Troubleshooting something totally bizarre

[faulknersj]

Jimmy, is there a crossover in these cabs? I recently had an issue that sounds almost identical to yours. At a low volume, the sound was totally clean. As I turned up I would gradually reach a volume where the speaker would fart out below an E octave, and sounded fine above that frequency. The culprit ended up being a loose solder connection and after replacing the crossover, walla, the issue disappeared.

 

[JimmyM]

nope, no crossover, just two 10"s and no tweeter.

 

[Jerrold Tiers]

Hah!

I am suspecting that your head does indeed have a problem......

The protection circuit could do that, depending on how they designed it, which I have no idea of, never having seen the circuit.

But, different speakers present different "reactive loads", so if the part of the protection that is supposed to let it drive reactive loads is not quite up to snuff, that *particular* amp might not like that *particular* combo of speakers, but still might drive other pairs, or speakers of same nominal impedance fine.

There are probably other cabs it would "not like", you just don't have them.

Alternately, if one output device out of several was not working right, that might cause a similar problem. But in that case it probably would not be quite so discriminating as to what it liked or disliked...... maybe, maybe not.

The cabs , or one of them, might even be different enough for some reason to cause that particular amp a problem. That pair might cause ANY Micro VR to not like them, where it would drive a good pair fine. And other amps might not be so fussy. I kinda doubt this one, but it still is not "ruled out", I think.

 

[xed]

Just a thought but are you sure that what you're hearing isn't something else virbrating and making sound other than the speaker cones? Next time you plug it up change everything about location. Move it all to another room. If you stacked head and cabs before then don't do it now. If cabs were vertical then set horizontal. Change everything about thephysical setup and see.

There have been a couple of times that i thought i had an amp issue and i really had a freaky odd vibration issue that was just loud and sounded like amp/speaker issue.

Probably the head but you never know.

 

[JimmyM]

xed, i'd be the first one to say that to someone else, but it's definitely coming from the rig. there's no mistaking that speaker snap sound.

jerrold, thx for the reply. since nobody else seems to have this problem but me, from what you're saying, i'm gathering that the protection circuit in the head might be defective, but it also might be one of the cabs that's a little off spec causing it?

it's times like this i wish i hung out in music stores so i'd have a friend who would let me try their floor micro vr rig along with my stuff so i could narrow it down. it's really perplexing and fascinating.

 

[LowB-ing]

Did you try each of the 210 cabs with one of your b15 cabs, just to see if one of them is causing trouble and not the other?

I think someone else proposed this earlier in the thread, but I never saw you mention that you tried it.

 

[xed]

xed, i'd be the first one to say that to someone else, but it's definitely coming from the rig. there's no mistaking that speaker snap sound..

crap, ah well. I had my fingers crossed that it would be something weird but easy for you to get squared away. Sounds like equipment has to be sent back and forth which is always a pain.

 

[JimmyM]

Did you try each of the 210 cabs with one of your b15 cabs, just to see if one of them is causing trouble and not the other?

I think someone else proposed this earlier in the thread, but I never saw you mention that you tried it.

the one thing i didn't do...didn't think it would matter, but now that you put it that way, i'm going to try it right now.

edit: worked with each 210av and a b-15 cab, didnd't work with the 210av's.

 

[lbwdog]

Hey Jimmy.....did you ever say whether the offending sound is coming from both cabs when paired?

 

[JimmyM]

Hey Jimmy.....did you ever say whether the offending sound is coming from both cabs when paired?

it is.

 

[Calaverasgrande]

Jimmy, is there a crossover in these cabs? I recently had an issue that sounds almost identical to yours. At a low volume, the sound was totally clean. As I turned up I would gradually reach a volume where the speaker would fart out below an E octave, and sounded fine above that frequency. The culprit ended up being a loose solder connection and after replacing the crossover, voilà, the issue disappeared.

there I fixed it.

 

[Foz]

have you tried using both cabs with a speaker cord long enough to keep the head remote?

 

[JimmyM]

yeah, foz. definitely not the head rattling in the footcups. so hoped it was, but it's not.

 

[Foz]

ya - was also wondering about emi/rf noise leaking from the head - we ran into this once trying to figure why a train wreck clone guitard amp was so noisy with one particular cab loaded with alnico blues... put head on cab and it squealed like a pig, move head off cab and quiet as a mouse, problem did not replicate on other cabs.

 

[stiles72]

Did you take a peak inside each cab to check and confirm if they are both wired the same and all conenctions are solid - either by removing the jack plate and looking in or by removing a speaker from the front?

 

[okcrum]

...Over to some smart guy to tell you how to futz about sweeping it manually with your meter and a PC tone generator.......

And the smart guys have no help on that

That's because you can't do it with just a signal generator and a meter, unless it's a vector voltmeter that measures phase, too. And then you'd still have to do the plots manually. That's about the minimum you'd need, plus an impedance bridge of some kind. The old school method.

You could put a cheap test rig with new gear together for about the cost of a new SVT head. You'd pay less used for heavier, bigger test equipment.

Real peoples' real lab measurements get made with audio network analyzers and the like, which start around the price of a decent car.

Or, if you only need to measure to about 10-20% accuracy, there are some nice software packages out there for free that will let you do it on a PC with decent sound hardware.

 

[aborgman]

That's because you can't do it with just a signal generator and a meter, unless it's a vector voltmeter that measures phase, too.

All you need if you just want to measure impedance (and not phase shift) is:

signal generator
true RMS AC voltmeter
a non-inductive ~1K resistor
a 4 (8/16) ohm precision non-inductive resistor for calibration.

And then you'd still have to do the plots manually.

Yep, you'll have to plot it manually.

see: http://www.cartchunk.org/audiotopics/ImpedMeasureFAQ.pdf

You could put a cheap test rig with new gear together for about the cost of a new SVT head. You'd pay less used for heavier, bigger test equipment.

Or you could drop $100 on a Dayton Woofer Tester

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Real peoples' real lab measurements get made with audio network analyzers and the like, which start around the price of a decent car.

Or more... but they'll also do a lot more than mere impedance sweeps.

 

[okcrum]

All you need if you just want to measure impedance (and not phase shift) is:

signal generator
true RMS AC voltmeter
a non-inductive ~1K resistor
a 4 (8/16) ohm precision non-inductive resistor for calibration.



Yep, you'll have to plot it manually.

see: http://www.cartchunk.org/audiotopics/ImpedMeasureFAQ.pdf



Or you could drop $100 on a Dayton Woofer Tester

[Invalid or Expired Link Removed]



Or more... but they'll also do a lot more than mere impedance sweeps.

You forgot to mention you'll also need to understand what it is you're doing.

Scalar measurements are always easier. They often don't tell you enough though, especially in weird cases like the one Jimmy's having.

 

[Petebass]

Or you could drop $100 on a Dayton Woofer Tester

[Invalid or Expired Link Removed]



Or more... but they'll also do a lot more than mere impedance sweeps.

I've got one, and yep it's a great little piece of gear. But if it were me I wouldn't want to spend money troubleshooting gear I bought new. The guys at Ampeg can do this, and probably will.

 

[aborgman]

You forgot to mention you'll also need to understand what it is you're doing.

I don't think you'll find too many folks sporting a function generator who aren't capable of understanding that basic little procedure.

Scalar measurements are always easier. They often don't tell you enough though, especially in weird cases like the one Jimmy's having.

Well, since it definitely looks like a combination issue involving head and cab - it's only going to be a hint where to look.