My amp is SQUEEEAAALLING after a tube change!

[nakleh]

Hey folks!

Recently re-tubed my ENGL Fireball with a set from Eurotubes.. 2 6L6GC and 4 12ax7. I retubed it last week, biased it up [biased perfectly within range, around 63%], checked it quickly, and everything seemed fine. I didnt get to really play it until yesterday..

Went to practice yesterday and once I got the volume up a bit, [volume 1/2 way, gain halfway] the amp started squealing and feeding back like crazy.. was really un-manageable, no matter how far I stood from the amp, no matter if the effects loop was in use or not.. the only way to stop the squealing was to dial the gain down to almost a bare minimum. the amp wouldnt feed back WHILE I was playing, only when I stopped playing.. It was even too much for my Decimator pedal to handle.

Would this be caused by a power tube or a pre amp tube? Unfortunately I dont have the old tubes anymore to check the power tubes.. I do have a couple of 12ax7s around so I can check to see if it's those, but i figured I'd ask for your invaluable advice to steer me in the right direction.

can anyone advise? i'd really appreciate it.

 

[bryvincent]

are you sure its not your guitar?

 

[Standard 64]

Probably a tube.What did you do with the old ones?

 

[nakleh]

I've had the same guitar for 3 years now, a LP Custom, and I've never had a problem with it before. Nothing was changed on the guitar, and only the tubes were changed. I can't imagine it's suddenly the guitar.

 

[alexb17]

maybe try tapping your preamp tubes with a pencil without the guitar plugged in. if one rings out and triggers the feedback it's probably microphonic and needs to be replaced. if you find it sometimes you can fix it by shuffling your preamp tubes around. That worked for me once but I ended up replacing it soon after. faulty gear is no good.

 

[dennistruckdriver]

Put the old tubes back in it.

 

[155]

preamp tube

 

[Drew224]

either you didn't seat a preamp tube well, or it's bad.

 

[NoStatic]

either you didn't seat a preamp tube well, or it's bad.

Likey this ^^. Reseat all the pre amp valves. If ythat doesnt work, swap them around. if that doesnt work- ones bad most likely.

 

[nakleh]

i dont have the old poweramp tubes, unfortunately. but i do have some 12ax7s around so I'll try those and see. will also reseat all of the preamp tubes first to see if thats it. hopefully its just a preamp tube. thanks!

 

[Splattle101]

That sounds like a microphonic preamp tube. And if it's dependent on the gain being up, it sounds like it's the first preamp tube.

Simply get one of your old preamp tubes and use it to replace each preamp tube, one at a time. You'll find one where the squealing stops. That's the culprit, and you need to send it back to Eurotubes.

 

[>Photi G<]

V1 is most likely the problem. If not, take a new tube, and swap it out with each of the other 3 preamp tubes, one at a time until the squealing stops.

 

[Jason]

My amp does that if I use the built-in reverb. If I don't, it's fine... something went wrong in my reverb circuit awhile back I guess. Does the same thing after a complete tube change.

 

[Splattle101]

My amp does that if I use the built-in reverb. If I don't, it's fine... something went wrong in my reverb circuit awhile back I guess. Does the same thing after a complete tube change.

That sounds like a parasitic oscillation, and the cause of them can be a royal bitch to find.

 

[nakleh]

So a quick update, all is not resolved:

+ I found the original power tubes and put them back in, this did not solve the feedback.
+ I tried rearranging the pre-amp tubes into different sockets, this did not solve the feedback.
+ One at a time, I replaced each pre-amp tube with another 12ax7 I had, this did not solve the feedback.
+ I unplugged the guitar from the amp, dimed the amp, and tapped each preamp tube with a pencil eraser. None of them started squealing or feeding back.
+ I tried another guitar [Gibson SG, normal guitar is a 2006 Gibson Les Paul Custom] and the SG squealed the same as the Les Paul.
+ I lowered the bias on the amp with the new tubes in it, just to see if that had any effect, it did not stop the feedback.

So I've narrowed it down to two possibilities:

+ I ordered a "high gain" tube set from Eurotubes. Maybe there is simply TOO MUCH gain in these tubes for my amp? My amp is a "high gain" amp to begin with, maybe these are putting it over the top? Unfortunately I dont have a full set of preamp tubes to replace the ALL at the same time, but maybe I should buy a new set and swap the entire set out?

+ Could the problem be my guitar? [or both guitars?] I've never had a problem with the guitar before [stock Les Paul Custom pickups] feeding back.. I've used the guitar with a JMP2204, JCM900, JCM800, Dual Rectifier, and the Engl Fireball. None of those amps in combination with this guitar gave me any problems. In my Fireball, I never had this squealing problem until after my tube change. Is it possible the guitar is the culprit? I'd assume the pickups are not potted, maybe this would help? Although I can't see why this would suddenly be a problem..

Any additional advice would be appreciated. If I have the same problem at my next rehearsal my bandmates are going to rip their ears off!

 

[Splattle101]

I wouldn't have expected the power valves to be the problem.

Regarding the preamp valves and tapping them with a pencil or chopstick, what you're listening for is not the squealing you were getting from the guitar. You're listening for ringing, jangling noises because something in the valve is rattling. If it rattles, it will cause microphonic feedback.

I don't think it's your guitar.

I don't think it's the power valves.

I think it's a preamp valve or something to do with the preamp.

Is this happening on either gain setting, or only on Ultra gain?

 

[nakleh]

Regarding the preamp valves and tapping them with a pencil or chopstick, what you're listening for is not the squealing you were getting from the guitar. You're listening for ringing, jangling noises because something in the valve is rattling. If it rattles, it will cause microphonic feedback.

...

I think it's a preamp valve or something to do with the preamp.
Is this happening on either gain setting, or only on Ultra gain?

Thanks, Splattle. when tapping the preamp valves, i didnt notice any noise other than the tap of the pencil eraser.. no ringing or jangling. plus, since i swapped out each tube, i'd think that would have solved it if *one* of them was microphonic, no? unless I have TWO faulty preamp valves?

it's only happening on ultra gain channel, but i can get it to preeeeeety much stop if I turn the gain WAY down on that channel while having the master at around halfway up the dial..

 

[Splattle101]

..... no ringing or jangling. plus, since i swapped out each tube, i'd think that would have solved it if *one* of them was microphonic, no? unless I have TWO faulty preamp valves?...

It's not impossible, but you'd be unlucky to get two duds in a new set.

it's only happening on ultra gain channel, but i can get it to preeeeeety much stop if I turn the gain WAY down on that channel while having the master at around halfway up the dial..

The fact it's gain related really makes me think it's a preamp valve. But you've checked 'em, so...

Have you spoken to the fella at Eurotubes?

 

[nakleh]

Yes, and he told me to "unplug the guitar, dime the amp, and see what happens. if it doesnt feed back, it's NOT the tubes, it's your guitar."

well, I did what he said, and it doesnt feed back.. but I can't imagine how SUDDENLY my guitar is a feedback machine after 3 years of ownership and no problems.

i think the only solution is to order a full set of preamp tubes, perhaps lower gain, and see if that does the trick..

 

[nakleh]

and hey, Splattle..

thanks for taking the time to try to help me. I really appreciate it!