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My son's garage band, think fifteen and sixteen year old kids, played a bass guitar through my Peavey Classic Delta Blues 30. Something is wrong. When playing bass strings of a straight electric now, it begins to squall or rattle uncontrollably. You can cease the vibration of the guitar strings, but the amp is now in charge howling to beat the band.

We unhooked the guitar and cord, but my son left the amp on while putting the other stuff away. Scooting it across the floor it did the same thing it had earlier with the guitar at volume. It started squalling, kind of like feedback. It was definitely connected to the device in the bottom of the amp.

I think it's a reverb device, hidden inside a naugahyde cover and screwed to the floor of the amp. Two wires go from that thing up into the head. Once I realized that the scooting vibration set off the amp, it made me check this device by rapping it with my knuckles. Sure enough, rapping it would make it take off squawling. Something akin to a feed back that will not stop until you kill the power switch.

Haven't had a problem until now. These boys also blew a PA speaker for me direct running the Bass Guitar into that system.

We now have a dedicated Bass Amp, so if it was the bass causing these troubles, it won't happen again the same way. I had to replace the PA Speaker complete, even a replacement crossover didn't fix that.

Okay, any amp repair folks have any ideas what I should check. I bought my older son one of these amps too and his has been trouble free except that once I remember changing some tubes. Great little amps, but you could fit what I know about them in a thimble with room to spare. Any ideas? TIA, Bruce

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http://www.dickeyguitars.com

EDIT: Deleted my response. Your profile has 3,000+ posts and you know a *LOT* more about guitars than I do, so I felt it would be easily misconstrued for condescension if I were to say the use of reverb could be causing unabating feedback. Please accept my apologies!

I mention that only because I can't use reverb when playing live-- my pickups are microphonic and they *will* feedback with reverb added. I like the more "open" tone of microphonic pickups, but I do have to be careful where I stand so as not to squawl.

John


EDIT 2: In the event that others might not be familiar with that scenario, that is the spring reverb tank in the bottom of the amplifier. When kicked, moved, or other things, if the reverb is on a high setting it can induce feedback. Really, he probably doesn't need to use reverb playing live anyway (unless he's using it for a specific effect).

Thanks John. I'm acoustic all the way and have little experience on electrics or amps. I knew I wanted a tube amp and copied a purchase a longtime friend had made who knows his business.

This amp was set to zero on reverb, yet the thing was howling. Again, rapping that device in the bottom of the amp, made it replicate the sound the regular guitar made earlier playing a volume level of 3 on the low E string.

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http://www.dickeyguitars.com

I'm not an amp tech, just a player, but I've been playing electric guitar for over 25 years. IMO it's not very likely that the reverb tank is the problem. It's basically a box with a spring in it, and an input and an output. Not much there to go wrong. It sounds to me like you have a microphonic preamp(or power amp) tube, and when you tap the reverb tank, the vibrations are causing the tube to squeal like a pig.
However, if they were playing a bass guitar thru the amp, it's possible this might have damaged it, as guitar amps are not designed to reproduce the extreme low frequencies of the bass guitar. But I would suspect you'd more likely blow the speaker than do permanent damage to the amp. However, this might have been the reason the tube went microphonic in the first place, if that indeed is what it is. Take it to a tech. If it's a microphonic tube it'll be fairly cheap to fix. And don't let them plug a bass into anything but the bass amp again.

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Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

Bruce, is it really high pitched and oscillating like a really bad, really annoying UFO sound effect? Or is it a constant, annoying high tone like regular feedback?

Okay, based on the questions, I went back and tried the amp.

I agree now it's not the reverb part secured to the bottom of the amp.

Turning all the knobs to zero and just flipping it on, it's fine.

But tap anywhere on the amp with your knuckles and a loud buzzing begins. Likely it's the vibration, just as when I scooted it on the floor a little, it started behaving badly with a loud buzzing. The only thing that stops it is flipping the switch off.

One other thing will set it off when everything is set to zero, pushing in the channel switch does it just the same.

There are four longer tubes and three shorter tubes. The four long ones appear all lit the same. I don't detect the shorter ones to be lit, if they are they are not as visible as the four long ones.

So, I may just have to find the local amp guy, ay? Thanks. Any other ideas would be appreciated. I could take the speaker out and inspect it, that was what I suspected, but at low volumes and before the buzzing begins the amp appears to work normally.

James, It isn't a high squeal, as to say microphonic, but a low buzzing, probably less like feedback and more like a loud constant transformer type hum.

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Bruce, the shorter tubes are called the heaters. They're on From the far side of the power tubes (the big ones), they're called V1, V2, V3, and V4.

My question about the sound of the feedback is important. Can you do your best to describe it in the terms I mentioned?

Those Peavey's are very nice amps. They're hidden jewels.

No.


No.

Once it initiates it's simply a loud buzzing sound, like the hum of a transformer only amplified. Not a rattling speaker sound either. A low pitched squalling sound or hum.

I could send you a short video?

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http://www.dickeyguitars.com

I thought it was something called parasitic oscillation. Parasitic oscillation usually comes when one of the ground connections from the shielded cable gets loose. I don't know what would produce a lower squeal

Parasites, they have those in guitar amps too. Darn.

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http://www.dickeyguitars.com

Mr. Orr is much more knowledgable than myself, but based on what ya'll have said, it really sounds like microphonic feedback to me and I'm betting it's a microphonic tube. Mr Orr-- is it possible that once the vibrations reach a threshold from the speaker it sets up a feedback loop causing the tube to vibrate, almost like the magnetic field of an e-bow keeps an electric string vibrating?

I bought my oldest son an identical amp.

I may just have him bring it over and start a parts swap. Tubes first.

They have been good amps. Banish the Bass!

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http://www.dickeyguitars.com

I'm not Mr. Orr until 8:20 monday morning

It isn't the speaker, but good problem solving. If it were a speaker issue, you'd just hear the farting around of the blown speaker. Let us know about the tube swapping, Bruce.

A posibility is a cracked speaker cone. These amps and many amps where not made for bass guitars. Speakers cant handle the low frequency. A tube could have also been damage by excess vibration caused by the bass guitar. As for the reverb tank a spring could have popped off it holder or the holder broke off. They are soldered on...Mike

Thanks Mike.

My first thought was a blown speaker.

I've heard blown speakers before and this is different, not rattle-y, more of a loud constant hum or howling.

I've also heard the loud squeal of a microphonic tube, that happened with our other one and new tubes took care of that.

Maybe swapping out tubes with the other one will point to the guilty culprit.

Thanks everyone for chiming in, two heads are better than one!

I'll come and repost the solution when we get it, good point James.

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It could just be that one of the preamp tubes is loose - check they're all seated properly in their sockets. Running a bass through it might have provided enough vibration to damage one. If you tap the tubes gently one by one (use something hard but non metallic) while the amp is running, any microphonic ones will ring like a bell. The amp doesn't need to be turned up loud for this - a microphonic tube will be obvious at low volume. Another thing it could be is a dry or damaged joint in the preamp circuit - that's a bit trickier to trace and is definitely a job for a pro.
You shouldn't have to get anywhere near any large currents check the tubes, but be very careful anyway - the chassis should be well grounded so assume it's the only thing that's safe to touch. Better safe than sorry..
Cheers,
Bri

Thanks Brian I'll do that now.

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Brian, You da man!

I checked each tube wiggling it upward into the head.

I can't say any were out of place, they didn't look like it.

Matter of fact these Peavey Delta Blues amps have a little wire holder for each tube. No way for them to fall out.

One drawback to an open back amp is that I've seen folks bang the tubes through carelessness. Some amp stands rest right on the tubes, needless to say, I made my own to fit for my boys.

Brian, the amp is fixed, evidently one of the tubes just wasn't seated or making contact fully, wire holders were all on as well. I'm baffled and mystifyied, but the little Les Paul went to wailing, just like normal. No hum, no microphonics, nuttin'. That is great.

Thanks for making that suggestion. Reseating the tubes by wiggling and thumping each one several times with the bells mallet, did the trick.

Since I only have two thumbs you are top-rated. Where did you learn to fix amps anyway? I'm impressed.

Thanks for all the great responses. I thought for sure this baby was blown out somehow with the bass guitar mistreatment.

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Awesome! Nothin' like a fix that didn't cost nuthin'! Good deal.

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Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

Great news! I'm glad it was such a simple fix. Ya'll want to tackle an all original 1960's Fender Super Reverb that had part of a resistor burn up and fall out of it, zapped the tar out of me, and still has the two prong plug on it?

Glad it worked for ya
I shared a repair workshop with an electronics tech for a few of years and picked up some tricks from him. He's building amps and pedals under his own name now
http://www.flynnamps.co.uk/
pharmboycu - I'd recommend him any day, but it would be long journey - he's in Glasgow, Scotland
Bri