Tube Problem or Horrendous Feedback?

[burntoast]

I just got a used Mark IV Combo and I'm having trouble with the sound.

When I run it on Full Power on the Lead channel I get this horrendous screeching that swells, but I can't tell if this is just feedback or a tube problem. When playing, the tone sounds flaky, and is riddled with fuzz. Oddly, I don't encounter this problem when I'm on the Rhythm 1 or 2 channels.

The Lead Gain is set to 7, and the Lead Output is set to 2. The Master Output is set to 3.

What do you guys think?

 

[Esromeron]

Sounds like a preamp tube to me. If you have a spare preamp tube switch it with each slot and see if it fixes it. If you don't have a spare try switching around different tubes in the preamp and see if the problem goes to other channels or all channels. You can eventually narrow down which tube it is in this method also, but it can take much longer and requires knowledge on what each slot does.

 

[MrMarkIII]

+1 on changing the tubes one at a time.
If the screeching continues after you unplug the guitar from the amp, it ain't guitar feedback, it's microphonic tube feedback, the same thing that happens with a mic and a P.A. The bad tube is sent into oscillation (Screech!) by the vibrations coming through the chassis from the speaker. Doesn't have to be a combo, but a separate head is a bit more insulated from the speaker. A bit. The speaker is still vibrating the air in the room, and that air is still vibrating the chassis and the tube.

 

[Platypus]

Definitely a bad preamp tube I'd say. Check the tube chart for the triodes that control the lead drive.

Alternatively just take a pencil with an eraser on it and tap the preamp tubes while your amp is on, the bad one will make a ton of noise.

 

[ANIMATED SUSPENSION]

If the squealing stops with the guitar's volume off it's not a preamp tube. You're probably using too much gain/treble and standing too close to the speakers ! Are using the 'harmonics' or 'mid gain' setting and what is 'lead drive' set to ??

 

[Platypus]

If the squealing stops with the guitar's volume off it's not a preamp tube. You're probably using too much gain/treble and standing too close to the speakers ! Are using the 'harmonics' or 'mid gain' setting and what is 'lead drive' set to ??

The swelling he described is what makes me sure its a preamp tube but your point is valid as well. Usually that oscillation is a good sign its preamp related

 

[burntoast]

If the squealing stops with the guitar's volume off it's not a preamp tube. You're probably using too much gain/treble and standing too close to the speakers ! Are using the 'harmonics' or 'mid gain' setting and what is 'lead drive' set to ??

My lead drive is set to 7 as well, and I am using the Harmonic setting. Whenever I move farther away from the amp, it seems to disappear, so it looks like you are right. Also, I tapped on all the preamp tubes and they seem to be fine.

However, I just noticed that one of my power amp tubes (the second one from the right facing the back of the amp) isn't glowing, so it looks like that might also be a problem.

What do you guys think?

Thanks!

 

[dodger916]

Switch to Class A and see if the problem persists. The tube you described would not be involved when in Class A, just the two outside tubes

However, if it were power tube related, I believe it would happen on all three channels, not just the Lead Channel.

 

[burntoast]

Thanks. It behaves similarly in Class A, but only when I'm in front of the amp.

I've also noticed the filaments in two of the three functioning power tubes are glowing weakly. Does this mean these tubes are nearing the end of their life?