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HF beaming from a combo


Tomm Williams

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I was playing through one of my amps the other day and noticed how tightly it beamed the highs. I have a number of amps and this particular amp does this to a much larger degree than any others. In my quest for a cheap fix (as I like the speaker that's in it) I've read about the "Mitchell Donut" which seems to have a lot of people claiming it fixes this issue quite well. My question is, the speaker in use is an Eminence GB12. Is the design of this speaker known for such behavior ? As stated above, I have numerous other amps with various other speakers (no other GB12's) and I've never encountered this (to this degree) in ANY amp I've ever owned.

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The amp is probably voiced like that. Could also coincide with something in the speaker / enclosure response.

Try another speaker or just obstruct the audio from the center part of the speaker. Attaching something to the cloth might work.

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While you wait for that answer...

What amp? Single speaker? Clean, dirty, all ?

 

If it's a small amp my guess is it's just voiced that way for cut and "clarity" and so it won't (wtf is wo not anyway?) mud out at volume.

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The speaker does have a pretty big peak between 3~5Khz as you can see in the specs here. http://www.parts-express.com/pedocs/specs/290-488--eminence-legend-gb128-specifications.pdf

 

I'd think the issue is either with the guitar you're using (single coil pickups) the amp being used, or possibly the pedal choices. Simple fix is to use an EQ pedal and pull down the peak that's most annoying. Speakers are supposed to project what you feed them, it may be you're feeding them too much highs and not enough mids and lows

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I looked up a few things about this amp. The amp came with a 15" speaker. Maybe it was modified for a 12" speaker. IDK. So it could have been eq-ed for the 15" speaker.

 

http://www.tdpri.com/threads/can-anyone-tell-me-about-the-gretsch-pro-bass-6170.87488/

They're a Valco made amp from the late 60's. I'm a bit dubious on that 35 watt claim, since all Valco made tube amps were cathode biased. Maybe. They used bigger OTs in their latter days, and the Pro Bass was started in '67 at the earliest. The company folded in '69, after being bought out by Kay. After that, the facilities were used to make lesser amps, mostly solid state.

 

The Pro Bass is identical to the last Thunderbolts, the Harmony 420 and the National Thunderball. That is, a great and simple tube amp with a few "quirks." Input stage is stifled to prevent distortion (remember- this 35watt amp was meant for "bass"), and the treble and bass tone controls are nearly useless for the guitar tone spectrum. Stock, it can sound good with a boost, but with a few simple mods, it can be a stellar amp- it is a cathode biased Valco with a 15" speaker! Usually cheap enough that you can do anything you want with it without guilt. Had one that I opened up the input resistance to Fender specs, and modified the tone controls to something a little more usable for guitar- fender brown Treble-Bass, if I remember right. Sounded much healthier.

 

I really know nothing about these gretsch amps, but the net also said they were made by Valco.

 

Gretsch68p21.jpg

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I bought it as a working chassis with no cab or speaker. I built a cab for it closely following the shape of the original with an open back but did not attempt to copy exact dimensions. The previous owner had done some mods to it (of some sort) as it needed some work, I took it to Skips Amps near Sacramento who specialize in vintage tube amps. Skip removed the mods and fixed a couple other things.

He told me he had worked on a number of these and this one had a surprising amount of gain after being returned back to its original form. It is a great sounding amp but a bit different. I have some foam enroute for the donut fix. I'm optimistic it will make the amp behave more towards what I want.

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I bought it as a working chassis with no cab or speaker. I built a cab for it closely following the shape of the original with an open back but did not attempt to copy exact dimensions. The previous owner had done some mods to it (of some sort) as it needed some work, I took it to Skips Amps near Sacramento who specialize in vintage tube amps. Skip removed the mods and fixed a couple other things.

He told me he had worked on a number of these and this one had a surprising amount of gain after being returned back to its original form. It is a great sounding amp but a bit different. I have some foam enroute for the donut fix. I'm optimistic it will make the amp behave more towards what I want.

 

I think you can do it, sounds like a fun little project.

 

http://www.backfromthesixties.co.uk/#!gretsch-pro-bass-6170/bq8ls

 

ad89e3_2da18eb49cba4804811d777bdd8adbcd.png

ad89e3_65e24be5746c403aba6fb52eca8e6ecc.png

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Dispersion is largely a product of the diameter of the speaker. Dispersion starts to beam when the diameter of the speaker cone (or dome) is equal to half the wavelength (http://www.passivecrossovers.com/index.htm). A 6.5" speaker starts to get beamy at around 1KHz so a 12" would start to get beamy at 540Hz or so. A bass has most (not all) of its energy at frequencies lower than 540Hz; the E on a 4-string bass has a fundamental of about 41Hz and the G fretted at the 12th fret has a fundamental of about 196Hz. A guitar doesn't go that low. Between the natural directional properties of a 12" speaker and the peak at 2.5KHz or so that WRGKMC mentioned, it's to be expected that you'll hear a lot of high end when you're "on axis" to the speaker.

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Its a bass amp so its probably not going to break up as easily. The voicing of the amp was likely optimized for bass, not guitar.

What I'd suggest is re-voicing the amp for guitar. This should get rid of the ice pick tones the amp is likely producing and be better suited at coloring strings an octave above bass strings.

 

The treble cap C6 is a .02. I cant hear the amp so I cant tell you which way to go with it. If the tone gets dark really fast and deep with the current one in there try a .01uf instead. If you need more high end rolled off at a lower frequency, try a .05uf cap

 

If you want to cut mostly highs and leave the mids use an orange drop cap. They have a fairly high Q so the frequencies below the cutoff point will be much better.

 

If you want to mess with the bass EQ, there's probably not allot you can do. With bass full up, you have no coloration being added, when you turn the bass down you're sending the signal through a band pass cap. which removes bass frequencies. The question again is weather it rolls too much bass off or not enough. You can increase bass loss by using a smaller C8 cap like a .001. If you want less bass roll off, you increase it to like a .005

 

I'm not sure how far these mods will take you. The amp has no midrange control which is want you really need to take down the peaks on the speaker. As Mikeo said, that amp originally ad a 15" speaker and likely rolled off around 3~4Khz. The amp was voiced to sound right for bass through a bass speaker. When you put in a guitar speaker with a big high frequency bump, its a matter of a bad speaker choice for that head more then anything else. You can try to tweak it but given the fact its a really simple circuit I don't know how effective its going to be.

 

My best suggestion is either get a darker sounding speaker like a Celestion green back which will give you a midrange boost, rework the tone stack and add a midrange control, or just sell it an get something better suited for guitar.

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Interesting the OP states the audio is fine from 6' out; dead on' date=' and that it's the off axis spectrum with the offensive peak(s).[/quote']

No, he doesn't:

. . . Standing 6' out dead center it sounds great but take take a large step to either side and it changes significantly.

He says the sound "changes significantly." He does not say there's a high end peak off axis. Apparently he likes the high end just fine but he doesn't like the way it drops off when he's off axis.

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No, he doesn't:

 

He says the sound "changes significantly." He does not say there's a high end peak off axis. Apparently he likes the high end just fine but he doesn't like the way it drops off when he's off axis.

 

Correct, this amp sounds nearly as good as any other amp I own...... From a limited position. Now of course mic'ing that great sound into a system is easy but hearing it just from the amp is the issue. As I've stated earlier, a simple gaff tape center plug showed immediate improvement so I'm looking forward to what the " Mitchell Donut" may produce.

As has also been noted, this unit was designed and voiced as a bass amp but it sounds very good with my guitars. Maybe it's an exception? Regardless it's a keeper.

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. . . I'm looking forward to what the " Mitchell Donut" may produce.

As has also been noted, this unit was designed and voiced as a bass amp but it sounds very good with my guitars. Maybe it's an exception? Regardless it's a keeper.

It helps that the Legend GB12 is a guitar speaker. Sure, the amp is voiced as a bass amp but the speaker doesn't know that and it's responsible for a large portion of what you actually hear. Some interaction of the amp's voicing and the speaker's response is giving you a sound you like. As long as it's a "keeper" I wouldn't worry about it too much. Something like the "Mitchell Doughnut" (a piece of felt with a hole in the middle) was used back in the 70's by EV to improve dispersion of HiFi tweeters so it could be just the ticket for your amp.

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No, he doesn't:

 

He says the sound "changes significantly." He does not say there's a high end peak off axis. Apparently he likes the high end just fine but he doesn't like the way it drops off when he's off axis.

 

 

 

Correct, this amp sounds nearly as good as any other amp I own...... From a limited position. Now of course mic'ing that great sound into a system is easy but hearing it just from the amp is the issue. As I've stated earlier, a simple gaff tape center plug showed immediate improvement so I'm looking forward to what the " Mitchell Donut" may produce.

As has also been noted, this unit was designed and voiced as a bass amp but it sounds very good with my guitars. Maybe it's an exception? Regardless it's a keeper.

 

 

Missed a step evidently. This in and of itself is not consistent with HF beaming / gaff taped grill.

Nuther thought, since it is a bass amp, the guitar speaker might be over reacting to a presence peak in the amp included for slap/funk clarity? In which case recapping the tone stack as per WRGKMC (WTFDTSF???) might be the answer.

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You can buy some beam blockers from weber and try them. Maybe its all you need. http://www.tedweber.com/wbb12-4 They fit over the speaker as you mount it and are supposed to block the highs.

 

You could probably make your own using a metal strap with a round piece of cardboard glued to it. The strap needs to be thin so it doesn't cause an air gap or bend the speaker frame. If you use something thicker you'd either need to notch the baffle so it fits flush or you could glue a piece of wood across the hole itself.

 

beam-blocker-video.JPG

 

If its a front mount speaker it shouldn't be a problem mounting one like this.

 

2012-09-13_22-41-00_83.jpg

 

 

 

 

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Correct, this amp sounds nearly as good as any other amp I own...... From a limited position. Now of course mic'ing that great sound into a system is easy but hearing it just from the amp is the issue. As I've stated earlier, a simple gaff tape center plug showed immediate improvement so I'm looking forward to what the " Mitchell Donut" may produce.

As has also been noted, this unit was designed and voiced as a bass amp but it sounds very good with my guitars. Maybe it's an exception? Regardless it's a keeper.

 

A few years ago one of my neighbours lost everything, including his guitar and amplifier, in a house fire. I gave him an old strat I had kicking around and a friend gave him one of those Gtresch bass amps. I had to replace the 6L6s but did not do anything else to it and it sounded just fine with the guitar.

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You can buy some beam blockers from weber and try them. Maybe its all you need. http://www.tedweber.com/wbb12-4 They fit over the speaker as you mount it and are supposed to block the highs. . . .

The "Mitchell Doughnut" is a simple, cheap DIY mod. As I mentioned previously, EV was using something similar, except made of felt, back in the 70's with HiFi Tweeters. Here's a pic of one: [ATTACH=CONFIG]n31738639[/ATTACH]

 

I'd be inclined to try it first and leave the "beam blocker" for later.

 

A few years ago one of my neighbours lost everything' date=' including his guitar and amplifier, in a house fire. I gave him an old strat I had kicking around and a friend gave him one of those Gtresch bass amps. I had to replace the 6L6s but did not do anything else to it and it sounded just fine with the guitar.[/quote']

Yeah, it matters a lot less what it says on the front of the amp than what comes out of it. The Fender "Bassman" comes to mind.

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I'd be worried about using foam like that on the inside against the speaker. The stuff is too flexible and would eventually sag and rub against the speaker cone.

 

I'm not sure its the best fix either because the treble beaming comes from the center of the cone and those leave the center open. I'd think it might make the issue even worse. Cost isn't a factor with either however. Either method will only cost a few dollars and you can even make your own for practically nothing. Having all that foam is going to blanket all the good frequencies you actually want to retain too.

Its like using one of those foam wind screens on a mic, it cuts wind noise but at a big cost.

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I'll make no claim of being an Acoustical Engineer here. The inventor of this device apparently has a substantial background in such matters and there is an exhaustive explanation of why it works on TGP. In regards to it rubbing on the speaker, he suggests attaching the ring to the grill cloth with light adhesive so it shouldn't be touching the speaker.

Regardless, I'm not against replacing the speaker if need be. I was looking for a cheap fix as the GB12 sounds quite good but perhaps it is beamy by design. I should have the foam hopefully today and I'll see what happens.

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I suspect the folks at EV knew what they were doing. The foam/felt/etc. won't block low frequencies and other "good frequencies" any more than a normal speaker grille but it will block highs. The hole forms an aperture that effectively reduces the diameter of the speaker so that only the center is radiating and a smaller speaker has better dispersion.

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