Amp Making "Whistling/Squeaking/Tea Kettle" Noise


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Lt. Shinysides
02-20-2010, 05:26 PM
This just started up today, but my Vox has been making a very strange sound. It happens when I turn the volume up to about 7:00 and any higher than that, and goes away when I put the volume to 0. It is a noise almost like a squeaky whistling noise, like the noise a tea kettle makes when it is boiling. I cannot tell whether it is coming from the tubes or the speakers because it is a combo. The amp is new (for me) but sat inside the guitar store since 2007, so I have no idea how much use the tubes have on them. Does anyone know what could be making this noise? I'm not sure whether it could be a tube problem, a cable problem (it makes the noise with every cable) or something worse.

EDIT: oh, and the noise does not increase in volume as I turn the volume of the amp up. the noise is always the same volume from when it starts right up to the highest volume on the amp.

Krieger91
02-20-2010, 05:39 PM
i don't know, sorry..i have a vox AC 30..i hope that doesn't happen to mine.
by the way, if you'ce got the same amp as mine and you boughy the pedal (th only one that works with it) how much did you get it for??

gumbilicious
02-20-2010, 05:41 PM
sounds like microphonics(aka strange noises) in the preamp tubes. microphonics can be from unshielded preamp tubes(a little unlikely) or from a preamp tube going bad(more likely). newer preamp tubes don't go through the QA like they did back in the day, so duds aren't uncommon. good news is this is an easy and cheap fix. i'd recommend getting a new preamp tube(or 2 if you really feel like you may get another dud) and replacing an indivdual 12ax-7 with the new one, if the noise doesn't go away then put the old one back in and move to the next one. this will be completely acceptable to do, you aren't exposing yourself to high plate voltages(keep amp unplugged) and no biasing is needed for preamp tubes.

if you go through all the preamp tubes and the noise doesn't go away then i'd be pretty surprised, but it would be tech time then cuz it may be a filter cap(though doesn't sound like what a filter cap does when it goes bad)

Lt. Shinysides
02-20-2010, 06:00 PM
the weird thing is, now that i shut it off and turned it back on it has totally stopped. i dont think its a microphonic tube since its not a "squeal" its almost like its picking up a radio signal

Lt. Shinysides
02-20-2010, 06:01 PM
i don't know, sorry..i have a vox AC 30..i hope that doesn't happen to mine.
by the way, if you'ce got the same amp as mine and you boughy the pedal (th only one that works with it) how much did you get it for??


I very much doubt its the same. Mine is not a custom classic. (there are no pedals, the channels are not switchable)

Shinozoku
02-20-2010, 06:03 PM
You've got a bad preamp tube. Get a chopstick or something and tap on each preamp tube til you find the one that makes the most noise. That's the one you'll have to replace.

Roc8995
02-20-2010, 06:38 PM
Can you reproduce it with the volume all the way down?
If you can, it may be a cap in the power section going bad.
If you can't, it may be a power tube.
A microphonic preamp tube will get quieter as you turn the volume down.

CECamps
02-20-2010, 07:40 PM
What amp is it specifically? And can you try to explain the issue again being as specific and detailed as possible?

311ZOSOVHJH
02-20-2010, 07:48 PM
^Vox AC30 handwired I believe

Can you reproduce it with the volume all the way down?
If you can, it may be a cap in the power section going bad.
If you can't, it may be a power tube.
A microphonic preamp tube will get quieter as you turn the volume down.


This


Shineysides - sorry man :(

CECamps
02-20-2010, 10:55 PM
This sounds like a tube problem from what he's explained. Specifically, it sounds like you may be experiencing some oscillation which can indicate a bad tube.

I'll assume it's an AC30 top boost. I'll also assume you were plugged into either the normal or brilliant input (not the vibrato/tremolo input). In this case, there is only one preamp tube that falls before the volume knobs in the circuit. Therefore any of the other preamp tubes as well as the phase inverter could be suspect.

So I would take a single, known good 12AX7 and test it in each tube position (except the tremolo driver because it is a 12AU7) one at a time to determine if one of your preamp tubes or phase inverter tube is bad.

If that doesn't reveal the problematic component, time to look at the output tubes.

Lt. Shinysides
02-20-2010, 11:38 PM
for everyone wondering it is a Handwired AC30.

I have been completely unable to reproduce this sound again, i've been fiddling for hours, i've tapped all the tubes, they all make the same noise, checked the capacitors for problems and they seem to be okay, i even checked the soldering joints and everything checks out. It has completely stopped making the sound which is even MORE worrying.

I am kind of hoping it's a tube problem though, because it gives me an excuse to stick some NOS bad boys in there.

EDIT: ^ it doesn't have a tremolo input, it has a 2 top boost inputs and 2 normal inputs, and it made the noise on all inputs and even made it when nothing was plugged into it. I'm starting to suspect that it was picking up a radio frequency or something because no matter what i do it wont do it again, I even tried running it on the full power input at 10 (yes my neighbors did come knocking :p: ) and it still wouldn't do it.

stratman_13
02-20-2010, 11:40 PM
What amp is it specifically? And can you try to explain the issue again being as specific and detailed as possible?
It's like an AC30 Heritage edition or something like that. It's all wood (no tolex), I think. It's been a while since I've seen the pictures.

imgooley
02-20-2010, 11:45 PM
the weird thing is, now that i shut it off and turned it back on it has totally stopped. i dont think its a microphonic tube since its not a "squeal" its almost like its picking up a radio signal
I was thinking it was a microphonic tube. Just test it with a pencil tap test.

Other than that, it could be a different microphonic component, bad shielding, or it could be picking up radio frequencies.

Lt. Shinysides
02-20-2010, 11:46 PM
What amp is it specifically? And can you try to explain the issue again being as specific and detailed as possible?

its a Vox AC30H2L (handwired)

what happened earlier today was, I turned the amp up about halfway, plugged into the Top Boost input, and played for a while, eventually I noticed that it was making a very strange, high pitched sound. the best way I can describe this sound would be an electronic "grinding" sound, like a squeak but at varying frequencies, or a tea kettle whistling. I turned the volume totally off (on the amp) and it stopped, but the second i put the volume up to anything audible the noise started up again. the noise did not increase in volume as the amp increased in volume, it was the same the whole way along the volume spectrum.

When I unplugged the cord it still made the exact same noise. I then shut the amp off, let it sit for about 5 minutes, and then turned it back on, and since then it has not made the noise again. It's been running continuously for close to 5 hours now and has not made the noise no matter how hard I push the amp.

I am starting to suspect it may have picked up a radio frequency or something like that because it has never made the noise before, and hasn't since, but it was a very strange noise.

stratman_13
02-20-2010, 11:56 PM
*explanation*
That, as weird as it sounds, sounds almost exactly what my iPod player does in my car. When the car is idling, and I'm using the FM adapter (you know, jacks the radio signal, and plays the iPod on it), everything's normal.

But, when I start accelerating and reach a cruising speed, this really faint whistle comes through the speakers. Not loud, but you can hear it. When I brake, the sound decreases in volume, and once I stop, it goes away again. Cycle repeats, etc.

It's probably just a stray radio signal. My amp picks them up occasionally; we live pretty close to a cell tower/radio tower.

CECamps
02-20-2010, 11:57 PM
I doubt it was a radio station. AC30's typically have 68K grid stopper resistors on all inputs which will easily block radio stations.

It sounds like you had a tube oscillating. If so, you'll likely hear it again at some point.

Lt. Shinysides
02-20-2010, 11:59 PM
I doubt it was a radio station. AC30's typically have 68K grid stopper resistors on all inputs which will easily block radio stations.

It sounds like you had a tube oscillating. If so, you'll likely hear it again at some point.

if that's the case, how do I fix that? just replace the tube? also, how can I tell which tube is the problem? Just do the usual pencil trick?

stratman_13
02-20-2010, 11:59 PM
I doubt it was a radio station. AC30's typically have 68K grid stopper resistors on all inputs which will easily block radio stations.

It sounds like you had a tube oscillating. If so, you'll likely hear it again at some point.
What exactly is tube oscillation? (Noob question alert, but hey, gotta learn sometime.) I know the tubes in my amp rattle a little bit, but my tech said something about tube oscillation after he'd gone over and through it after I bought it.

ethan_hanus
02-21-2010, 12:02 AM
You wernt using a cell phone near it were you, I know my amps goes crazy when there is a cell phone near it, mainly when the cell phone is sending or receiving anything. Either that or you somehow perfectly timed it right and got a 60 cycle feedback hum going by pure chance.

IDK. Microphonic tube sounds more likely, could of got the tube just right causing it to hum.

CECamps
02-21-2010, 12:04 AM
if that's the case, how do I fix that? just replace the tube? also, how can I tell which tube is the problem? Just do the usual pencil trick?

The fix is replacing the tube.

The pencil trick is for microphonic tubes, which is a different problem. Microphonic tubes create a resonant feedback tone which you can typically stop by touching the tube with a pencil eraser or chopstick.

Whenever I'm trying to pinpoint a problem tube, I take a known good tube and try it one at a time in each preamp tube position (and the phase inverter in a push/pull amp) until the problem goes away or gets less prominent.

CECamps
02-21-2010, 12:24 AM
What exactly is tube oscillation? (Noob question alert, but hey, gotta learn sometime.) I know the tubes in my amp rattle a little bit, but my tech said something about tube oscillation after he'd gone over and through it after I bought it.

Describing oscillation within a tube is tough to make simplistic, but I'll give it a shot.

Basically, high frequency oscillation (high-pitched squeal) can occur within a tube based on certain factors even if the circuit the tube is in was designed in such a way to eliminate them (which the AC30 and any other amp is). The grid/input inductance, plate/output inductance, along with other external components and the design of the circuit itself all can contribute to it. It's basically a resonation of a particular frequency.

Normally, high frequency oscillation is controlled with something called a grid stopper resistor which is a resistor placed just before a particular tube's grid (hence the name). Depending on the value of the resistor, it works in conjunction with the tube's Miller capacitance (an inherent capacitance value that the tube exhibits) to create a low-pass filter which rolls off highs at a specific frequency.

When oscillation occurs despite this type of design, it's indicative of faulty components within the tube itself. The normal operational characteristics of the tube change. In this case, my idea was that inter-electrode capacitances within the problem tube may have changed.

Sorry, I know that wasn't very simple. But I hope you get the idea. Boiled down, oscillation is a high-pitched squeal type of noise.

stratman_13
02-21-2010, 12:28 AM
Ha, yeah, I get it. Thanks man :cheers:

oneblackened
02-21-2010, 01:40 AM
Holy crap. I'm not the only person who's had this problem!

I had it with my old VS30 (which I traded in for my Vypyr) - and after lugging (ish) it in for service, it was fine.

Tedward
02-21-2010, 02:24 AM
That, as weird as it sounds, sounds almost exactly what my iPod player does in my car. When the car is idling, and I'm using the FM adapter (you know, jacks the radio signal, and plays the iPod on it), everything's normal.

But, when I start accelerating and reach a cruising speed, this really faint whistle comes through the speakers. Not loud, but you can hear it. When I brake, the sound decreases in volume, and once I stop, it goes away again. Cycle repeats, etc.

It's probably just a stray radio signal. My amp picks them up occasionally; we live pretty close to a cell tower/radio tower.



Mine does that also.


I'm pretty sure the accelerator spinning the gears creates some kind of magnetic shitstorm and causes the device to squeal, like some kind of oscillation.