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Whistling Vox...


Teahead

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I've not had a great afternoon. I returned from a few days away and decided to jam with the 'tongue. My Roger Mayer Voodoo Vibe didn't want to play... :evil: but I could live without it and we had a great session.

 

Then I get home and try to test the 'Vibe to see if I could get a good description of the problem before I contact Roger. During this I noticed a nasty little "whistle" occurring behind certain notes coming from my amp. I plugged straight guitar and amp, trying both guitars and a couple of leads, the problem is definitely the amp. It's present on all channels too.

 

I'm thinking it's a tube, because it sounds like the kind if noise that a blown light bulb would make when shaken. It tinkles and whistles with a consistent tone, but isn't present on every note, only for around four frets at a time as you climb the fretboard, then it disappears, only to return a few frets later. :mad:

 

I have never had this trouble in practice, but I do remember encountering a similar problem once when playing at home. I'm hoping that someone can point me in the direction of which valves I should replace first to try and fix it. I'm presuming either Power, PI or rectifier since it's common to all channels, any idea how to narrow it down?

 

Thanks.

 

Tea.

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It seems input level related, billster. The "whistle" blooms as the note does, but only on specific notes(!)

 

I've heard that the rectifiers are the most unreliable piece of the amp and mine still sports the stock one, which is around six months old and had seen moderate use.

 

I have changed the output and preamp valves, but not the rectifier, so I may have strained it in doing so. Thanks for your help...

 

Tea.

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The fact that it varies with what notes you play indicates that sympethetic vibrations are causing a tube (or possibly another component, but most likely a tube) to make the noise.

 

(For what it's worth, try both of your guitars, just in case there's something in the one that recently got rewired...)

 

With the amp on and the gain and/or volume up, and nothing plugged in, take a chopstick or a pencil or similar non-conductive probe, and lightly tap on each tube, trying to get each one to make the noise. Check all of the tubes, even though you noted that it's common to all channels; if a tube is sympathetically "excited" into making the noise, it may be possible for it's oscillation to go "downstream" regardless of what input your guitar is plugged into. If neccessary, repeat the procedure with the guitar plugged in, as well. Chances are, one or more tubes will sing out and 'fess-up.

Ask yourself- What Would Ren and Stimpy Do?

 

~ Caevan James-Michael Miller-O'Shite ~

_ ___ _ Leprechaun, Esquire _ ___ _

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I tried both guitars, the LP actually increased the problem becasue it cause more of a "booming" sound. "Sympathetic Ringing" does sound very likely as it blooms and dies naturally.

 

I may have trouble poking the tubes, I'll have to remove the chasis to get at them and then plug the amp in to check for the ringer. I'll leave it until tomorrow because it's a lengthy process and I should have all my wits about me before I attempt it. Thanks for the advice!

 

Tea. :thu:

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The "send" and "recovery" tubes for the reverb will do this, too, so check ALL of the tubes.

 

Most likely, it is one of the nine-pin miniature double-triodes in the preamp or function section, or the "PI".

 

Sometimes a bad input-jack can do this, but since you have noted that it doesn't depend on which input you're plugged into, it is more likely a tube.

 

To check the inputs, insert a dummy plug; you can make one by connecting the two leads on any 1/4" phone (instrument) Tip/Sleave mono plug, or taking a mono patch-cord and temporarily jumpering the tip and sleeve of one end somehow. (This works best if the "whistle" will sustain on its own). If the "whistle" is eliminated completely when the dummy plug is inserted, or agitated into recurring when the dummy-plug is wiggled while plugged-in, the input jack is faulty.

 

Whatever happens, let us know what's up!

Ask yourself- What Would Ren and Stimpy Do?

 

~ Caevan James-Michael Miller-O'Shite ~

_ ___ _ Leprechaun, Esquire _ ___ _

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I don't have any 'verb! Which is a good thing, considering I have eleven tubes to worry about already(!)

 

I've been wondering why I didn't encounter this in practice, only in my spare room? It sounded better than ever today at full throttle, then I find this at home volume. I do lift the amp off the floor in pracice, but not at home, influential?

 

Tea.

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Weather, voodoo, sunspots, psychic convergence with ghosts and U.F.O.s...

 

If it's not a tube or component itself, it can also be a loose pin contact conection or bad solder-joint; both susceptible to movement, temperature, and sympathetic vibration. But I'm still betting that it's a tube.

Ask yourself- What Would Ren and Stimpy Do?

 

~ Caevan James-Michael Miller-O'Shite ~

_ ___ _ Leprechaun, Esquire _ ___ _

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Well I opened the amp, to discover a grey resistor(?) rattling around inside. It's one of two identical ones and it has obviously been a rushed solder job that has loosened and eventually came away. http://www.voxshowroom.com/northcoast/images/products/new_amplifiers/ac30service/backofamp.jpg

 

In this picture, they are towards the bottom right of the board as it appears, parallel to the bass and treble pots of the TB channel, which are the second and third from the left.

 

Printed on the board as R70 and R71. It's the R71 that has came off. I've checked the shematic that Myles has available and I'm presuming these to be resistors that control the tone of the TB channel?

 

Would this account for the noise I was hearing? Certainly it could have been ringing against the metal chasis. Could I have damaged the amp by using it like this? Is it simply a matter of having a competent person solder this back on? Thanks for your help...

 

Tea.

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How long have you had that amp? I can't remember, did you buy it new, or used?

 

If there's a warranty still in effect, have it taken care of care of your (authorized) Vox dealer. Express your concerns about the amp being given a good once-over, literally scoped-out, too, instead of just resoldering that resisitor. Better safe than sorry, right?

 

That probably is the cause of the "whistling"; radical changes in the resistor values in some preamp-section applications can cause such noises, and I'd say that going from it's resistive value to infinite resistance as the leads went in and out of contact with the PCB's pad would count as a radical enough change!

 

If it isn't still under any warranty, having a competent person put R71 right again would be a good (and fairly simple) start, but I'd want the whole amp visually inspected and checked out by someone who is familiar with these PCB Vox AC30TB amps, just for good measure. Again, better safe than sorry, right? (It probably didn't hurt anything else, but, it's a good idea to check things out now, as a matter of course. It cou;dn't hurt, but it could help!)

Ask yourself- What Would Ren and Stimpy Do?

 

~ Caevan James-Michael Miller-O'Shite ~

_ ___ _ Leprechaun, Esquire _ ___ _

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Kevin, you're absolutely right in all you say. I bought the amp new, just about four months ago, so it's certainly under warranty. I hope they don't insist this component has been disturbed through "careless use?" You know the old, "Customer Damage" routine? :rolleyes:

 

I try not to have an image of the laziest guy in the shop putting my amp together, it could have been the best workman, or woman, who just missed one little thing. The optomist in me will consider the latter foremost in my mind.

 

The amp has performed well, it sounds great and while I know they have a reputation for 'reliabilty' issues (i.e they break often!) I have no other cause for complaint. I've posted for advice on the Vox Forum at PP, so hopefully someone will have specific and first-hand knowledge of what I may or may not have damaged.

 

If it is, as I suspect, one of the TB tone controls, then I may be able take some comfort from the fact that I haven't used it for some time. We'll see, but for now, I'll get in touch with Korg UK in the morning, they seem pretty approachable judging by the contact info on their site, so I hope for an authorised repair centre where I can get the amp thoroughly checked as you so wisely emphasised.

 

I've been consoling myself, luckily my brother Gareth left his 10w practice amp here yesterday, with a line of Keeleys in front it yields a pretty good home tone! I'll go and pour out some blues... :P

"Woke up this morning, lost all faith in my Vox!" :(

 

Tea. ;)

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Originally posted by Teahead:

"I try not to have an image of the laziest guy in the shop putting my amp together, it could have been the best workman, or woman, who just missed one little thing. The optimist in me will consider the latter foremost in my mind."

Chances are, the printed circuit board was loaded-up with its components and then sent through what is called a wave solder machine. There, it is conveyed over a pool of temperature-controlled molten solder, close enough to the surface that the component-leads dipped into the solder. Then the solder wicks up the leads and onto the pads, usually forming very good extremely, consistantly uniform solder-joints.

 

If a component is cocked or otherwise poorly oriented on the board, or if one or both of its leads are cut too short, a poor solder-joint can result. That should then be caught in post-wave inspection/touch-up, but, then, no-one is perfect, right?

 

Further chances are, whether you used it or not, nothing very dangerous or worrisome would have happened, as long as the component didn't roll around and physically short some other circuits togther. Going intermittent like that was more like switching back and forth between "fine" and "bad resistor choice, that; sounds lousy and squeals like Beatlemania".

 

http://www.musicplayer.com/ubb/icons/icon3.gifhttp://www.musicplayer.com/ubb/icons/icon2.gif By the way- what are the colors of the stripes on that resistor? That will tell me its value; if you can send me or post the schematics, I can get a good idea of what it's for, or at least what section of the amp it's a part of.

 

"I've been consoling myself, luckily my brother Gareth left his 10w practice amp here yesterday..."

Gosh, I wonder where he got that from... ! ;):D:thu:

Ask yourself- What Would Ren and Stimpy Do?

 

~ Caevan James-Michael Miller-O'Shite ~

_ ___ _ Leprechaun, Esquire _ ___ _

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Your post explains the difference between the original PTP design and the PCBs of the reissue, I presume? The originals and the Handwired Edition don't seem to have the boards inside, again I'm assuming, but is this what accounts for a large percent of the extra cost?

 

The labour involved, should the AC30 be built without PCB boards, would obviously be too costly for a $2000+ price tag, eh? That comes with the twice as expensive Limited Edition! Oh well, at least for $2000+ I'll get a darn good PCB won't I? :eek:

 

There is nothing near the liberated resistor, that could have possibly disturbed it. So your "bad solder gets missed in QC" headline sounds like front page news thus far. Thanks for your help here Kevin, it's reassurance, at the very least, to know your knowledge is at work. I'm such a bad "parent" to my amp, it's hard to feel deserving when it's reduced to a puzzle of wires, resistors and...erm, whatever those other things are... that aren't tubes! :rolleyes::D

 

The resistor is coloured... Brown-Black-Brown-Gold... and on it's sibling, R70, the Gold stripe is nearest the top of the amp, if that helps any? The schematics I looked at were on the site of our honourable moderator, but on second looks, I'm not even certain it was specific to the Korg RI because ot seemed to be a JMI schematic from the Sixties. :evil: Cloink here and then I scrolled down to the schematics links.

 

I'll phone Korg first thing in the morning, after I phone the Citizens Advice folks to find out exactly what my entitlements are. I've only had one reply on the Vox boards, urging me to return it for warranty repair. Not because of specific knowledge, more to enforce the terms of the warranty and have them acknowledge responsibilty, which makes sense.

 

I'm hesitant to lose my amp for weeks and pay huge shipping, only to have Korg solder R71 back in, but this is my baby here and I want it to get the very best care available. I would like the piece of mind that comes from hearing a qualified technician say my amp is 110%

 

The irony of Myles' last post, touting Jimmy Wiggles replacement boards, is far from lost on me. I recall posting on that thread, "I don't know why I need it, but I do!" :freak:

 

Tea.

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That would be a 100-Ohm resistor, with a +/- ("plus-or-minus") tolerance of 5%; that is, its actual resistance could range anywhere from 95-ohms to 105-ohms if measured. (Many old vintage amps used components with a tolerance of +/- 20%, so don't feel let down!)

 

(1st)Brown = 1, (2nd)Black = 0, (3rd)Brown = multiplier of 10

 

-So-

 

"1" & "0" make "10", multiplied by "10" equals a value of "100", 100-Ohms.

 

The color of that fourth band indicates the "tolerance".

 

Judging by its size in the photo, I'd guess it to be a two-watt, though it's hard to be certain, it might be a one-watt. (This just refers to the heat capacity of the resistor.)

 

Its orientation doesn't matter, that is, they're not polarity specific, they "do go both ways", as the scarecrow in the Wizard of Oz said. So it doesn't mean anything which way the gold-band lays.

 

Hopefully you can find someone who's within driving distance! Either way, see if you can ship just the chassis, minus the cabinet and speakers.

 

At the very least, if you really don't want to part with it or ship it, it could likely just be correctly soldered in place, and be a done-deal.

 

Be prepared for the ironic voodoo mojo mystery and twisted sense of humor inherant in tube-amps... don't be too terribly surprised if it's still a tube that was causing the "whistle"! If so, be thankful that it just wanted to draw your attention to that loose resistor there...

 

Oh, yeah... it's not "wiggle"... quoting Myles, "By the way ... Jimmy Wigle is not "wiggle" it is

pronounced WYgle, sort of like Zakk Wylde."

Ask yourself- What Would Ren and Stimpy Do?

 

~ Caevan James-Michael Miller-O'Shite ~

_ ___ _ Leprechaun, Esquire _ ___ _

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Ooops! I can never stop at just one "G!" ;) Thanks for the heads-up Kevin, I could probably do with staying on Jimmys good side, considering my predicament! :rolleyes: This from the Vox PP Forum...

 

According to the Bill of materials for the Korg AC30TB, R70 and R71 are two parallel 100ohm /5W resistors that are the cathode biasing for the el84's.

 

It may be that one has been purposely lifted to adjust the current down. In recent AC30's I've worked on I've found that 100 ohms alone was just about the right value to get the original JMI spec of 125mA total standby current. The amps sound really nice when setup like this, and run cooler and safer, especially if you're using EI's on the output. So this may not be such a bad thing to leave alone.

 

I'd have a look in the preamp section for this problem. Try interchanging the front end, PI, and topboost 12ax7's with the Vib/Trem oscillator 12ax7 one at a time and see if you can pin it down that way. You can put the microphonic one back in the oscillator spot and it should be fine there.

 

Recent experience has also taught me to check the internal rear frame of the speakers for foreign objects. Don't go throwing small things into the back of the amp for storage. They could land inside the speaker frame and vibrate at certain frequencies. Leading you on a wild goose chase.

 

Quickly followed by...

 

You wouldn't leave the resistor loose if you were solely using one 100ohm wirewound - I say send it back, it 'may' be a simple solder job, but there could also be an underlying problem that caused this which could cost more.

 

Also, if you argue the case, you should pay absolutely nothing for shipping when sending an amp for warranty repair, either take it to the shop where you bought it or get them to arrange for a courier to pick it up from you house. Mackie did this when one of my SR1530s went down.

 

I don't know if any of the information is trustworthy and as the posts seem to contradict one another, I'm back to square one. I think I'll copy the facts and considered opinion to post on the "Ask Myles" thread, I'd feel better knowing a little more of Myles and I don't doubt he would be well enough informed to shed some light.

 

Tea.

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Yeah, Myles seems to be pretty up on Vox and Marshall amps and all their incarnations. He'll know the score!

 

You may want to copy 'n' paste the entirety of those Vox Forum posts over to Myles' thread- or at least, the pertinent parts- and ask him what he thinks of those statements (just be sure to ID them clearly as being from an unofficial forum, at first read I mistook them as coming from "Vox" and thought they were awfully informal...).

 

I wondered if that was a cathode resistor for the output section, as it is a larger one that would stand the extra heat there; but PCBs can have some seemingly squirrely component layouts that don't always put parts where you'd expect to find 'em, and there are a number of 100-ohm resistors in a Vox AC30 schematic!

 

Chances are, you didn't hurt anything by running the amp that way, as long as the wayward resistor didn't cross wires elsewhere (and that would have "let the smoke out", and you'd have known); although, I'd want to know if there were any other surprises laying in wait.

 

Again- this might not have been the cause of the noise you were experiencing in the first place. I think an exceptionally friendly pooka may have been toying with you there...

Ask yourself- What Would Ren and Stimpy Do?

 

~ Caevan James-Michael Miller-O'Shite ~

_ ___ _ Leprechaun, Esquire _ ___ _

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I agree Kevin, at least I'm trying to look on it that way. I left the amp with my local tech at lunch time, he's not up to date with the Korg RI as he expected the resistor to be one of the old value, having seen his fair share of JMI Originals.

 

He's going to put the resistor back on, he seemed shocked at the "clean" break it had suffered. Even asking me if I had snapped it off whilst poking around! I explained the noise that I'd experienced and he said he fully expects the resistor to be the cause, but that nothing can be guaranteed and we'll know for sure in the morning.

 

He's going to test it as extensively as he can, I've come to trust him in the last year so I'm confident in both his abilities and his honesty. He told me of a customer who in 1985 brought his AC30 to him for it's first service since he had bought it new in the late '60s!

 

Apparently he lives quite local and still owns the amp, which is in mint condition having never been gigged. Maybe one day I'll get to compare my RI to the famed original?

 

I posted what I thought to be the relevant information on the "Ask Myles" thread, you don't happen to notice any glaringly obvious details that I've excluded do you? I think I covered it.

 

Tea. :thu:

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I feel good about my calls now...

 

Originally posted by myles111:

"No problems other than some interesting tube wear ....

 

Just have a tech put the resistor back. I guess flow soldering does have it's own list of issues."

As for the "clean break", it was probably very tenuously connected by a mere breath of solder, perhaps on one lead and pad alone. It wouldn't have taken much to fracture it at all.

Ask yourself- What Would Ren and Stimpy Do?

 

~ Caevan James-Michael Miller-O'Shite ~

_ ___ _ Leprechaun, Esquire _ ___ _

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Check out the very latest issue of MIX magazine...

 

...Eddie Ciletti is repairing a VOX AC30 for Bob Dylan, I beleive, that seems to have a similar problem as yours...some sort of frequency specific noise.

 

I don't think he has it solved yet in the article...but it might give you some clues...and you might even send him an email about it.

miroslav - miroslavmusic.com

 

"Just because it happened to you, it doesn't mean it's important."

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Whoa, that is quite a coincidence!

 

I'm glad to have been wrong in suspecting other reasons for the whistling. Cool!

 

Strange that both amps had that bad soldering and all on the cathode resistors! I wonder what the cause was? Perhaps the way the boards were clamped onto a rack for transport through the wave solder bath? I can see that easily happening... although, the "weak legs" poses the image of a poorly prepped/trimmed component, or maybe a clamp squishing it down and an assembler merely coaxing it back up by bending the leads; hmmmnn...

Ask yourself- What Would Ren and Stimpy Do?

 

~ Caevan James-Michael Miller-O'Shite ~

_ ___ _ Leprechaun, Esquire _ ___ _

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