Hum from new tube amp, what can it be?

[Dr Balalajka]

I just bought the Epiphone Century Amp, the 75th Anniversary edition that just came out. Cute amp and it sounds fine for what I do and it will most likely work for some of my gigs. It dopes however hum quite a bit. At gigs this probably isn't loud enough to be a problem, at home it's a bit of a pain though.

 

Trying to describe it... it comes as the tubes heat up, so it's not there right away when you turn the amp on but after about 30 seconds or so. It's doesn't come from the pick-up or the cable as it is there before anything is plugged in and does not get better or worse when you do plug in. The volume and tone knobs don't affect it to any degree. It's not the electricity in my apartment, not the outlets since the same thing happens in the store.

 

So, what is it? Fautly tubes? Something else? Just a poor construction?

 

It's brand new and I have a warranty. They will look at it, probably send it in for service. Maybe it can't be fixed in which case I can again decide if I want it or not. Thing is, I want it but I'd prefer it didn't hum.

[bluesmann]

You've got 2 6V6 power tubes and 2 12AX7 preamp. 99% of the time if you have a issue with a tube amp its a tube causing the problem. Being its new take it back to the store and replace the 12AX7 one at a time. then the 6V6 for another matched pair. That should do it. Make sure you have good cables too.

[guitarcapo]

Swap out tubes and see if the problem goes away. It sounds like you have a bad tube based on your description. Some other common reasons an amp will hum are:

 

1. poor lead dress...less likely because these amps have PC circuitry.

2. An interaction between the power transformer and output transformer

3. A problem with the center tap or virtual center tap of the filament supply

4. Improper star grounding

 

Short answer: Swap out the tubes. If it still hums return it because you can't fix the other possible causes easily.

[t_e_l_e]

on the other hand, a bit of hum from a tube amp without anything is plugged in is normal.

when you start playing (together with band) you don't notice it anymore. but in a dead silent room with nobody else in it, it can be annoying as hell...

 

..but yeah its normal...

 

if new tubes do not make it better, it could be that it is what it is

[onelife]

Does the hum change when you move the volume control on the amp?

[Dr Balalajka]

Thanks guys!

 

I guess the store will try to swap tubes and if that doesn't work they'll send it away for repairs. Can I live with the hum? I guess I could becasue the amp was bought specifically for what I consider my louder gigs, not for home use or even for recording. On stage the hum won't be very noticable, if at all actually.

 

It does change a bit when you turn the volume and tone knobs drastically, the extreme settings have more hum, but it's a not a huge difference.

 

/Terje

 

 

[onelife]

The reason I asked is because one of the things that could cause hum is a problem with the filter capacitors.

 

If the main filters are bad the power supply ripple will reach the early, pre volume control stages, and be amplified by the high gain of these stages. This will cause a noticeable difference in the level of the hum when moving the volume control.

 

If the bias supply (assuming the amplifier uses fixed bias) filter is bad then the hum will only reach the power tubes and not be affected by the volume control.

 

Sometimes, as seems to be the case with your amp, the noise generated by the tubes in the early stages of the amplifier is referred to as hum but this noise it not due to bad caps.

 

As mentioned earlier in the thread, the most likely (and easiest to test ) cause is a bad tube.

[WRGKMC]

Since most of the hum does change with volume, It sounds to me like the hum's coming from the cable, pedals or guitar.

 

If the guitar is unplugged and you turn the amp up, do you still get a hum? If you do then it could be inadequate power supply filtering or the tubes need to have the bias matched. As tubes burn in and age the bias on them can vary and develop hum. When power tubes are balanced, the hum is nulled. I don't think that model is very old so my guess is its the power tube bias is you have the hum with no input.

 

There's also the possibility the amp just never had good shielding too. I've had many where I had to beef up the shielding where the head mounts to the cab, add grounded tube cans and stuff like that.

 

The Epiphone Valve JR had major issues with hum when they were first released. They used AC voltage for the tube filaments which increased the hum. They later modded the filament voltage for DC voltage which fixed the problem. May be the Century amps have a similar issue and a similar fix. The schematic does look like it uses AC for the filaments. http://www.ampwares.com/schematics/epiphone/ea_25-century.pdf

 

Sovetec does make a 12AX7 LPS tube that has the filament enclosed behind a plate which is supposed to limit AC heater hum. They are good sounding tubes too but they aren't the best tubes for a combo because they can become microphonic from the vibration.

 

If the hum disappears when you unplug the input, then you obviously have hum coming in on the input jack and you'll have to tackle that separately. Single coil pickups, unshielded two conductor pickup wiring, guitar cavity shielding, 90% or better shielded guitar and patch cables, zero hum AC pedal adaptors are all items that need to be gone through and have the hum minimized. Even then with a gain pedal cranked and no notes being played you can still have allot of noise with all that gain amplifying nothing but AC radio waves sneaking into the signal path.

[Dr Balalajka]

The hum is not related to the cable, pick-up or instrument. It is there before anything is plugged in and not really affected by an instrument being plugged in. The hum changes a little when you turn the volume up or the tone to the extreme settings but the change is not drastic. Thanks for the schematics!

[guitarcapo]

One that can really drive you crazy is an interaction between the power transformer and output transformer. The best fix is to MOVE and install the output transformer to a different location/orientation in relation to the power transformer...which can be very difficult to do.

 

Sometimes replacing the OT with a better shielded one or one of different construction solves the problem.

 

I had a vintage Magnatone 440 that had that problem. None of the other Magnatone amp designs I owned had hum... and after going over everything, I determined that the problem lied in the fact that the 440 was the only Magnatone that had reverb and was also compact. The extra reverb tube required forced the OT too be close to the PT and hum resulted. I replaced the OT with a shielded one but couldn't move it. No room. But most of the hum went away.

[AJ6stringsting]

I use a Rocktron Super C noise reduction unit in the effects loop of my amp, so it can close like a noise gate inside the pre amp of the amp. I did this with my Carvin, Mesa Boogie, VHT and Peavy amps. Works like a charm. Just set the N.R. unit so it will not close too soon on your signal.

[Dr Balalajka]

Thanks again for all tiåps ansd suggestions.

 

The amp was returned to the store and they probably sent it back to the dealer to have it fixed or replaced. I say probably becasue they haven't really told me what happened. Their tech hasn't bveen in when I've called or visited the store and since this is not my main amp on most of the time it's not something that worries me.

 

If the hum can't be fixed I will get a chance to reconsider buying the amp.

[dick wiggins]

Its propably the tubes got burned out if it was a demo model

[Yan M]

Hello everybody,i'm new on this forum,my name is Yan and i live in France.,well,i've just bought this nice amp two weeks ago and...same problem!Hum(or buzz)is a common issue with these kind of amps(and single ended as well)it is often due to filaments heaters (6,3V AC)after opening the patient and thorough inspection,the amps looks very well built and isolated on the whole.OT&PT are at the opposite side of the frame and the shieldings looks good;i didn't spot any reasons to have ground loops anywhere either.The filaments supply is provided by a separate small tranny with (apparently)no center tap.the heating power runs through a twisted pair of B&W wires connected onto the filaments pins of the tubes.After measuring it,the power seems to be well balanced,but...it hums through! There is a well known easy trick for ,if not "curing" the problem,at least reducing a large amount of hum.As i said,the separate power tranny doen't seems to have a CT.One can create a"Virtual"one by adding on each side of the end of the circuit(last 12AX7)black&white wires a pair of 100R resistor(2Wis a good value)connected to ground.Sometimes one side is enough.Some use a 100 or 500ohms pot(2W)with each side pins on the circuit's end ,and wiper connected to ground and then adjust the hum by ear.this is a mod within the reach of basic skills in electronic(think about discharging the filter caps before!!!)Before soldering,one can try the circuit(resistances or pot)with two or three crocodiles so as to keep the warranty OK

Anyway it DID solve the problem on mine,hope it will help others! regards Yan

PS:I repeat the usual warning:BE VERY CAREFUL,HIGHT VOLTAGES IN PRESENCE!

[WRGKMC]

Hello everybody,i'm new on this forum,my name is Yan and i live in France.,well,i've just bought this nice amp two weeks ago and...same problem!Hum(or buzz)is a common issue with these kind of amps(and single ended as well)it is often due to filaments heaters (6,3V AC)after opening the patient and thorough inspection,the amps looks very well built and isolated on the whole.OT&PT are at the opposite side of the frame and the shieldings looks good;i didn't spot any reasons to have ground loops anywhere either.The filaments supply is provided by a separate small tranny with (apparently)no center tap.the heating power runs through a twisted pair of B&W wires connected onto the filaments pins of the tubes.After measuring it,the power seems to be well balanced,but...it hums through! There is a well known easy trick for ,if not "curing" the problem,at least reducing a large amount of hum.As i said,the separate power tranny doen't seems to have a CT.One can create a"Virtual"one by adding on each side of the end of the circuit(last 12AX7)black&white wires a pair of 100R resistor(2Wis a good value)connected to ground.Sometimes one side is enough.Some use a 100 or 500ohms pot(2W)with each side pins on the circuit's end ,and wiper connected to ground and then adjust the hum by ear.this is a mod within the reach of basic skills in electronic(think about discharging the filter caps before!!!)Before soldering,one can try the circuit(resistances or pot)with two or three crocodiles so as to keep the warranty OK

Anyway it DID solve the problem on mine,hope it will help others! regards Yan

PS:I repeat the usual warning:BE VERY CAREFUL,HIGHT VOLTAGES IN PRESENCE!

 

It would be much better to just convert the filament voltage over to DC to eliminate this problem.

 

The Epiphone Valve Jr had this issue when it was first sold. People started modding them to DC voltage by installing a bridge rectifier to get rid of the hum. Later Epi changed their design to eliminate the problem.

 

Other things that can help. Using cans on your preamp tubes. Old amps like Fenders all had spring loaded cans that you'd screw over top of the preamp tubes ground hum out and the spring helped hold them in place. http://www.ebay.com/itm/like/221548644974?lpid=82&chn=ps

 

Installing them to the old style would require replacing the tube sockets but There is a new version that only requires a ground screw hole to be drilled. http://www.thetubestore.com/Parts-Accessories/9-Pin-Chassis-Mount-Sockets/The-Tube-Cage

 

Lastly you may want to try preamp tubes with spiral filament's to reduce hum. https://tubedepot.com/products/sovtek-12ax7lps-preamp-vacuum-tube

[Yan M]

Yes you're right,it worth the try;no big deal:two diodes ,a cap and a resistor to drop the increased voltage to the right amount.BTW,on the century reissue, the preamp tubes are already in spring loaded cans(epi did a good job)As for the sovtek "spiral filament" tubes,some says they are microphonic:to be verified...the solution i used worked for me,so...anyway one can make the 100R resistor test on each end of the supply;just to be on the safe side. One last thing,here in Europe,some small tube amps used to have a hum issue due to the difference of power frequency(PT designed for 60Hz where we use 50Hz,but well,now the trannies seems to handle both frequencies,the filtering is better too)About this lovely sounding amp,this hum issue shouldn't be a give away,a red flag.DON'T return them,this particular problem is well known &can be solved pretty easily.This is the main drawback with brand new amps warranty :no one(including techs) dares" tinkering"easy repairs.This amp has everything to become a sought after collector,meanwhile it can provide hours and hours of happiness.

[Yan M]

P.S: The point is: wether you use AC or DC fil supply,you will have to ground reference it,otherwise it will hum.For some(unexplained) reason, this amp's IS NOT...

[Guest admos]

Cool.

[WRGKMC]

I have a couple of the Sovtek tubes. I haven't found them to be microphonic at all in a head. I can see where they can become microphonic in a combo. Just about all 12AX7's do become microphonic in combos eventually due to the constant vibration of the speaker. Tubes are cheap however and getting one to see if there is a difference can at least help to diagnose the issue. If you change one and there is less hum, then its a clue to work with.

 

The reason why ground on the DC heater removes hum is because that half of the circuit is ground right up to the filament. It will carry any stray emf to ground right up to the filament itself. Without the ground it will act like an antenna loop and draw in any stray hum present and the tube will amplify that hum.

[Yan M]

I will try the Sovteks then ( stock tubes are cheap indeed)according to "Occam's razor":always start with the simplest explanation(and fixing)thanks for the amendment!...

[WRGKMC]

That's what I'd try. I don't know how much those long filament model is suppose to help. I've used them but not for a hum issue.

 

I'd likely inspect the chassis interior as well. Make sure all the wires are as close to the chassis as possible to minimize hum. Just be careful in there. Caps will hold their charge and sting you even when the amps unplugged. Be sure you go easy. You don't want to create a problem cracking solder joints either. If you can tack the wires down then the chassis help to absorb any stray AC hum they may be carrying.

[Phil O'Keefe]

FWIW I switched the tube filament power from AC to DC on my Fender Princeton Reverb II and it made a significant difference in the noise levels of the amp - for the better. I wouldn't bother with the mod for a stage amp, but if recording is your main use for an amp, and you want to have the lowest noise levels possible, it's a worthwhile mod to do - or have done, if you lack the skills to safely DIY.

[crsnot]

 

Hello everybody,i'm new on this forum,my name is Yan and i live in France.,well,i've just bought this nice amp two weeks ago and...same problem!Hum(or buzz)is a common issue with these kind of amps(and single ended as well)it is often due to filaments heaters (6,3V AC)after opening the patient and thorough inspection,the amps looks very well built and isolated on the whole.OT&PT are at the opposite side of the frame and the shieldings looks good;i didn't spot any reasons to have ground loops anywhere either.The filaments supply is provided by a separate small tranny with (apparently)no center tap.the heating power runs through a twisted pair of B&W wires connected onto the filaments pins of the tubes.After measuring it,the power seems to be well balanced,but...it hums through! There is a well known easy trick for ,if not "curing" the problem,at least reducing a large amount of hum.As i said,the separate power tranny doen't seems to have a CT.One can create a"Virtual"one by adding on each side of the end of the circuit(last 12AX7)black&white wires a pair of 100R resistor(2Wis a good value)connected to ground.Sometimes one side is enough.Some use a 100 or 500ohms pot(2W)with each side pins on the circuit's end ,and wiper connected to ground and then adjust the hum by ear.this is a mod within the reach of basic skills in electronic(think about discharging the filter caps before!!!)Before soldering,one can try the circuit(resistances or pot)with two or three crocodiles so as to keep the warranty OK

Anyway it DID solve the problem on mine,hope it will help others! regards Yan

PS:I repeat the usual warning:BE VERY CAREFUL,HIGHT VOLTAGES IN PRESENCE!

 

Hi Yan....can you describe in more detail your fix with the resistors for this amp - maybe a picture?

I have the same amp. Same hum .

Different preamp tubes helped a little and biasing cold (15mV) reduced further, but the hum is still there.

It doesn't change with volume much. Sounds to me like some sort of ground issue.

If your fix with the 100R 2 Watt resistor worked then I'd like to try it on mine.

The schematic posted is not the correct one for this new Century amp - the Century 25 is different.

I have attached the actual schematic (which was a real struggle to find!).

I'd also like to change the Dark input to high impedance...5.6 Meg....suitable for a crystal microphone ....I'd appreciate anyones input on how I can do this. Most aps its just a resistor to ground of the inputs but the schematic has me a bit baffled.

[Yan M]

Ok,let me take a few pictures: more détailled comments to come...as i said, the question is:is it possible to reduce the hum of this very amp for someone with basic skills(and tools)without modifying everything? i think it is.It worked for me anyway.the hum hasn't totally disapeared but is now within acceptable standards for this conception an amp.(just enough to wispers:"I'm NOT a digital technology with a vintage look!")BTW the overall conception looks good and well build.I made a Vs test with an other "pre-war" tube amp much more expensive(around 1400$),a boutique handwired ,same power,push pull Stimer reissue (made by Maurice Dupond)with an old Manouche Favino(pickup same make:Stimer)the Epi is more responsive,mellower,but well,that's subjective i know.Ah yes:the Stimer hums too! for that sort of money...Anyway,this Epi is worth the (little)effort to fix it.

[crsnot]

Thanks Yan , I look forward to seeing how it is done. The amp is definitely worth the work .

I pulled the amp out of the cabinet last night and had a look at the wiring also. According to the web experts the heater wiring is done in the worst way for inducing hum...they are loosely twisted , don't run at right angles, don't go to the corners of the chassis and worst is that they loop around the valve base instead of coming from one side ( exactly as the old picture attached shows right and wrong way). I'll attach a couple of pictures of the wiring and the correct way apparently. Maybe this is where the hum is coming from ?- seems like a fairly simple fix if so!