Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Marcus on January 15, 2014, 11:54:38 am
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I left my amp on overnight and it now squeals badly. It's not feedback. As soon as I take it off standby it starts. Turning the volume knob on my guitar just changes to tone of the squeal. I still get signal. Where should I start?
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First, I'm wondering if you have a tube amp? If it is, I would start with the tubes. If you got some known good tubes. Install one tube at a time starting with preamps tubes to power tubes. If the new tubes don't make any difference, put the old one back in and go to the next. If at any point the squeal stops when a tube is replaced, you've found your problem.
If it is a solid state amp--well we have a solid state thread too. Platefire
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I replaced all the tubes and it's still doing it. It's and old Mesa Mark IIB head. I guess it time to break out the ole DMM.
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Negative feedback wire broken / circuit ? Do you had any touble with speaker output jack or presence pot ?
Remove V5 Phase inverter tube ; do you have the squeal ?
Shematic here ;
/www.freeinfosociety.com/electronics/schemview.php?id=512
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Is there any one tube you can pull to stop the squeal, say the first preamp tube? That'll at least help you narrow down the search with the DMM. Good luck, let us know how it goes.
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I left my amp on overnight and it now squeals badly. It's not feedback.
It is feedback, just maybe not from speaker -> strings -> pickup -> amp -> speaker, like what you're thinking of.
And leaving the amp on all night should not matter. I've got some hi-fi amps which were intended to be installed (in perhaps a studio) and left on 24/7.
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You might have cooked something.
Have the filter caps ever been replaced? If not, 1 or more of them might have been on their last leg and they gave up their last hurrah.
Or a R could have cooked.
Brad :think1:
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Negative feedback wire broken / circuit ? Do you had any touble with speaker output jack or presence pot ?
Remove V5 Phase inverter tube ; do you have the squeal ?
Shematic here ;
/www.freeinfosociety.com/electronics/schemview.php?id=512
Pulled the PI tube and the feedback stopped. With the tube installed I can touch the tube shield base and it stops. I can't tell which resistor is the NFB. Also I just bought new filter caps but I would like to solve this little problem before I put them in.
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Then keep the phase inverter tube in its socket and remove the next closest tube towards the input jack.
Keep working towards the input jack until you find which tube stops the noise, while the tube next to it does not stop the noise.
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Also I just bought new filter caps but I would like to solve this little problem before I put them in.
That might be the problem.
Are you saying that all the filter caps in the amp are the originals from the factory?
Brad :icon_biggrin:
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Does it squeal with no guitar or cable connected to the input? I ask this because sometimes it's our guitars that are the culprit. I picked up my Strat today and plugged it in and got nothing. This is a new guitar I built recently. New pickup and new volume pot. I think it's the pot. Make sure the amp exhibits the problem all by itself. That 'touching the tube shield base' comment caught my eye.
Leaving the amp on shouldn't be a problem. I have one amp I built that has no indicator lamp. The only way I know it is left on is the light from the heaters. I've accidentally left it on for days on end before without a problem.
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Hmmm, Check the tension on your input jacks. Make sure they are grounding out when nothing is plugged in. Maybe the extended run caused some heat build up and it was just enough to lift the contact.
Jim
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HBP said it too...did you try sequentially pulling tubes working back toward V1 to find out where in the preamp this feedback is coming from?
So touching the phase inverter tube base V5 stops the squeal...hmm... dampening microphonics or body capacitance effect?
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HBP said it too...did you try sequentially pulling tubes working back toward V1 to find out where in the preamp this feedback is coming from?
So touching the phase inverter tube base V5 stops the squeal...hmm... dampening microphonics or body capacitance effect?
+1000
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It is feedback, just maybe not from speaker -> strings -> pickup -> amp -> speaker, like what you're thinking of.
Can be every bit as bad and annoying if coming from wife or kids! :laugh:
(just a little passing time humor until Marcus gets back to us on this)
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I left my Silvertone 1482 on all night one time. I grieved about it but the amp was fine. Platefire
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I left my Silvertone 1482 on all night one time. I grieved about it but the amp was fine. Platefire
An amp in good chape can stay power on 24 hour a day
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So I changed the filter caps. Problem solved! I sounds huge. I was having a few other problems and they went away with this cap change. Thanks guys!
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So I changed the filter caps. Problem solved!
Alrighty then, good job!
I was having a few other problems and they went away with this cap change.
It would have been helpful to post the other problems that you were having as it well could have pointed to the filter/electrolytic caps.
Brad :icon_biggrin: