Loud Hum - JCM900

juno75

oooo, 2 posts in one day! Boards been blocked in work for a year but I got back in so I got questions stocked up!

Lately my JCM900 has had a quite bassy hum, not to noticeable but its getting louder and I had to turn the amp off last night, woz wrecking my head.

I'm farly sure the valve's need swapping out.They would be reasonably well used by now

BUT

The hum is not a typicall symptom of knackerd valve's AFAIK, The tone is pretty good still and there is no nasty screeching or anything
so does any one have any other suggestion on a potential cause of the hum before I shell out on a bunch of Sovteks?
I know it's most probably the valve's but if any of you guru's have a suggestion.

I also get a nasty crack when I put it into stand by.

And when ever my frige kicks it I also get a crack but I need a surge suppresser to clean the feed I reckon. Bedsit= Amp and Fridge in same room

Doctor J

Aye, tube amps and **** power = noise. When I was jamming with dabhoys he used to get fierce noise through his Mesa which wouldn't appear when he plugged in at home, the noise was purely a result of noisy power and interference in the room we were playing in. Try a different power outlet in your gaff.

If you can bring it somewhere else to play just to check the amp isn't borked, of course do that too

feylya

The fridge kicking in would suggest dodgy leccy line alright. Has this hum just appeared suddenly or has it increased over time? Any new electrical appliances installed in the house of late?

vox

Your best bet is if you have a spare tube try swapping it with the ones currently in the amp to see if the noise stops. It could be a number of things like the power tranny to the filter caps aswell. If you are thinking about a full retube have a look at tubestore.com and look at some of the JJ or Electro Harmonix tubes. They are fairly priced plus a nice exchange rate. Delivery takes around 1 week plus i have never been stung for customs yet, probably because its such a light package.

Eoin Madsen

Does it hum when there's no guitar connected? Does it hum when in standby?

Damaged or worn valves can make very different noises, so I wouldn't discount a valve problem. First thing I'd try is swapping out the EL34s, one by one. If it's one of them, then just replace the quad. If not, try the same with the 12AX7s. It also could be a borked filter cap, but that would be unusual if your amp is less than about ten years old.

You didn't drop it or anything, did you?

For valves, www.watfordvalves.com in the UK have a great reputation. Though they're presently closed till the 18th, I think.

dabhoys

Another test to try does the hum increase or decrease when you put up either the master volume or gain or both.

When I had a low end hum in my amp it was caused by a burnt out resistor. Theres a few things you can do to test it out.

Another one to be sure of is the amp grounded properly. It could be mains hum too.

Try out the level controls and let us know if it effects the hum and see what else we can do...

juno75

Cheers for the suggestions chaps-

Try a different power outlet in your gaff.

possibly, I think I have already but will try agian tonight to confirm.

The fridge kicking in would suggest dodgy leccy line alright. Has this hum just appeared suddenly or has it increased over time?

Yeah, the fridge has made me suspicious for sure, he hum was bearable but for home practice its getting to loud, pain in but. The lamps get bright /dark also sometimes!

Your best bet is if you have a spare tube try swapping it with the ones currently in the amp to see if the noise stops

When swapping valves do you think its better to do em all in one go or is it a case of one dodgy one causeing the fuss? Would the tubes be 'unbalanced' if there was one new one? BTW, they are electro harmonix in there already, nice valves. Gonna try Sovtec's as Ive heard good things about em.

Does it hum when there's no guitar connected? Does it hum when in standby?

It hums with/without the guitar on both channels and the hum is equal on both. When you go to stand by there is a crack when you hit the button but then no hum.

also could be a borked filter cap, but that would be unusual if your amp is less than about ten years old.

Pray tell - Is a filter cap the thingy that holds the valves in with the springs? Are they cheap enuff to replace? I think its pushing 10 years, I need to check the serial number. I aint dropped it but my brother bought it down to Cork in his van recently for me so I would not discount a bit of ruff handeling (hay,its a Marshall,rock!!)

Another test to try does the hum increase or decrease when you put up either the master volume or gain or both.

No, gain or channel does not affect the hum. As for volume I think the hum is constant regardless of volume. I'll go home tonight and pi$$ of the nieghbours for 10 mins and see.

burnt out resistor.

That sound scary man I dont mind spending a few sheckels maintaining the thing as its a well used, vintage and in a few more years a classic of sorts.
Burnt out resistor sound nasty and expensive.

The dirty current in my gaff seems to be a prime candidate,deffo, possibly in comination with the valves. Would a surge suppressor sort that out to you think? I reckon those spike dont help the valves anyhow

Thanks for all the ideas, ver informative

Eoin Madsen

A burnt out resistor or resistors would actually be the cheapest thing to fix - a resistor costs less than 50c. But it would be tricky to diagnose.

A filter cap is a large capacitor, looks a bit like a small coke can. They're mounted similarly to valves. The make up part of the power supply which filters the DC current after the rectfier. They are expensive-ish to replace - €8-12 per capacitor.

The preamp valves (12AX7s) you could just swap one out. The power valves (EL34s) should be replaced as a set, for best results. You can get matched quads, which will give the most consistance performance. Randomly buying 4 EL34s is not a good idea. The amp will need to be re-biased after changing the power valves as well.

Is this a 50watt or 100watt JCM900? I've been assuming 100 Watt. The 50 Watt only has two EL34s.

A surge isn't going to cause anything as consistant as a hum, so I'd imagine a surge protector will be pretty ineffective. I had this exact problem in a second hand amp, which I tracked down to being one of the EL34s.

juno75

kool, I know the bits your on about, good to put the correct names to em.

im gonna swap out all the valves I reckon, Its a second hand amp and Im not sure exactly how old the valves in it are.

The amp is the 2x12 100w version.
Wieghes about the same as constipated elephant.

As long as I can remedy it as I'd hate to have to bin the amp.

dabhoys

Its not bad. I think it cost me €40 to get it fixed.

The fact that the gain or volume don't effect the level of the hum would indicate it has nothing to do with the valves at all. If it was the valves the hum would change level when you boost the gain or master volume.

It sounds like you have the same problem I had. I did the same crap of switching tubes etc and it did nothing.

Take to an amp tech he'll sort it out fairly handy for ya.

Getting new tubes would be a good idea anyway. Older tubes are probaly the cause of the resistor going.

Having nothing to fear. Just take it to the amp tech it sounds like alot more then the tubes....

Eoin Madsen

dabhoys wrote:

The fact that the gain or volume don't effect the level of the hum would indicate it has nothing to do with the valves at all. If it was the valves the hum would change level when you boost the gain or master volume.

Well, not really, valves are a little more complicated than this. As I said, I had a problem with an EL34 that was identical in symptoms to this. I presume the problem was with the heater filament rather than one of the signal carrying electrodes.

dabhoys

Well the first thing to do is too change out all the tubes and if the problem is still there take it to an amp tech.

I took mine to Mark Leven and he did an excellent job and set the Bias properly for me too which was nice.

juno75

Well the first thing to do is too change out all the tubes and if the problem is still there take it to an amp tech.

That sound like the sensible thing to do I reckon. Id be confadent to do most thing on my guitar but I'll probably electracute myself if I do any thing more than the valves :eek:

Where is this Mark chap located? does he have a webby or anything?
Cheers

feylya

Amp Repair In Dublin
Aladdin's Amps
Phone: +353-1-4935553/+353-0-862490940
Email:aladdinsamps@yahoo.co.uk

musician.ie

Anyone know an amp repair place in Cork? My transistor amp ( fender pro chorus ) has the same problem - low bassy hum.

Johnny Storm

Again apologoies in advance if this is a stupid suggestion...

You mention it gets worse when the fridge kicks in and sometimes the lights go dim.

A few years ago I had a problem where the voltage to my house was below standard. The only way I knew this was when a very low droning boom used to come out of the powered sub-woofer on my PC. This lead to some interesting phone calls to Dells support centre in Bangalore. I couldn't figure it out but found out my neighbour had the exact same problem with his PC. After a few weeks the ESB got their act together and the problem stopped never to recur.

So - I would take your amp to someone else's house and give it a blast before replacing any parts

Eoin Madsen

Not a bad idea, but there's nothing to lose by replacing the valves anyway, it could probably use it.

juno75

yep I think dirty current is the culprit.
I aint had a chance to bring amp outside my gaff yet but I have noticed the level of the hum going up and down in level.
I could hear the lightswitch next door being truned on - pop from the speaker.
Not good I reckon!