Microphonics on my 6L6 tubes

[suislidE03]

Hey guys, just figured i would post this. Ive had my amp for a couple months now, and i realized that when i turn it around 7-8 on the volume, the amp produces the high pitch feedback with a tremelo effect. Ive read a couple of articles, and they say this is what microphonics are.

The thing is, they all say that it is from vibrations, mostly coming from the speakers putting out all that sound. In this case, I bought a weber minimass attenuator a while back and even with it soaking up max power, the amp still does it around 7-8 on the volume. It doesnt get loud at all, its still at bedroom levels. So why do i still get microphonics?

Also, my tubes are GT 6L6B's and they get that blue glow on both power tubes. I notice that the higher the volume level, the more blue i see, and when i get microphonics the blue color kind of giggles or vibrates slightly. Any ideas?

 

[muttonchopsrule]

Re: Microphonics on my 6L6 tubes

someone correct me if i'm wrong, but i'm pretty sure blue glow is normal, to a certain degree anyway. why do you need to turn it up that much anyway? i found that the sweet spot on my amp is when the volume is at about 4 or 5 and anything above that just sounds muddy and oversaturated. but perhaps that can vary from amp to amp.

does it do it at all at any point before the volume is up that high?

 

[Edgecrusher]

Re: Microphonics on my 6L6 tubes

The blue glow is normal. What i would be worried about its if the plates themselves (not the filaments inside them) were glowing red hot. As far as microphonics go that usually happens to preamp tubes more than power tubes. If your careful you can take something (I prefer a pencil with one of those big add on erasers) and tap on the tubes if you have a microphonic one you will hear it ringing.

Edit: I should clarify all preamp tubes are microphonic to a point. Even with a good tube when you tap on it you will hear some sound and chiming. its when you get an inordinant amount that you know a tube has gone bad. Yes the vibrations from speakers can cause tubes to ring microphonically but if the tube has gone bad they can ring without this also. Remember even with the mass your still running the amp internally just as hard.

As a side note this is why i keep extra tubes on hand so that if my amp starts having strange feedback like this i can pop the new set in and see easily if its tubes or not. And if it is i just work backwards putting the old ones in one at a time till the problem comes back to find the culprit. Ok I'll get off my soap box now.

 

[Edgecrusher]

Re: Microphonics on my 6L6 tubes

someone correct me if i'm wrong, but i'm pretty sure blue glow is normal, to a certain degree anyway. why do you need to turn it up that much anyway? i found that the sweet spot on my amp is when the volume is at about 4 or 5 and anything above that just sounds muddy and oversaturated. but perhaps that can vary from amp to amp.

does it do it at all at any point before the volume is up that high?

Yeah that really depends on the amp. My MKIV sounds best at about 5 also. But my 50w MV Marshall sounds best about 8.

 

[Lewguitar]

Re: Microphonics on my 6L6 tubes

It's rare that the output tubes are microphonic. Most likely it's one of your preamp tubes. They're probably 12ax7's or something similar. Like Edgecrusher suggested: buy a few extras and keep them on hand. If the tubes in your amp start to ring, then tap each of them with your finger and see if you can find the one with the problem. Replace it with a new one and see what happens. This kind of thing is very commen.

It's not always tubes though. You could have a bad cap or loose solder joint somewhere. It's usually preamp tubes though when you hear that ringing sound.

The blue glow is gas. It's normal, especially as the tubes get a little older.

Lew

 

[suislidE03]

Re: Microphonics on my 6L6 tubes

Found the problem. Two of the 6 12AX7s are microphonic with both gain knobs maxed with the volume at 8 or above. Thanks for the help guys.