Can I get rid of ghost notes?

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Can I get rid of ghost notes?

Postby boola on Thu Apr 14, 2011 8:51 am

Hi,

I recently changed the filter caps in my 100w super lead and it hasn't changed the ghost noting one little bit. I get ghost notes around the 12th fret on the D string.

I haven't changed the bias filter caps, could changing these help or is there anything else that I can try?

Thanks
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Re: Can I get rid of ghost notes?

Postby thornybank on Thu Apr 14, 2011 10:48 am

Old Gear Page thing....

When its bad and difficult to live with, its usually appears to be the most exaggerated on the D, G and B string above the 10th/ 12 fret, tho it can crop up about anywhere. Ghost notes caused by inadequate power supply filtering usually have a freq at around 120hz modulating the signal (60hz ripple in the supply supply X 2 based on full wave rectification = 120hz) and creating the ghosting. If youre in Europe it would be a 100hz freq since the standard is 50hz in the supply. Its also somewhat related to something called "intermodulation distortion" which occurs in all tube driven amplifiers being pushed hard at higher volumes, but is not nearly as noticeable as the out of tune ghost notes created by an underfiltered power supply (also more evdident when the amp is being pushed toward the max end of its output.)

A poorly designed bias circuit can be an ingredient in the ghosting problem mix also, for the same reason (being underfiltered and allowing ripple in the supply to pass.) There are also proponents of bad grounding and lead dress, the way wires are routed and components are situated, adding to the problem with an underfiltered power supply. In some amps with triode/pentode switching, the problem can be so pronounced in the triode mode thats almost or is, unuseable without more filtering.

In essence, in some amps it can be tolerable and be considered part of the overdrive/distortion mystique....some players tend to live with it better than others while its waaay annoying to others depending on a number of variables. If its a problem, as said...it can usually be helped with added filtering.
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Re: Can I get rid of ghost notes?

Postby demonufo on Thu Apr 14, 2011 9:58 pm

There is also the possibility that the choke isn't up to the job...
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Re: Can I get rid of ghost notes?

Postby JimSS on Fri Apr 15, 2011 3:18 am

I've had this problem with all my 100 watt "plexi" heads, some vintage and other home brew. When I installed a PPIMV and run the master at 3/4 ghosting disappears and retains all the goodness. My 50 watt '71 heads do not seem to have this problem with ghosting
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Re: Can I get rid of ghost notes?

Postby boola on Fri Apr 15, 2011 6:45 am

JimSS, that's interesting I was wondering if 50W models might not ghost as much as 100s.
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Re: Can I get rid of ghost notes?

Postby geetarpicker on Fri Apr 15, 2011 8:35 am

Years ago I put my '68 superlead power supply on a scope and it was obvious how much ripple occurs when the amp is pushed hard. There was a time during the 80s where I modded my '68 Superlead 100 with well over 2000uf (yes not 200uf, but 2000uf!) of power supply caps! I used HUGE computer grade Mallorys and believe it or not the tone wasn't changed that much but the ghosting was totally gone. That said it just about filled the topside of the chassis near the preamp as each cap was about 6" x 2" and I used 4 of them stack up on their sides. The caps were so big the amp was finally almost balanced on the carrying handle! These days the caps alone would cost hundreds, and I wouldn't even considering such a mod on a vintage amp. These days I live with stock filtering in my current plexi, a '68 superbass. Over the years I've definately noticed that 100 watters all ghost to some extent, and seem to ghost more than 50 watters. IMHO it's something you have to learn to live with. One minor mod (if it really bothers you) that makes a modest difference is to simply double up the capacitance on the first two filter caps by simply putting an additional pair in parallel. With a couple brackets you could add these internally with only needing to drill (horrors, sorry) a couple small mounting clamp holes.
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Re: Can I get rid of ghost notes?

Postby 5 Trumpets on Fri Apr 15, 2011 3:49 pm

In my experience increased filtering did the trick on my 66 BFSR. In the case of my Marshalls, what I heard playing by myself did not translate when I played with a band. :wink:
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Re: Can I get rid of ghost notes?

Postby cerrem on Fri Apr 15, 2011 9:57 pm

A little caution should be taken when increasing the filter cap vaules, especially the first set.....
The peak currents will go up and can damage the diodes unless replaced with something bigger such as 1N5408 types...
ALso...the power transformer winding will heat up more from these peak currents....not really a good idea...
The ghosting is due to the screen filtering circuit not the plate filtering...Marshall wired up the screens incorrectly and it has been "monkey-see-monkey-do" ever since...
Marshall also used the same size choke and caps on the 50W as well as the 100W ...not really the best idea...
Since the bias circuit "tilts" in one direction when the amp is cranked....this will put the output stage also tilted in one direction and cause a big imbalance in currents...This also makes the ghosting worse....It also tend to make one side of tubes have slight red-plating....The cause of this is mismatched time-constants on each side of the phase-inverter....the PI is fine until the blocking distortion kicks in....

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Re: Can I get rid of ghost notes?

Postby boola on Mon Apr 18, 2011 8:05 am

Thanks guys, some great information in this thread. geetarpicker, that is a funny story and just the kind of crazy thing I would try but my amp is unmolested so I couldn't bring myself to do that.

So the consencus is that ghost notes happen and there aint too much that can be done about it....

As a side-note, do you guys think I should change the bias filter caps? Not to get rid of the ghost notes, but are they something that should be changed like the big filter caps?
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Re: Can I get rid of ghost notes?

Postby 5 Trumpets on Mon Apr 18, 2011 11:48 am

It's a good idea to change any electrolytics.
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Re: Can I get rid of ghost notes?

Postby bwplexi on Tue Apr 19, 2011 12:50 am

boola wrote:So the consencus is that ghost notes happen and there aint too much that can be done about it....

Increasing the mains filtering will usually kill ghosting but the amp will "feel" stiffer and less musical. At one point I increased mains to 200uF and it fixed ghosting but the feel/tone sucked.

As someone mentioned, using a PPIMV set around 75% or less will usually remove ghosting also. If you don't want an MV you could, in theory, use large grid stoppers on the power tubes to simulate an MV set at about 75%. 47k to 56k would probably work but may reduce some treble also. I haven't tried this because I use a PPIMV.

A stiffer choke may help also.
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Re: Can I get rid of ghost notes?

Postby zaphod_phil on Tue Apr 19, 2011 3:33 am

bwplexi wrote:A stiffer choke may help also.

Or a bigger filter cap feeding the screen resistors. That will help suppress ghost notes, while not significantly increasing rectifier peak current.
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Re: Can I get rid of ghost notes?

Postby Ogoki on Wed Apr 20, 2011 5:32 pm

After a lot of experimenting, I have accepted that ghost notes are really a fact of life with these amps.

A while back I messed around with a few stiffer chokes from Hammond. They really didn't do much for the ghosting and gave the amp an unfocused feel which made me pull them right away. I ended up with the original choke back in place and extra holes in my chassis.

I often go with 1.5k or 2k screen grid resistors, which should limit current to the screens a little. Bumping the screens filters to a pair of 60 or 68uF can also help a little and retain a pretty stock feel. Again, these tweaks only help a little with ghosting...

Chris brings up a really important point about one side of the PI driving harder than the other. I have started paying a lot of attention to this after witnessing some xf2s redplating at very modest biasing settings under hard playing. I played around with various power tubes and always got redplating on the two power tubes driven by the 82k side of the PI. (Believe or not, the RFTs wouldn't replate when Mullard, Amperex and Roznov Tesla EL34s would!) In short, the plate load is too low and the voltages are too high. Simply running a one-step upward mismatch (i.e. amp on 8ohms with a 16ohm cab) should make things more bearable for the power tubes. Raising the value of the PI tail resistor (e.g. from 10k to 27k) also helps. And, run your power tubes which draw less current in the two slots closest to the preamp.
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Re: Can I get rid of ghost notes?

Postby boola on Thu Apr 21, 2011 3:21 pm

Ogoki wrote:After a lot of experimenting, I have accepted that ghost notes are really a fact of life with these amps.

A while back I messed around with a few stiffer chokes from Hammond. They really didn't do much for the ghosting and gave the amp an unfocused feel which made me pull them right away. I ended up with the original choke back in place and extra holes in my chassis.

I often go with 1.5k or 2k screen grid resistors, which should limit current to the screens a little. Bumping the screens filters to a pair of 60 or 68uF can also help a little and retain a pretty stock feel. Again, these tweaks only help a little with ghosting...

Chris brings up a really important point about one side of the PI driving harder than the other. I have started paying a lot of attention to this after witnessing some xf2s redplating at very modest biasing settings under hard playing. I played around with various power tubes and always got redplating on the two power tubes driven by the 82k side of the PI. (Believe or not, the RFTs wouldn't replate when Mullard, Amperex and Roznov Tesla EL34s would!) In short, the plate load is too low and the voltages are too high. Simply running a one-step upward mismatch (i.e. amp on 8ohms with a 16ohm cab) should make things more bearable for the power tubes. Raising the value of the PI tail resistor (e.g. from 10k to 27k) also helps. And, run your power tubes which draw less current in the two slots closest to the preamp.


OK please bear with me as a bit of a novice, I REALLY appreciate all this help.

I have seen exactly this kind of redplating with modest bias settings (I'm also using Mullard el34s) I have always felt with this amp as though the el34s are getting too much current and are breaking up way to early on the volume control. I would prefer to have more PI breakup than el34 breakup at lower volume settings.

Ogoki, I don't understand the function of the PI tail resistor. Does that essentially reduce the signal current going to the power tubes?

Increasing the size of the screen grid resistors sounds like it will do exactly what I want, reduce the dissipation of the el34s, reducing the amount they can breakup while also helping with ghost notes. Am I understanding that correctly?

Before I get the iron out, I will definitely also try switching the amp to 4ohm (I am using 2 4x12s)
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Re: Can I get rid of ghost notes?

Postby Ogoki on Thu Apr 21, 2011 3:44 pm

Raising the PI tail resistor will diminish headroom in the PI - i.e. more breakup there and less signal hitting the power tubes. It should also help to make the PI a little more "symmetrical". Are you indeed experiencing the redplating only on the two power tubes closest to the preamp tubes, like I was?

There is some great info on these issues here: http://forum.metroamp.com/viewtopic.php ... 6&start=75

Increasing the screen grid resistors will limit current to the screens, which is the weak spot on these EL34s and also the culprit when it comes to ghosting. It won't cure ghosting by any means, but should help, while contributing to tube longevity.

Running an upward "mismatch" is definitely the first thing to do, in my opinion. It's really not much more of a mismatch than running a "matched" load (and is a lot easier on your power tubes), as a lot of mistakes went into the "designing" of these amps, due to poor comprehension of tube data sheets, amongst other things....
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