FBT 200 - strange noise

[pamelahute]

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)

[nickb]

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.

[glebert]

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.

[pamelahute]

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.

[nickb]

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.

[pamelahute]

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?

[vintagekiki]

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)

[pamelahute]

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....

[Randall]

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.

[pamelahute]

Thanks Randall. I will change the rectifier tube (already ordered one) and if it’s 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 don’t feel I have the expertise to open things up and test further myself.

[vintagekiki]

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.

[pamelahute]

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.

[ATTACH=CONFIG]55564[/ATTACH]

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.

[g1]

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.

[pamelahute]

I actually changed all three...

[SoulFetish]

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.