Unwanted noise - intermodulation distortion (ghost notes)?

[Gunner]

Hi guys,

Just finished my Metro SL build. The amp works grate, from first power up. But I have a problem with a discreet noise, or bad sound. I can’t figure out where it comes from, why it occurs. If you have been working with tube amp a lot you have probalay noticed this too. It is not a problem only in this particular amp. I hear this sound more or less in many other non-master amps when pushing the power stage, for example in my JTM45, Bassman and Marshall 1987. But I suppose, in this very loud 100w amp the sound becomes more obvious.

The noise, or bad sound, is a low "groaning", maybe in low G freqs (100Hz or a multiple from that freq). As a subharmoinc tone. It starts off when you play particular notes on the guitar, preferebly higher up, from G-string and higher. (It might occur when you play lower notes too but it might be mixed with the guitar tone.) You have to turn the amp up a bit so the power stage is runing warm.

I have tried a lot of changes too reduce this. With no success.

Here are som short clips from what I’m talking about. The first is very short, repeated single notes. The second clip – here I play different notes. I have eq-lifted the low freqs (100-300Hz) a to make it more obvious. You might need headphones to hear it clear.

http://www.kickback-rock.se/songs/sound2.mp3
http://www.kickback-rock.se/songs/SLnoisetest3.mp3

It does not come from the guitar or speaker. I have of course tried different guitars and cabs.
It doesn’t seem to come from the preamp. I have separated the preamp signal into another amp and the noise is gone.
It is not a tube problem. I have repleced all tubes with no effect on the noise.
It is not a gorunding problem. The amp is absoluley dead silent.

First of all I thougt it might be a resultat of bad eletroytics in the B+ supply. I have replaced all cap cans with F&T. No difference, even though they are nice blue…

We have discussed this problem deeply on a swedish tube amp forum. Several different theories. One was about too low filtering on power tube screens. To reject that I have tried extra 22uF filtering on each power tube screen. No diference.

Someone thought the choke could be more or less bad wired. I don’t know. Haven’t tried that.

The main remaining theory is that it has to do with “non-linear intermodulation distortion”, maybe in the OT or the whole power stage. Well, I can spell it, but I really don’t know what it means and have no idea how to reduce the effect.

http://en.wikipedia.org/wiki/Intermodul ... plications

So, the stage is yours you guys who have been digging in theese amps more than me. Give me a clue? Is this a “phenomenon” we have to live with in this kind of amps?

/Gunnar

[54strat]

I listened and nothing is jumping out at me. Maybe a little fizzy, but I can't hear any low "groaning". Mostly I hear some pretty good Metro/Marshall tone. I notice stuff when playing at home that magically goes away on a stage with drums, crowd noise, and other amps. New build you say? Give it some time to settle in.

[Gunner]

No one else hear this subharmonic sound/noise ? To me it is quite obivous. Listen in headphones if you don't hear it in a comoputer speaker.

I played another couple of hours on this amp and maybe it not as obvious any longer. Maybe the electrolytic capacitors and/or the transformers need some brake in.

Still I'm interested in knowing about this phenomenon, since I hear it in many other tube amps. There must be many of you who have similar experiences? And hopefully some of you also know why it occurs. And how to reduce it, of course...

/Gunnar

[Mars Hall]

No one else hear this subharmonic sound/noise ? To me it is quite obivous. Listen in headphones if you don't hear it in a comoputer speaker.

I played another couple of hours on this amp and maybe it not as obvious any longer. Maybe the electrolytic capacitors and/or the transformers need some brake in.

Still I'm interested in knowing about this phenomenon, since I hear it in many other tube amps. There must be many of you who have similar experiences? And hopefully some of you also know why it occurs. And how to reduce it, of course...

/Gunnar

Look up older posts on "ghost notes" or "ghosting" and see if any of those desriptions ring true to your experience. This is a common issue with these amps and in most cases the sub harmonic frequecies can not be heard in band situations.

I would describe ghosting as out of tune notes riding the notes actually being played.

[Gunner]

Well, maybe that is what it is all about. But I thought ghost notes rather did follow the notes from the guitar. The sound/noise I hear is more constant in frequence, somewhere in the 100-300Hz range.

Are there any sound clips form similar sounds identified as ghost notes? Could be interesting to compare.

Is the solution to increase the main filter values? I have the stock double 50+50 cap cans. Other solutions?

Do I maybe have a choke problem? Which choke could make an improvement in my case? I have actually a Dagnall choke from a 1959RI amp in my build. The reputation on these irons are...

/G

[Gunner]

Searched the forum for information on wich choke I should choose as replacement, but I'm a little bit confused. Can someone give me an advice here?

I suppose it should be able handle more than 150mA. But higher or lower Henry value? There are several alternativs in the store recommended för 100w amps. What differences can I expect from these chokes?

- Metroamp 352-114 Choke 5H 100mA
- Dagnall C1999 Replica Choke 8H ?mA
- MM 10 Henry Choke 200mA
- MM MAR100 Choke, 3H 250mA

The amp has 50uF everywhere, EL34, B+ around 475V, Dagnall C1998 replica OT.
I want a rather tight SL-sound, not the more brown low filtering 12.000 sound.

Thanks / Gunnar

[Mars Hall]

Searched the forum for information on wich choke I should choose as replacement, but I'm a little bit confused. Can someone give me an advice here?

I suppose it should be able handle more than 150mA. But higher or lower Henry value? There are several alternativs in the store recommended för 100w amps. What differences can I expect from these chokes?

- Metroamp 352-114 Choke 5H 100mA
- Dagnall C1999 Replica Choke 8H ?mA
- MM 10 Henry Choke 200mA
- MM MAR100 Choke, 3H 250mA

The amp has 50uF everywhere, EL34, B+ around 475V, Dagnall C1998 replica OT.
I want a rather tight SL-sound, not the more brown low filtering 12.000 sound.

Thanks / Gunnar

If it is in fact ghosting and it is too annoying to deal with, as most do. You could raise the H value of the choke. It would have tha same affect as raising the filtering. It would tighten up the bass response and give the amp a stiffer feel, while helping eliminate the ghost.

[demonufo]

Standard chokes with 100watters were around 4 to 5 henries, and 100mA rated, which can be barely enough with many 100w amps flat out (50watters used the same chokes, and rarely have issues, but there may be other reasons aside from choke ratings), which is probably why Mercury Magnetics tend to rate theirs at 200mA for most Marshall applications.

I'll second trying to raise the H value of the choke to see if that makes a difference. After all, they're not exactly the most expensive item going. Personally, I'd try the C1999 first, to see what that does.


I've actually got an "as new, never fitted" MM MC10H sitting unused on the shelf here if you want one of those.

[Gunner]

Standard chokes with 100watters were around 4 to 5 henries, and 100mA rated, which can be barely enough with many 100w amps flat out (50watters used the same chokes, and rarely have issues, but there may be other reasons aside from choke ratings), which is probably why Mercury Magnetics tend to rate theirs at 200mA for most Marshall applications.

I'll second trying to raise the H value of the choke to see if that makes a difference. After all, they're not exactly the most expensive item going. Personally, I'd try the C1999 first, to see what that does.


I've actually got an "as new, never fitted" MM MC10H sitting unused on the shelf here if you want one of those.

Great! I suppose we can make a deal on that one. The MM MC10H is worth trying. I'll send you a PM.
/G

[Gunner]

Well, a new MM 10H/200mA choke (thanks demonufo for fast action ) made the amp sound different - tighter bass and less "wild" in the treble freqs. But, the ghosting is still a problem, or at least quite present.

Then I tried extra caps on the first filter caps. Added another 100uF+100uF in parallel with the two cap cans. And finally - that modification almost eliminated the ghost notes!! Not completely, but almost. And the treble freqs sounds much more clean without any influence from ripple. But the amp loses the airy, harmonic sound and feel. It became... boring. So I swapped that caps immediately.

I was a little bit confused since so many topics on this ghosting problems focus on the screen filtering and the choke. No one seems to have raised the importance of the first filter.

Hm, don't know what do to next? Maybe just accept this is a part of the sound in these amps? Or do anyone have other interesting measures to take?

/Gunnar

[demonufo]

Perhaps raising the mains filtering (again) at the same time as lowering the other filter caps maybe?
Some one will chime in sooner or later.

[VintageCharlie]

Though this is an old topic, i thought i'd post anyway. To my ear it sounds great. I hear the low-mids groaning thing going on on the dirty clip, but it is so hard to hear out. Maybe it's more obvious live, but on a recording it's practically non existent - in band context it will be gone. The semi-clean clip has some slightly fizzy overtones, but i think it still sounds great and, afaik., this is what especially el34 amps do - they seem to me to have some fizzy overtones to the high mids. What brand of filter caps did you use for the build? It seems they contribute something to the tone. I've heard good things about rifas. But i doubt that changing the caps to another brand would change the things you want out of the tone - i think it's part of the tone altogether. And what i hear in the overdriven clip is a rich and nice drive - like the tone a lot - don't make yourself crazy because of something that can be barely heard with studio headphones in an isolated recording without band. Would like to hear more clips (not just "test clips") - do you have a link?

All the best,

VintageCharlie

[Gunner]

Thanks for your comments VintageCharlie!

Of course ghosting is not a big problem when playing live. And maybe you can "hide" it in a recording situation. But I'm also sure very few people can hear I'm playing on a hand wired Metro amp instead of a poor 1959 RI... I suppose we are talking about tweaking your amp into pefection here?

Here is the solution I ended up with (actually some small changes)



When I add these extra filter caps I get almost no ghosting at all and no annoying ripple inte the high freqs. I can lower the resistor value to 10R (it gets rather hot with 20R...). And I have put back my 5H choke (I'll also try with a 3H C1999 choke later).

When doing this the amp feels slightly stiffer, not as loose in the bottom and the top freqs. But I can recover that cool desirable feeling without ghosting! Like this: I just lower the filter cap value for the screens and the phase inverter filter cap - just simply remove the paralleling of the cap cans. Then I get 50+50 in series = 25uF for the screens, and 50Uf instead of 100uF for the phase inverter. Still with no increased ghosting!!

The problem is you now get so many parameters to tweak. You get into deep water not knowing if you are swiming in the correct direction...

VintageCharlie - I started with the cap cans Metro supply. I changed them to F&T with no big difference when it comes to ghosting.

/Gunnar

[demonufo]

And I have put back my 5H choke (I'll also try with a 3H C1999 choke later).

I thought you might do that eventually.
Nothing wrong with the Mercury as such, I just think 10H is too high for getting the right feel.

[Gunner]

And I have put back my 5H choke (I'll also try with a 3H C1999 choke later).

I thought you might do that eventually.
Nothing wrong with the Mercury as such, I just think 10H is too high for getting the right feel.

Yes, you were right there. Actually I was surprised how important it is with these low gain amps not filter away all noise since you lose harmonics.

But I'm working with another project that can benefit from the choke I got from you! It is an old cool Johnson amp, 100w 4xEL34. A major make over will convert it to something like a SLO clone, point-to-point...

/G