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Laney Cub 10 has two problems: buzz, plus bias trimpot won't allow 70%

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UsableThought

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EDIT: I am rephrasing my question. You can read the original post below for background, but I have refined the nitty-gritty of my question to something much simpler:

Does anyone have a tube amp, in the vicinity of 8 to 20 watts or so that does NOT hum or buzz noticeably (it's really a buzz, not a hum) when played clean (NOT cranked or high-gain) at moderate, bedroom or living room volumes? That is what I am trying to find out, so as to make a comparison with the Laney Cub 10 that I have.​

If you DO have a bedroom/living-sized tube amp that is quiet when played clean, please do tell me about it!! And conversely, if you have come to believe there is no such animal, please let me hear why you think that, too!

- - - - - - - - - - - - - - - - - -

(ORIGINAL POST BELOW)

I hope this is the right forum to post this - I am brand new here.

I am brand-new to tube amps and am concerned about a Laney Cub 10 I bought. I'm fairly experienced in a handyman sort of way with using a multimeter & fixing electronic stuff. I bought a simple octal bias probe from EuroTubes & was able to use it plus my meter to follow the bias-setting procedure described in quite a few places, including on EuroTubes. I have set gone through this procedure for two different tube sets on the Cub 10: the stock power tubes, which are Rubys, and a matched pair of Electro-Harmonix 6V6GT fromTheTubeStore.com. I also have gone back and forth between the stock preamp tubes, again Rubys, and a pair of Sovtek 12AX7-LPS.

And now the problems . . .

First, the amp SOUNDS great when the guitar is played, especially with the Sovtek pre-amp tubes. However in between playing notes & getting this great sound, whenever I am NOT playing, the amp exhibits a prominent mix of buzz and hum - not so loud that it would clearly indicate a massive failure somewhere, but loud enough to be irritating. With the volume at a moderate living room level, e.g. 11 o'clock, and the gain at about noon, I can clearly hear the buzz/hum whenever I am not actively playing notes. Even with both gain and volume at zero, I can still faintly hear the buzz/hum if I am within 6 feet of the speaker. Whereas I have read various statements online by amp techs and amp makers that a good tube amp should be almost silent when not being played, unless you put your ear up to the speaker. So I am wondering, is it just that the Laney is a budget amp and so they didn't put the money into a better power supply? Or do I have a lemon? The noise occurs even if the guitar is not plugged in. And it occurs with both sets of power tubes and both sets of preamp tubes. The amp also has a trimpot for minimizing hum and I have played with this - it is set correctly, in the center where the tubes work to cancel each other out; but it ain't enough to get rid of the buzz/hum I am hearing.

And now, about the bias: I cannot get the bias adjust trimpot to give me a 70 percent value with either power tube set! It was easy to get the plate voltage, assume 14 watts for the Electro-Harmonix 6V6GT (I looked it up & ) or 12 watts for the stock power 6V6 Ruby (I looked that up also). With the Electro-Harmonix, plate voltage was 305V, so plugging everything into the formula, 70 percent = 32 mA. But I couldn't get the bias adjust trimpot to give me anything higher than 31 mA even when turned all the way right to its maximum; so I left it at 31 mA. In trying to solve the hum problem I put the stock power tubes back in and re-baised. In this case, the plate voltage measured 280, thus the 70 percent figure is 30 mA; but now I couldn't get the bias adjust to go below 33 mA! I ended up setting it at 35 mA which is in the middle of the trimpot's range. With either set of tubes, the amp SOUNDS fine and I don't see any red-plating . . . but the buzz/hum is always there in the background. I don't know if there is any relationship between the hum & this odd behavior with the bias adjust trimpot.

So I can wrap this up in the form of two questions:

1) Is it commonplace to have a very noisy tube amp with nothing to be done about it, especially if it is a budget amp like the Cub 10?

2) What is going on with that bias adjust trimpot that I can't adjust it to get what would seem to be an optimum value!?
 
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pdf64

Member
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7,643
Which if any of the vol / tone controls affect the hum/buzz?
Your building's electrical installation may be less than ideal, or there may be a high level of electromagnetic noise in your local environment; have you tried it in a different building, ideally alongside a buddy's amp?
How old is the amp?
1/ As system gain is increased, noise problems will become more prominant.
2/ It may be that the power tubes you've got are at either end of the normal range. The settings you've achieved seem reasonable, and as the amp may use surface mount technology which would be tricky to mod, it may be best not to be concerned about it.
 

UsableThought

Member
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1,649
Thanks for replying. It may not have been clear in my original post, but let me say it more clearly now: The amp is brand new, and all the tubes are brand new. And the noise is affected by both volume and gain controls. I'm aware that noise gets worse with increased gain in pretty much every application; my point is that I don't find the current level of noise acceptable given that it occurs at very moderate playing levels, NOT with high gain. And I am asking if it is typical for tube amps.

Aside from a problem with the amp itself, the only other possibility would be electric noise. The power conditioner may not be dealing with any if such exists; I will probably be returning it. I have ordered a different kind of line filter and I may or may not bother with taking the amp elsewhere. I play it where I live and if it is noisy where I live then I don't want it.

Maybe I should put my question differently: Does anyone have a tube amp, in the vicinity of 10 watts or so (like the Cub) that does NOT hum or buzz noticeably (it's really a buzz, not a hum) when played clean (NOT cranked or high-gain) at moderate, bedroom or living room volumes? That is what I am trying to find out, so as to make a comparison.
 

J M Fahey

Member
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2,663
What happens if you set volume and gain to 0?
What if you set guitar volume to 0?
What guitar/pickups are you using? (single/double coil/humbuckers?)
Do you play straight into the amp or use some pedal?
If using pedals, unplug them and go straight into the amp, any change?
If you are starting and rotate your body/guitar, does the buzz change?
Do you have a cellphone on you? (no kidding)
Is there any light dimmer in that room or one by it?
Are you close to a TV or old style tube PC monitor?

EDIT: almost forgot: why would you want to rebias a new amp?
Don't "fix" what's not broken.

In fact, you can mess the result.
A single ended Class "A" amp does not like a random idle current setting but there is one specific value, set at factory of course, which allows maximum power and symmetrical clipping.
To boot, the OT is designed for that specific current.
 
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UsableThought

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1,649
I don't mean to be rude, but you are asking questions to which I gave the answers in my original post. Please, if you want to be helpful, read before responding.

As for why I might want to rebias, this is useful and many contend even necessary when putting in new power tubes - at least for those with the tools and experience; others might just want to take the amp to a tech. This is a very common procedure among tube amp enthusiasts and if you do not know about it, there is a wealth of information you can learn from. Just Google "biasing tube amp" for starters. As an aside, if there were a good tech near me I would gladly ask my questions that way; but unfortunately I live in a small town & there aren't a lot of amp techs around, if any. Thus I am reduced to hoping that someone on this forum will actually be able to answer my questions as I have put them.

And finally this is a class A/B amp, not a class A. But please do read up on biasing; it's an interesting topic. Here is a good article to get you started:

http://www.aikenamps.com/index.php/what-is-biasing
 
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J M Fahey

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2,663
Ok, you are clearly more interested in arguing than in solving your problem.
Maybe the fact that 2 Techs asked the same question (posts 2 and 4) means your first post is poorly written or incomplete after all.
To which I added the full standard questionnaire asked to all people who complains of such problems.
You might have simply answered "yes-yes-no-no-yes" or whatever so we can proceed with diagnosis instead of throwing your little tantrum.
Oh well.
 

UsableThought

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1,649
Well, J.M., since you are still with me, I am going to forgive your hard words and assume you do have some mild interest in the affair. Here is what I have decided on:

- I have ordered an RFI line filter from Mouser.com, which is where I get a lot of my parts for pedal builds, solid state amp repairs, etc. I read a page on noise reduction that suggests this will reduce mains noise if anything will - here is the page - the recipe for putting the RFI line filter into a convenient plug-n-play form is down near the bottom: http://ethanwiner.com/dimmers.html

- And I have ordered two JJ 5751 preamp tubes to replace the Sovtek 12AX7-LPS. I'm doing this because I just read an article asserting that a good way to bring down overly aggressive gain in an amp, and in the process lessen noise while opening up more detail, is to put in a matching tube type that has less gain. The 12AX7 type is considered to have 100 percent gain and the 5751 type is considered to have 70 percent of that. These JJ's get good reviews. And maybe it will work for me - fingers crossed. I like to play clean, and the reviews make it sound like this is a good tube for playing clean and can work well in a small amp such as the Cub. Here is the article that mentions this concept of "pre-amp tube gain factor" - I'm sure you'll find it interesting: http://www.thetubestore.com/Gain-Factor

We'll see. If neither of these things work out the way I want them to, then I'll return the amp, either for an exchange or a refund. The reason I might possibly want to exchange it is that I got it from American Musical Supply as an "open box item," which I think means they might have had it on the floor, or possibly someone else bought it and returned it. So it cost $25 less and they give you the same 45-day trial period as with a new amp. Maybe the fact it was "open box" means that someone else bought it, didn't like the buzz any more than I do, and returned it just as I am thinking of doing! In which case a truly brand new Cub might be worth taking a chance on.
 
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RussB

low rent hobbyist
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Does the amp buzz & hum with nothing plugged into it?

70% is a maximum, not a target. Less is always better if it sounds good because the amp will run cooler
 

UsableThought

Member
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1,649
Does the amp buzz & hum with nothing plugged into it?
Yes. In fact with nothing plugged into it and the volume and gain turned down to zero - even then a buzz is detectable. It's really a buzz and not a hum. Sounds like it may be 120 cycle noise.

Anyway I have just dropped off the amp at the local UPS store to be returned to the online place I bought it at, for a full refund. It was an "open box" sale for $25 less than a new unopened Cub. Once I get my refund I will go ahead and buy one of those un-opened Cubs and hope that I have better luck. And even if it does have a bit of noise, I can see what it sounds like when I put a pair of matched JJ 5751 tubes in it for preamps - these get good reviews as a replacement for 12AX7 tubes if you like to play clean (as I do), and their lower gain can reportedly help ease noise problems. That's a similar strategy to your suggestion of going for less than 70 percent bias if it sounds good.
 

RussB

low rent hobbyist
Messages
11,158
Good move.

I have never seen a good review of the JJ 5751 though, quite the opposite in fact
 

UsableThought

Member
Messages
1,649
I have never seen a good review of the JJ 5751 though, quite the opposite in fact
Since this thread was active, I have acquired another Laney Cub 10 and am playing around with it. It has a similar noise issue, but I am going to be more patient this time - I have found the noise is less, for example, if I roll the tone control on the amp down (previously I had it wide open and controlled tone with a tone pedal).

Aside from that I did try a pair of the JJ 5751s in it just to see. Very clean and clear, as you would expect with less gain; but they did nothing for the noise. And they do not have quite the depth or warmth of a Sovtek 12AX7 so I am going to stick with the Sovteks and work out my noise issues slowly, over time, as I get the urge to tinker a bit here & there.
 

StanG

Member
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4,669
In my experience, a small tube amp is always going to make noise when the tone control is all the way up--nature of the beast.

As a side note: when you have less than 20 posts, you may want to refrain from insulting/talking down to the guys with many posts, who have shown themselves to be both knowledgeable and helpful. Just my opinion, but it makes you look like a know it all jerk.
 
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Mate, I think that being caught up in trying to do things by the numbers you may just have missed out on a few crucial details, some of which J M Fahey may just have picked up on.

This Cub amp puts out a max of 10W clean, right? As you can tell it isn't going to bias like a regular class AB which would put out twice this biased at 70%. This could infact make it a "hot class AB", much in line with some Vox style amps running the valves so hot people believe they are infact class A. And they behave much like class A amps, although of course the bias source is still of class AB topology.

I'd ask Laney just how hot this amp is meant to be biased before continuing on this pursuit of yours. Either it's special circumstances or the current should have to be doubled or halved to make sense. Once that is taken care of we can look into this noise issue of yours.
 
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