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Thread: FBT 200 - strange noise

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    FBT 200 - strange noise

    Hi all,

    I'm new here, and not a super tech myself, but I have been facing a weird issue on my FBT200.
    It's an italian from the 60s, it sounds really great. It has been totally serviced a while ago and works really perfectly except that I have very strange vibration noises on certain notes.

    Hear by yourself here :
    https://www.dropbox.com/s/c9yah7hz5k9bwva/FBT.mp3?dl=0
    https://www.dropbox.com/s/05gymz8p8l...FBT-2.mp3?dl=0

    Here is what we tested (me and the tech who worked on the amp, and who is not a newbie) and everything was unsuccessful.

    - changing the speaker -> no influence / the speaker is fine when tested on another amp and putting another speaker in the combo doesn't fix the problem
    - Reinforce the base of the tubes
    - Change all tubes one by one

    Last but not least, the vibrating noise appears after 5 min or so, when amp is warm. There is no weird noise when it's powered on (although I can play), it only appears after a few minutes and stays forever. This is rather distrurbing.

    I have to say I'm not sure what we should/could test next. The sound really seems mechanical, and again, all the circuit and condos / capacitors have been changed, it's brand new inside.

    Any suggestion, help, idea, would be very helpful.

    Thanks in advance for your help guys,

    Best,

    Pam.

    (I'm in Paris, France, but can't update location on my profile / insuffiscient privileges)

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    I had an issue like this on a (almost new) Fender Hot Rod that Fender could not figure out where the problem was. It turned out to be a crack in the cabinet under the amp chassis where it was hard to see, We took the whole thing apart before we found it. It was quite two hour saga.

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    Echoing nickb's suggestion, since you know that the speaker is good I would run the amplifier output to an external cabinet. If the problem goes away when the vibration is isolated from the amp the you know it is mechanical.

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    Quote Originally Posted by glebert View Post
    Echoing nickb's suggestion, since you know that the speaker is good I would run the amplifier output to an external cabinet. If the problem goes away when the vibration is isolated from the amp the you know it is mechanical.
    Hi guys, thanks or the prompt answers. Yes, when the amp is plugged into an ext cabinet, no vibration whatsoever. When another speaker remplaces the current one, vibration comes back.
    So it's somehow mechnical, for sure and linked to the combo itsel. The weirdness is that it starts after 5 min, when the amp is warm. If it was a crack in the cabinet, wouldn't it be immediately perceptible?

    Nickb, I understand you opened up the whole fender amp apart until you found out? once it was visible, how did you fix it?

    Tks for your answers. Really helps.

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    Quote Originally Posted by pamelahute View Post
    Hi guys, thanks or the prompt answers. Yes, when the amp is plugged into an ext cabinet, no vibration whatsoever. When another speaker remplaces the current one, vibration comes back.
    So it's somehow mechnical, for sure and linked to the combo itsel. The weirdness is that it starts after 5 min, when the amp is warm. If it was a crack in the cabinet, wouldn't it be immediately perceptible?

    Nickb, I understand you opened up the whole fender amp apart until you found out? once it was visible, how did you fix it?

    Tks for your answers. Really helps.
    It needed a new cabinet. It was under warranty.

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    Quote Originally Posted by nickb View Post
    It needed a new cabinet. It was under warranty.
    Ok. I did open up the whole amp, and I see zero flaws. Fixations are solid and it's all in good wood.

    However, here is the latest recording I did : https://www.dropbox.com/s/dik41bnxbw...FBT-3.mp3?dl=0
    When it warms up, you can hear this little oscillating hum, appearing at the same time as the weird frequency on the notes.
    The hum disappears slowly after a few minutes but not the vibration I'm complaining about.

    Could this be non mechanical after all?

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by pamelahute View Post
    the weird frequency on the notes.
    Could this be non mechanical after all?
    With a gentle knock with finger on the output tubes check if there are any audible sensations.
    If the output tubes worked for a long time, the electrodes in them move slightly (usually in rhythm of sound)

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    Quote Originally Posted by vintagekiki View Post
    With a gentle knock with finger on the output tubes check if there are any audible sensations.
    If the output tubes worked for a long time, the electrodes in them move slightly (usually in rhythm of sound)
    Tks for the quick suggestion vintagekiki.

    So here is what I did : wait until the amp warms up, play the note to get the weird vibration, knock on the tubes and.... seems like something happens on the rectiier, the EZ81.
    Please listen to the soundclip below.

    https://www.dropbox.com/s/r0rwjo8gfo...FBT-4.mp3?dl=0

    Thoughts?
    Could changing the rectifier tube simply fix the issue? I already did in the past, I'm almost sure, but never ran those tests though....

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    Last edited by pamelahute; 10-14-2019 at 01:22 AM.

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    It could absolutely be the rectifier tube, and swapping it out is the fastest way to know. If you are at the point of ruling out tubes, then pull the chassis and poke and tap with a wooden chopstick or drum stick and see if you can identify a loose solder connection. You'll need to find a way to wire it up so a speaker is connected with the chassis removed from the cabinet. BE CAREFULL! It's dangerous in there. If it's not a tube, my guess is you will find a loose connection that is sympathetic to certain notes being transmitted as vibration through the cab and chassis.

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    Last edited by Randall; 10-14-2019 at 02:26 AM.
    It's weird, because it WAS working fine.....

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    Thanks Randall. I will change the rectifier tube (already ordered one) and if itís not enough I wIll bring it to a tech to investigate what you suggest. Does it sound like microphonia to your ears? Have you heard the sound clips?
    I donít feel I have the expertise to open things up and test further myself.

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    Since sound sensations do not exist when the head is separated from the cab, the cause should be sought in the head.
    In the background when you're not playing there is a modulated sound like a silent microphonia.
    Remove the input tube (12AX7/ ECC83) and check whether there are still audible sensations.
    Check rectifier tube, 3 x 20uF and 25uF in the cathode output tubes.

    Click image for larger version. 

Name:	802933_20150208135426.jpg 
Views:	26 
Size:	40.3 KB 
ID:	55564

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    Quote Originally Posted by vintagekiki View Post
    Since sound sensations do not exist when the head is separated from the cab, the cause should be sought in the head.
    In the background when you're not playing there is a modulated sound like a silent microphonia.
    Remove the input tube (12AX7/ ECC83) and check whether there are still audible sensations.
    Check rectifier tube, 3 x 20uF and 25uF in the cathode output tubes.

    Click image for larger version. 

Name:	802933_20150208135426.jpg 
Views:	26 
Size:	40.3 KB 
ID:	55564
    Hi all! So I changed the rectifier tube, and bingo, it fixed the problem! Thank you so much for the help and suggestions in this inquiry, it was of great help.
    best.
    Pam.

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    To me, there also sounded like some power tube rattling in there.
    If the problem comes back, change both the EL84 tubes, not just one at a time.

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    I actually changed all three...

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    Quote Originally Posted by Randall View Post
    It could absolutely be the rectifier tube, and swapping it out is the fastest way to know. If you are at the point of ruling out tubes, then pull the chassis and poke and tap with a wooden chopstick or drum stick and see if you can identify a loose solder connection. You'll need to find a way to wire it up so a speaker is connected with the chassis removed from the cabinet. BE CAREFULL! It's dangerous in there. If it's not a tube, my guess is you will find a loose connection that is sympathetic to certain notes being transmitted as vibration through the cab and chassis.
    Without listening to the audio (yet), this was the first thing I thought of as well. I just worked on a Vibro Champ and ended up chasing down a problem similar to this. Ended up being the new 5Y3 rectifier tube we installed. The real bummer was that all 3 Sovteks we had left were exhibiting this problem and I fear we may have been sent a bad batch.
    I was going to start a thread on the subject and see if anyone had been experiencing this issue with 5Y3s.
    I was able to damp the noise with a gloved hand, but nothing else seemed to work. I tried re-tensioning the tube socket pins and retainer clip to no avail, and nothing we had on hand seemed to help.

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    Quote Originally Posted by SoulFetish View Post
    I was able to damp the noise with a gloved hand, but nothing else seemed to work. I tried re-tensioning the tube socket pins and retainer clip to no avail, and nothing we had on hand seemed to help.
    PT transfer own vibration to the chassis, and this vibration via tubes socket transfer to the tubes.

    It happens over time that the electrodes in the (old) tube slightly mechanical relax , and some of the electrodes (g1, filament ..) start to vibrates. This manifests as a sound sensation in the amplifier. The only solution is to replace this tube.

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    Quote Originally Posted by vintagekiki View Post
    PT transfer own vibration to the chassis, and this vibration via tubes socket transfer to the tubes.

    It happens over time that the electrodes in the (old) tube slightly mechanical relax , and some of the electrodes (g1, filament ..) start to vibrates. This manifests as a sound sensation in the amplifier. The only solution is to replace this tube.
    I don’t doubt that this happens. But its not what happened in this case. All of the tubes were new, and it wasn’t oscillating at mains or rectified frequencies. I suspect the speaker/upside down mounting in the combo amp setup had more to do with the problem in this case

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    Hi guys. I continued the tests.

    So the rectifier created the problem for sure, but if I keptthe old EH EL84 I had installed, the vibration was still there.
    I had to change all three the get rid of the issue.

    Here's a quick clip of me playing the notes which were generating the vibration (E / F mostly) - https://www.dropbox.com/s/dd3ivilcsu...honia.mp3?dl=0

    If you listen carefully, you can still hear a tiny thing but it's nothing compared to what it was.

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    Last edited by pamelahute; 10-20-2019 at 05:13 PM.

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    Quote Originally Posted by pamelahute View Post
    If you listen carefully, you can still hear a tiny thing but it's nothing compared to what it was.
    Your amplifier is a music amplifier, it's not a HiFi amplifier.
    The best way to see if an amplifier is working properly is to compare it (one to one) with another amp of the same model. (FBT 200 to FBT 200)

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    i know it's not hifi haha ! of course ! I'm used to noisy vintage stuff
    You heard the initial noise on my sound clips, I was really complicated to use, even for musical purposes and especially for recording.
    Now the amp can totally be used normally and musically.
    I don't think I was over reacting in the first place...! was I?

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    You have not overreacted, but you expect too much from the old man.
    Can you record an audio clip with the amp on without a guitar, let's hear what's really going on.

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    It's absolutely clean with the new EZ81 and 2xEL84. Nothing noticeable.

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    here is a clean recording for you : https://www.dropbox.com/s/ops6ybrd76...aying.mp3?dl=0
    Seems perfect to me.

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    As I think there may have been a couple different issues, I'd appreciate if people would listen to the sound files and comment on those issues.
    My comment was with regard to the first file in post #1, and post #6 at around 1m42s.
    I have never heard a rectifier tube do anything like this and I would like to know if others have. To me it is the sound of a rattling power tube. A bad rectifier I expect to make glitching sounds like zzt etc., but I don't expect it to affect actual notes as it is not directly in the signal path.
    I would like to hear about bad rectifier artifacts and which of the sound files you hear them in.

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    Quote Originally Posted by pamelahute View Post
    here is a clean recording for you : https://www.dropbox.com/s/ops6ybrd76...aying.mp3?dl=0
    Seems perfect to me.
    Clip doesn't work for me: no sound.

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    Quote Originally Posted by g1 View Post
    As I think there may have been a couple different issues, I'd appreciate if people would listen to the sound files and comment on those issues.
    My comment was with regard to the first file in post #1, and post #6 at around 1m42s.
    I have never heard a rectifier tube do anything like this and I would like to know if others have. To me it is the sound of a rattling power tube. A bad rectifier I expect to make glitching sounds like zzt etc., but I don't expect it to affect actual notes as it is not directly in the signal path.
    I would like to hear about bad rectifier artifacts and which of the sound files you hear them in.
    Hello G&, thanks for your comment.
    I think the 2 EL84 and the EZ81 were all problematic and changing them definitely fixed the whole thing, as you can hear on the two last sound clips.
    Although I'm not a tech, I'm trying to think in a very simple way : Testing, changing and listening.

    It's true that besides the resonance on certain notes, which was my main problem, the amp was making a weird hum sound when heating up. Again, it disappeared when the 3 tubes where changed.

    - the amp made no weird hum nor resonnance when powered up - the issue appeared when it was warm - 3 min after powering approx. Which made me think it was not only the chassis.
    - Hitting softly the tubes did nothing except with the EZ81 where I obviously could produce the same kind of resonnance I was trying to cancel
    - Changing the EZ81 only did not fix the problem although the original tube was clearly defectuous
    - Changing the EL84 pair as well as the EZ81 fixed everything : hum & microphonia

    Hope this summary helps a bit,

    I'll be happy to make more tests if needed.

    Best,

    Pam.

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    Quote Originally Posted by Helmholtz View Post
    Clip doesn't work for me: no sound.
    You have to put the volume up. It's the amp recorded without me playing to see the sound of the amp powered on, as vintagekiki requested.
    It's perfect, the amp is super silent.

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    Quote Originally Posted by pamelahute View Post
    here is a clean recording for you : https://www.dropbox.com/s/ops6ybrd76...aying.mp3?dl=0

    Seems perfect to me.
    FBT is absolutely silent, when you play (the previous clip) sounds solid.
    All the audible sensations which you hear when the guitar is plugged in are normal because the PU detects all EM radiation in the room.

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    Quote Originally Posted by vintagekiki View Post
    FBT is absolutely silent, when you play (the previous clip) sounds solid.
    All the audible sensations which you hear when the guitar is plugged in are normal because the PU detects all EM radiation in the room.
    Yes! It has never been so silent, almost hifi

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    I'm glad that you are reanimate an old man.
    He deserves to be gladden with new tubes.
    Now take the guitar in your hands and Name:  s0508.gif
Views: 37
Size:  4.8 KB

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    Quote Originally Posted by vintagekiki View Post
    I'm glad that you are reanimate an old man.
    He deserves to be gladden with new tubes.
    Now take the guitar in your hands and Name:  s0508.gif
Views: 37
Size:  4.8 KB
    Will do!

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    Oh yeah, 'turn it up'!
    Really enjoyed 'The Radio'.

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    Quote Originally Posted by g1 View Post
    Oh yeah, 'turn it up'!
    Really enjoyed 'The Radio'.
    Thanks for the kind words!

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