, and they're actually about the same length?

 

I might be interested in trying a different reverb tank, but I want VOX/KorgUSA to make my amplifier right before I start messing with it.

[TVinYpsi]

The Accutronicsreverb.com website provides all the codes and letter/number meanings for all their current products. It's a pretty useful website. Makes me wonder if it might be worthwhile upgrading to a 4-spring unit: Type 4 - 4EB3C1B would provide the same impedances, install orientation, and decay time.

 

But, as I'd said before, Job One is getting my new amp to function as designed before beginning to mess around with it.

[WRGKMC]

You may like this link on vox amps

[WRGKMC]

Here's the second part.

[WRGKMC]

Vox has been bought out around a dozen times. I'm sure Korg does its best to produce amps comparable to the originals but its difficult to know exactly how those vintage amps sounded. Any vintage amp you hear today will have aged parts which changes the sound and the original manufacturers of tubes, caps, transformers etc. don't exist any more. They are built with todays budget components which can produce different results as a whole.

 

This is not to say the originals were better or the new amps are worse. Budget components of the past had an even higher failure rate then todays computer designed parts, but there may have been a few unique components that made a big difference. The amp could be made with much better components today but that could easily double or triple the cost.

 

Modding mass produced amps to improve them is surely nothing new and its up to the individual to decide if its something worth the cost. Its no different than an auto mechanic taking a taking a stock street car bought at a dealer and hot rodding it into a race car. You can find many articles on this particular amp being improved with better Tubes, Caps, Transformers Speakers and Reverb Tanks. Just comes down to what you want to spend to get that additional edge.

[TVinYpsi]

RESOLVED

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WRGKMC: Thanks for those VOX videos. The first album I ever bought was a Beatles record. Hearing this history told by folks who lived it is a delight.

 

So, the thing is, you called it more or less correctly in one of your earlier responses to my Original Post, suggesting a possible cause for my reverb buzz was actually somewhere in the line power. My current working theory is that it's RFI or EMI induced by a nearby router and battery back up. I say nearby because there are two outlets in the vicinity of the router, each on a different circuit, the two running into different breakers. The buzz arose regardless of the outlet I plugged into ... until I moved to a different room. Would shielding on the reverb tank eliminate the problem? Possibly.

But the main point here is that, although my video did not lie, and those who watched it -- including the KorgUSA Customer Service people by the way -- agreed that the noise was outside the range of normal, I no longer believe my replacement amplifier is defective. [No question that the first amplifier was defective, since it would not reverb at all!]

 

I guess I can close this out with a hearty thanks particularly to WRGKMC and Phil for doing your best to help me out!

[Phil O'Keefe]

Glad to help - that's what we're here for.

 

Tom, you can try using a power conditioner with EMI / RFI filtering or even a UPS (uninterruptible power supply) unit to clean up the power that's feeding your amp, and thereby reduce the noise. If you have a electric motor or other noise source nearby (fridge, AC unit, the router and battery backup you suspect, etc. etc.) they can cause noise issues on the power line, and both of those types of units can help reduce or eliminate it.