Noise problems with an ENGL

[owenofhexxed]

Hi guys I'm new here, cool forum! I'm hoping some of you nice people can maybe help me.

Recently bought an Engl Fireball, I'm aware that it's a noisy bugger due to the high amounts of gain it can produce but I'm getting a noise problem regardless of gain or channel.

This problem always seems to occur after playing for about 15mins, it's a noisy consistant hum. Even when I turn the gain setting and (most frighteningly) the master volume the problem is still there. Normally then I knock the standby and power off for a while and when I turn it back on sometimes the problem reoccurs however after playing for say an hour the hum disappears.

I'm perplexed, the amp is new and some people are telling me that valves need 'bedded in', I personally think it might be the power supply and/or wiring in the practice room

Any help, suggestions or advise greatly appreciated.

Cheers guys

[sumgai]

owen,

Hi, and to the forums!

It would be too coincidental for two topics in a row, on the same day, to both have bad filter caps, but wouldn't you know it, guess what? Yep, you've got one going on the blink. It may not be so much a bad cap, as the solder joint connecting it to the circuit board may be bad, or a solder joint on one of the resistors in that filter cap chain..... there are some possibilities here, but don't discount the caps themselves.

Is this amp new? Any warranty protection here?

..... Valves need to be "bedded in".

What a crock of crap. Introduce me to your mates, I have a bridge I'd like to sell 'em. ;D You were closer in guessing the power supply being the probable trouble spot, two points for you!

HTH


sumgai

[owenofhexxed]

Cheers Sumgai

Yep the amp is new, received it just over a month ago so it shouldn't be a problem getting it repaired or replaced. I'm just reluctant to send it off as I'll be ampless for a while, I live in Ireland but bought the Engl from a German company.

Anyway perhaps you would be good enough to explain a little about 'filter caps' - sorry it may just be a terminolgy thing but I've never heard of them - then again this is my first proper 'valve head' and I'm a bit behind on such things.

What a crock of crap. Introduce me to your mates, I have a bridge I'd like to sell 'em.

Hehehe indeed sir

[sumgai]

owen,

Where are you at, in Ireland? Anywhere's near Cork? I'm an extreme Rory Gallagher fan!

Filter caps, the short story
When we use electricity from the outlet of our home ("power"), we are tapping an AC line. That AC stands for alternating current. The current actually alternates between the electrons flowing in one direction, then the opposite direction. In the USA, this happens 60 times a second, or 60Hz. Most of Europe and Asia, this happens at 50Hz. (Hz is short for Hertz, the scientist who first theorized electricity as having a cyclic nature.) There's a chart on the web if one needs to learn what the standard voltage and cycle rate is for each country.

In order use AC inside of our amp (valve or solid-state), we need DC, or direct current. This should be obvious, but I'll state it anyway, that means there's no alteration in the direction of electron flow. This gives us a greater energy density per cross-section of conductor, which in turn giv..... oh, sorry, I guess I'm getting carried away here. Ahem. Back on track.

To obtain DC from AC, we rectify the incoming AC. But the sad thing here is, that rectified AC is now pulsating DC - and yes, it's pulsating in time to the AC cyclic rate. In order to reduce those pulsations, we filter that newly-minted DC. The object, of course, is to make it pure DC, but in reality, that almost never happens. The resulting product still has a very small (0.01% of the total) amount of AC, which we call ripple. Large filters equals lower ripple, but there is a catch in that. But first.....

Capacitors are used as filters. They have the propensity of storing a charge, then releasing it slowly (given all other things being equal). Now, if we continually re-charge the capacitor (at our 60Hz rate), the cap should, in theory, never have time to discharge. But at the same time, we are drawing power for the valves, right? So, the power is coming through those caps, and thus they are discharging into those valves (and other components). From that we see that the incoming power is trying to keep the caps charged, and the valves are trying to keep the caps discharged. (Bugger all!)

This action is akin to "pumping", the voltage (and current) is going up and down by some small amount, all the time. The lower we can keep that pumping action, the more power will be available for the valves at the time of demand. But wait, there's more! Paradoxically, the more steady the available power, the more sterile an amp sounds. Huh? you're going. Right. The deal is, when a valve requires more power, and it isn't there (yet), the valve sounds as if it has compressed the signal. This is the source of all that you've ever heard described as the beautiful singing tone from a valve amplifier. It is simply the fact that the power supply filters can't keep up with the demands of the output amplifier valves. BTW, the usual moniker for this action is "sag".... the power supply is said to have sagged under the load/demand place upon it.

OK, that should do it, any questions?


sumgai

[owenofhexxed]

Cheers Sumgai

Right I’ve read through everything you’ve said (more than once lol) it all makes sense (I actually did a module in Uni on electronics but it was all PLC’s n’ stuff and it all seems too long ago )

Anyway what you’ve said makes sense however in my first post I was erroneous in describing the problem. Perhaps (if you’d be kind enough) you could do a re-diagnosis and suggest a course of action to solve this hellish problem… cheers doc

Ok the amp is an Engl Fireball, 2 channel, 60watt, all valve head.

When I power up initially all is good. But after playing at band practice volume for about 10mins this horrible hum starts.

The hum is present when the volume is rolled off on the guitar.

The hum is greater on the dirty channel but still present on the clean.

The x-files part is that when I move my hand toward the power/stand-by buttons the hum sometimes stops momentarily.

If I turn the amp on and off the problem temporarily goes away, sometimes it returns.

Usually after playing at band practice volume for over 45mins the problem disappears.

I’m due to play my first gig with this amp in a few weeks… if this bugger starts humming (especially with a mic in front of the cab) it will not be good… please advise Doc Sumgai (or anyone else for that matter) and many many thanks once again.

Oh yeah Sumgai I’m from Belfast but Cork is a beautiful and wonderful City, one of my favourite in Ireland. And as for Rory… well us Irish knock out a few jems every once in a while the man was a god.

[sumgai]

owen,

OK, I was all up to speed with you until that part about moving your hand towards the switch, and the hum goes away for a bit. Sounds like leprechaun work to me.

Seriously, I believe you have a heat-sensitivity issue. More than likely, the last filter cap in the chain, which would be the one that feeds into the first two pre-amp valves, is not up to snuff. It could be a bad connection issue, but I'm leaning towards the cap itself being the culprit.

There is one other possibility. The second stage of each channel very likely shares a common cathode bias circuit, just as Fenders have done since time began. If this goes bad, the gain should drop a bit, but you don't mention anything about the gain changing when the hum stops, after 45 minutes. I'd not point to this capacitor first, for that reason.

That bit about waving your hands like magic, that freaks me out. I have no clue why that should happen. Hmmm, have you taken this amp to another location and tried it out?


sumgai

[owenofhexxed]

Sumgai

Seriously, I believe you have a heat-sensitivity issue. More than likely, the last filter cap in the chain, which would be the one that feeds into the first two pre-amp valves, is not up to snuff. It could be a bad connection issue, but I'm leaning towards the cap itself being the culprit.

I think you're right with this one. I don't think it's that second possibility because (as you said) there is no drop off in the gain level.

I'm kind of relieved, the amp is now out of my hands, boxed up and sent off today. I did mention in the 'fault description' letter that I (well your goodself and others) suspected it was the filter capacitors but I didn't elaborate, it's their job to sort it. However I did ask them to inform me as to what the problem was, just for my own interest. The amp is little more than a month old so it should be perfect condition and considering it's an expensive amp too... I (with my limited knowledge) wasn't about to start messing around with filter caps lol

Anyway thanks for your help and insight Sumgai, I'll keep you posted on the outcome.