Mk IV Feedback in Lead Channel

[GmanJeff]

Clean and crunch on my Mk IV are fine; didn't change any settings, but my lead channel recently began to feed back immediately upon switching to it. I used the on-board spare pre-amp tube (12AX7), swapping it for V1, V2, and V3 without any improvement. V4 is a 12AT7 ECC81, and the earlier version of the manual indicates it's for the reverb only. A second version of the manual says it's "V4A = Second Lead High Gain Stage; V4B = Reverb Send", which I assume means it serves two functions simultaneously? Given the reference to a "second lead high gain stage", and because swapping V1 through 3 didn't help, I suppose I need a replacement 12AT7? Has anyone else had this experience, and did replacing the 12AT7 resolve the problem? Thanks in advance for any shared experience or suggestions!

 

[mrd]

I don't see any reason why you couldn't use a 12AX7 here. It actually seems odd to have an AT7 in there when its labelled "high gain stage. And even if its designed with an AT7 in mind, I'll eat my **** hat if putting an AX7 in there hurts anything. I say try an AX7.

 

[GmanJeff]

Taking MRD's suggestion, I did a bit of research about the differences between the AX7 and the AT7, and found that they can be safely interchanged. The feedback sound is still there, although when I reduce the lean drive and gain below 5, and keep the push-pull controls pushed in, it's greatly reduced. It's puzzling, because I was previously able to set drive and gain in the 7-8 range, and could keep the push-pull knobs in the pulled positions, without excessive noise and feedback. :?:

 

[mrd]

Well, you've swapped V1, V2, V3, V4... try V5. If that doesn't fix it, start eliminating other things (if you haven't already)... guitar's, pickups, etc.

It might be the power tubes? Do you have a spare set you can swap with? I'm not sure I've ever heard of them going microphonic like that, I always thought that was a pre-amp tube thing but someone feel free to correct me if I'm wrong.

 

[Fronzil]

maybe you know this, but how loud are you playing? lead channel will squeal if the treble, gain and drive are to high, mixed with high volumes

 

[GmanJeff]

Haven't tried replacing the power tubes, and don't have a spare set, but the clean and crunch channels work fine, and I'd think there'd be some issue there if the power tubes were implicated? Ditto for swapping out the guitar, cable, anything in my pedalboard (everything on there is off, except for a vocal processor that the amp has always worked fine with before), etc.

V5 is the "phase inverter", whatever that is (sounds like something in the drive system of the Starship Enterprise); I don't have the amp in front of me at the moment, but if V5 is a AX7 or AT7 I'll try swapping it out with the on-board spare. If it's a different type of tube, I'll have to go buy one.

Volumes are moderate, probably 1.5 - 2 on the Master, and around 7-ish on the channel volume and gain. I hadn't changed settings before the problem started. The lead channel was always a little bit noisier than the other two, which I think is only to be expected, but now it's clearly gotten a good bit louder/noiser and goes quickly into feedback. There's a pronounced increase in the noise floow when the push-pull controls are pulled out, too, which never used to be the case.

 

[mrd]

Well, 7 on the channel and 2 on the master is decently loud, but it shouldn't be feeding back that easily. Keep in mind, pulling the push-pull knobs out adds MORE GAIN. I won't tell you how to dial your amp, but... I'll tell you that pulling both the "fat" and "bright" knobs is, for most intents and purposes, way, way too much. Try one or the other, experiment with how each one affects the tone. To my ears, the fat knob just adds overall gain to the entire harmonic structure of the signal, whereas the bright knob just pushes the very top end up a bit. I never pull the bright knob, I find it to be too much.

Just for shits, try pushing both those knobs in and running your gain lower, around 4 or 5. Does it still feedback?

Also... what size is the room you're in? How close are you standing to the cab when you play?

The phase inverter tube has something to do with feeding the signal to the power supply. I'm not entirely sure of how it works, but I think it converts the signal back from DC to AC (don't quote me). It should be an AX7, swap it out and see if it helps.

Just for future reference, don't be so worried about changing tubes. Mesa amps are tough ****... unless you do something stupid like run the amp with tubes that are very obviously not supposed to be there (if they even fit the sockets, that is) or run the amp with no power tubes... you catch my drift. Mesa's #1 drive is to make amps that are reliable and sturdy. You can switch pre-amp tubes with the amp on if you want (you might blow the speakers from a "pop!' as the tube comes out, but the amp itself will be just fine.) AU7s, AT7s and AX7s are generally always interchangeable, at least in Mesa amps and at least to my knowledge and experience. They are all essentially the same tube, they just have differing levels of gain.