Newbie needs help - new tube amp hum problem

bricksalt · « **on:** 28 Aug 2009, 12:32 am »

Hi all. New to tubes here. I purchased a used Prima Luna prologue 3 preamp that I have used for two months with a ss amp with no problems. Prologue is quiet as can be and sounds very good to me. Recently purchased close out, unused, Melody 330B monoblocks. They just arrived. Speakers are Gallo 3.0 and cables are Kimber. Power cables for the amps are the ones included with them. I now have an audible hum that is rather loud and annoying when either or both amps are turned on. The hum is there whether the preamp is on or off. It is also there whether I use the 4 ohm or 8 ohm taps for the speakers (although I did not think that would make any difference). The hum is rather loud, even audible during soft passages of music. Any help or suggestions would be appreciated.

Thanks - Dave

bricksalt · « **Reply #1 on:** 28 Aug 2009, 12:34 am »

Correction - the Melody monoblocks are 300B not 330B.

hmen · « **Reply #2 on:** 28 Aug 2009, 12:54 am »

If your amp is like most 300B amps it probably uses AC to heat the 300B's and that will cause a hum. A lot of these amps will have hum pots that you can adjust to minimize the hum. Check your owner's manual to see if you have these.

JakeJ · « **Reply #3 on:** 28 Aug 2009, 01:09 am »

Hi Bricksalt,

Welcome to AC, this circle, and to tubes.

First thing comes to mind is a possible ground loop problem on the AC side. I'd take a couple of cheater plugs and put them on the amp's power cords (turning everything off to do this then back on, safety first). If the hum is gone then it's a ground loop issue.

REMOVE the cheater plugs. DO NOT operate any appliance that has a three wire plug via a cheater because it is dangerous!

Now if it is a ground loop you just need to figure out why. Is the entire stereo all on the same circuit? Not the same receptacle but just the same breaker. If not this can be the culprit. If the system is all on the same circuit then the problem is elsewhere. Some amps (like mine) have a ground lift switch. This is how I must run my VAC amps because despite have nice tight 3-prong receptacle the wiring in the walls is still 2-wire.

It may be the amps themselves. I am not familiar with these particular amps nor the 300B tube.

Maybe someone with some experience with Melody equipment can chime in.

Good luck,
JakeJ

bricksalt · « **Reply #4 on:** 28 Aug 2009, 01:19 am »

Thanks for the input guys. I will investigate. I know all of my equipment is on the same breaker. Are the cheater plugs the two prong plug that you can plug a three prong into and then you plug it into the wall? Like an adapter? Thanks

Niteshade · « **Reply #5 on:** 28 Aug 2009, 01:28 am »

Another easy test: Take the driver tube out and turn the amp on. This will divide the circuit so you can tell whether the hum is coming directly from the 300B or the driver stage.

Ericus Rex · « **Reply #6 on:** 28 Aug 2009, 01:28 pm »

JakeJ is absolutely right that you need a ground in the system. However, you only need one component grounded. All others in the event of a catastophy (how DO you spell that word?) will ground through that grounded component. You can have ground loop hums if more than one component is grounded. So I'd say, in addition to the other great suggestions here, fool around with cheater plugging different components (always leaving only one grounded) until you've eliminated, or at least minimized the hum.

Hope this helps!

JakeJ · « **Reply #7 on:** 28 Aug 2009, 02:10 pm »

Good point, Ericus Rex!

And I'll stick to my OP that you should be absolutely certain you don't have any ground loops. Then you can look to the amps as the possible source of the problem. Although they may still be innocent.

Ground loops are pretty common and once solved usually leave a happy music lover once again.

Blair's idea is good and he can possibly help you troubleshoot the problem via this thread with a few simple tests and maybe a couple of photos.

Don't forget to let us know what's going on along the way.

BR,
JakeJ

avahifi · « **Reply #8 on:** 28 Aug 2009, 02:32 pm »

If your Kimber Cable interconnects between the preamp and the amp(s) are of their standard twisted three wire configuration, you should know that they are MUCH more sensitive to hum pickup than normal coax shielded interconnect cables because they have much less shielding capability.

While they will work just fine in many applications, in others, especially if they are routed close to AC power cords, power transformers, long runs, etc., it would not be unusual to hear audible hum.

I suggest you try replacing them with standard old Radio Shack interconnects for a quick test to see if this is your issue.

Regards,

Frank Van Alstine

bricksalt · « **Reply #9 on:** 28 Aug 2009, 09:50 pm »

Thanks for all of the great suggestions. I am embarrased to say I don't know which tube is the driver tube and which is not. The configuration of these monoblocks is two smaller tubes in front and two larger tubes directly behind those. I will try to replace the kimber kables- they are the less shielded version, and will try to use cheater plugs to leave only one plug grounded and will report back. thanks again. I greatly appreciate the help. Dave

Ericus Rex · « **Reply #10 on:** 28 Aug 2009, 10:58 pm »

Do you have the manual? One of the two smaller tubes would be the driver.

Niteshade · « **Reply #11 on:** 29 Aug 2009, 12:26 am »

Typically amps don't hum if there's nothing on the input. If it does, try shorting out the input. (Tip of RCA to ground.)

You can try pulling all the small signal tubes and leaving the rectifier & 300B plugged in. It it stops humming, there may be an internal ground issue, a bad cap, cold solder joint, etc... Since it's the same with both amps- I'm a little at a loss. Things don't usually go bad in pairs unless something is shared.

bricksalt · « **Reply #12 on:** 29 Aug 2009, 02:24 am »

Hi all. Problem solved, I think. I switched the kimber kable wires from between the pre and amps and that did not fix the problem. Then I put a cheater plug on every amp and on the preamp. These are all plugged into a hospital grade power strip/surge protector (that I actually "borrowed" from the hospital in which I work) -whose three pronged plug is then plugged into a monster power "power center." The power center is then plugged into the ac outlet via a three pronged plug. This silenced the hum. Yeaa! I do appreciate all of your help. Blair, you were correct in that the hum was not there if the amps were disconnected from the preamp, but it was there if connected to the preamp with the preamp's power turned off.

So, now can I leave it like this? Is it a problem to leave the cheater plugs in the system. Does it make more sense to plug directly into the Monster power Power Center and skip the surge protector? Or, should I plug the power strip/surge protector into the wall and then run the Monster Power from the strip? I don't think I tried plugging directly into the Monster Power with all of the plugs without the cheater plugs. Should the Power Center take care of the ground loop?

Thanks again to everyone and to the creators of the Tube-o-phile Circle. Dave

JakeJ · « **Reply #13 on:** 29 Aug 2009, 03:58 pm »

Hi Dave,

OK, so it was a ground loop issue. It appears to be between the amps and preamp. Now you need to isolate it to a one or the other. Just use the cheater plugs to figure out which it is. If I had to use a cheater in my system I'd prefer it was on the low current device, ie. the preamp.

Definitely try just the Power Center or just the strip and swap them around as that may be the source of the problem (some odd incompatibility between the two).

Let's make sure you find the source before stopping. Ultimately we want the system to operate with no cheater plugs at all.

Also you might want to get one of these to check the wiring at the receptacle. Just to make sure your polarity is OK and there is good ground available.

Dave, I think you're close to nailing it down so keep it up. Good work so far!

Regards and keep us posted,
JakeJ

bricksalt · « **Reply #14 on:** 30 Aug 2009, 07:21 pm »

Thanks Jake. Last night I did further experimentation. I used either the power strip or the Monster Power Center alone and did not use them together. Moving the cheater plugs around and in cominbation I found that it did not matter if the cheater plug was on the preamp or not. If the cheater plug was on either amp (I tested them one at a time and together) the hum went away, if a cheater plug was removed from either power amp the hum returned. The results were the same for both the power strip and the Monster. I will order an outlet tester as you suggested. It also turns out that I blew a fuse or the breaker on the Main and Spare Amp outlets of the Monster Power "Power Center." So, I can only use the Monster at this point for a preamp or a CD player (according to the labels on the outlets). The higher current outlets are not workng even when I reset the breaker button.

So, where does that leave me now. I assume I am best off buying a Power Conditioner of some sort that will handle higher power items. Although, the Monster Power manual indicatest that two 50 watt amps should not have been too much for the unit to handle. Perhaps I shorted something out with all of the switching around. At least I did not short out one of the amps. I appreciate any help you all can provide. You have been great so far.
Dave

JakeJ · « **Reply #15 on:** 30 Aug 2009, 08:12 pm »

Hi Dave,

Let me make sure I understand this. Only the amps are causing the hum, and the hum occurs when both amps are directly plugged into the wall. If either one amp or both have cheaters the hum goes away. Is this correct?

I was really banking on the Monster Power Center or the surge strip to eliminate the problem. It could be the way the amps are grounded internally, dunno without seeing them.

I hate to say it but do what works. I have to run my Dynaco Mk 3 monos with a 2-wire cord also or they hum too. I had to make 2-wire cords specifically for them as even having the third wire in the cord yet unconnected it caused a problem. Just so you know others have these problems too.

Hope you got some help here!

Maybe somebody has some more ideas throw in.

Happy Listening,

JakeJ

bricksalt · « **Reply #16 on:** 31 Aug 2009, 01:19 am »

Hi Jake, Yes the mono amps hum if they are plugged directly into the wall or into either the surge suppressor or the Monster Power if they amps do not have the cheater plugs. The humming stops only if the cheater plug is used for both amps. I tried them one at a time and together. If one of the mono amps is without the cheater plug it will hum. So both amps need to cheater plug regardless of where they are plugged in.

Do you think a PC Audio high grade outlet replacement would make a difference? I can't imagine that a recently built, modern tube amp that costs $3800 retail for the pair should have an internal grounding problem. Does it seem that would be a product defect? I am not opposed to creating a two pronged plug for them, but why should I have to do that for a new product.

I have not removed any tubes to see if the problem lies there. Is it possible that the problem is with a tube?

Thanks - Dave

JakeJ · « **Reply #17 on:** 31 Aug 2009, 12:21 pm »

Hi Dave,

OK, thanks for the clarification.

No, a fancy receptacle will not fix it. I think maybe it is not so much a defect as perhaps a design issue. The amps probably work fine on the designer's bench with their AC.

The cheap fix you have already discovered. I too have to use two wire cords for some of my amps (vintage and modern).

An alternative would be if you have a good repairman in your area you could take the amps to them and see if they could figure out why and fix it. Personally I wouldn't worry about it and just do what you need to so you can listen to music. There will be some that don't agree saying that a three-wire cord is a must but if you go out an buy a new kitchen gadget like a blender or a coffee grinder there is only a two-wire cord on those so it can't be that big a deal.

Hope this helps.

BR,
JakeJ