Microphonic Tube In Mark IV?

[alantig]

Just want to see if I'm on the right track here. I think I have a microphonic tube in my Mark IV, but I didn't have any luck tracking it down, and I don't have a ton of experience w/tube amps.

I just got a Bad Cat Leash attenuator and was testing it out w/the Mark IV (widebody 1x12). I got a couple of speaker cable to use to hook up the Leash. When I tested it with my normal amp settings (tweed power), no issues, everything was fine, the Leash cut the volume just as I'd expect.

Then I turned up. Started w/R2. Normally I have the master for that around 3. Turned it up to 6-7. No issues. Went to the lead channel (roughly the same initial setting) and turned it up and got a pretty vicious howl. My first thought was that my guitar was feeding back - even though the volume was pretty low - because when I played anything, the howl was gone (or at least covered up). But when I turned the volume down on the guitar, the howl was still there. So I moved the amp to where I could get at the tubes and started tapping - nothing affected the howl.

Anytime I turned the lead channel over 3, I would get the howl. It would change if I flipped any of the switches - pentode to triode, simul-class to class A, harmonic to mid. Then I turned up the output volume while on R2 and got the howl, although at a slightly lower level.

So, am I on the right track that this is a microphonic tube, or at least a bad tube? Would I be better off to replace all four 6L6s or just the bad one (assuming it's one of those)?

Gonna call Mesa Boogie tom'w, but I'd like to go into this as informed as possible.

Thanks!

Alan

 

[mrd]

To me it just sounds like feedback. As far as I know, power tubes either don't or can't go microphonic, or at least I've never heard of it. It's usually pre-amp tubes. A way to test it is this: turn everything on, leave your guitar volume on zero, and go back there and tap (VERY lightly) each tube (pre-amp and power amp) with a pen or a pencil or something and if you get any sort of noise due to the tap, then that tube is your best bet.

Failing that, replace each pre-amp tube one by one with a known good one and see if that helps the problem. If you're replacing power tubes (I highly doubt you will need to in this situation), make sure you replace them in matched pairs. The outer two tubes are one pair, and the inner two tubes are another pair. You can't replace only one, so replace 2 at a time or all 4 at a time. I think all tube amps are like this, at least Mesa's are.

Anyway, like I said, I think it's just feedback. When I play loud, my guitar will feedback insanely even if my volume is barely, barely above zero. A good thing to remember is that when you start turning up the amp (attenuator or not), scale down the gain as well. Running your gain at 8 is fine for bedroom levels but if you are seriously rockin' out, turn it down to 6 or so. It will really help with the squealing, etc. There's a note about this in the manual if you wanna read further.

 

[stokes]

Do the tests as mrd says.Just a word on the 6L6 thing,power tubes can be microphonic,in fact all tubes are microphonic to some degree,even dead quiet ones,they just arent problematic until they become so microphonic that they are audibly so.The reason preamp tubes are more prone to becoming a problem is that,for instance,if V1 becomes audibly microphonic it is then amplified by each following stage.If a power tube is microphonic,it isnt amplified by any following stages.But they can and do sometimes become a problem just rarer than a preamp.

 

[alantig]

Thanks for the tips guys, but...

It seems the problem is from introducing the Leash into the system. For those who haven't seen one, the Leash has a cooling fan. When I take the Leash out of the equation and crank up the volume, there's no feedback. When I unplug the guitar at the amp, there's no feedback with the Leash. But plug the guitar in (or for that matter, just the cable), and the noise from the Leash is enough to induce the howl. So it was a good idea, but I think it's going to have to go back. Slightly expensive experiment, but at least I'll be able to return it and just eat the shipping.

Alan

 

[stokes]

Seriously doubt the cooling fan has anything to do with the problem,just disconnect the fan and you'll see.Its just the excessive overdrive causing feed back.You say its not there with the guitar unplugged,with no pickup,theres nothing to feedback.I never liked those attenuators,cause too much heat in your output,the fan will keep your attenuator cool,but does nothing for your output tubes and OT.Everything in the amp gets stressed to the max.I would bet if you turn down the gain and masters a touch you can control the feedback.

 

[alantig]

I don't think I can disconnect the fan w/o opening this thing up, and I don't really feel like doing that. I'm going to give it another shot w/the gain a little lower, but it seems like that kind of defeats the purpose that I was shooting for. I did try it w/my Budda amp and had the same thing happen when I turned the volume way up w/that amp.

The feedback explanation makes sense, although it seems a little odd to me as far as what would be picking up the feedback with no guitar plugged in. That's why I leaned towards it being noise-induced.

My hope was to be able to work the tubes in a different way and push things a little differently, but it looks like it's not going to pan out the way I'd hoped (and with my ears, I don't know if I'd be able to tell the difference - I'm no Eric Johnson, or even Eric Johnson's brother's mailman's third-cousin's next door neighbor when it comes to tone!).

Alan

 

[bryan_kilco]

is it a low or high-pitch sound?

I used to get a really high-pitched squeal from my old tubes.

When I would turn volume up, gain/drive up, or pull bright, I'd get the squeal. I just retubed the whole thing and it was fine.

 

[stokes]

Sorry,I missed where you said with no guitar and just the cable that you do get the feedback.Like I said all tubes are microphonic to some degree,you can test this by tapping the tubes,even tho you wont get an out of control squeel from a "good' tube you will get some sound.I would bet that if you dont have the Leash hooked up,and turn your controls up where you get the feedback with the Leash,you will get it without the Leash,albeit you wont be able to stand the volume.Boogies have such high gain characteristics its kinda the nature of the beast.Turning the treble down may tame it some too.