Microphonic Tubes?

[AeroG33k]

I'm not really sure about how Microphonic Tubes affect your sound, how bad it is for your sound or how you can tell you have one, but from what I know it sounds like I have this problem and it's getting worse. My amp always had a kind of "bell-like" ringing out of the notes which, at first, wasn't bad, but now it's getting very harsh, especially in the trebble/upper-mid frequencies. When I slightly tilt or bump the amp you can hear the spring that holds the (power?) tubes rattling through the speakers. The amp lost most of it's smoothness, and it's an old amp which supposedly never had it's tubes replaced. Are new tubes in order for me?

 

Another destinct possibility is that it has something to do with the guitar as the neck humbucker seemed more affected by it by itself than the mid (Both) or bridge positions. Some of you might remember I had that weird pickup "bleed through" effect...my bet would still be that it has something to do with the tubes though. HELP!

[Ricochet]

Tubes are fragile. Rough handling of amps loosens internal parts of tubes so they can easily be moved around by vibration. If it's in a preamp tube, the sounds produced by vibrations in that tube get amplified by every amp stage following it as well. Take a pencil, chopstick, or such and tap on each tube and see where the problem is. All tubes are microphonic to a certain extent, but you'll soon see if some make a lot more noise than others, or if you get a "ringing" that will be sympathetically excited by playing certain notes. Replace the offending tube(s).

[AeroG33k]

Well I haven't really handeled the amp roughly at all, other than a slight hit it got when I first got it and moved it in my car. Like I said, the microphonic sounding ringing was always there, but lately it's been getting very harsh, especially on the high notes, especially notes on the high e-string when chording. However, when I roll down the guitar volume it get's slightly better for some reason, while the EQ really doesn't do much other than choking high notes making it sound very flat. I'm new to tube amps, but since the tubes are all old and new tubes seem fairly cheap, any suggestions? I'm thinking a new set of groove tubes for late break-up (clean) and warmth...

[Ricochet]

I never replace tubes that haven't identifiably gone bad. I would never retube an amp with a complete new set. I'd tap around, find the offending tube and replace it. If it were one of a pair of output tubes I'd replace the pair, but I wouldn't pay a dealer to match them, either. Small-signal tubes like 12AX7s will normally outlast several sets of power tubes. Most tube failures occur in new tubes during their first 100 hours or so of operation, so changing them as "preventive maintenance" actually increases your chance of a problem. Old ones gradually fade away, losing cathode emission so they distort more easily, become noisy and won't put out full power. When they don't sound right, that's the time to replace them. If you retube it, be sure to buy spare tubes to have on hand. You should have those, anyway.

[AeroG33k]

Well, I have to bring this back up. I tapped the tubes as best I could reach them and I really didn't hear much through the speakers...maybe i have a different problem. I found something that may be more descriptive though: When I hit a note or strike a chord and immideately "choke it" to stop all vibration in the strings, the amp produces a dull 'echo' (not so much sustain as a ringing). It *appears* that this echo seems to enhance the trebbly frequencies and makes clean chords sound muddy (I dont have the amp for a while but I'm sure it sounded better initially!!). Is this echo normal? Is it indeed microphonic tubes? Could it be the fact that I play a semi-hollow guitar (I thought about this, but I don't think the pickups would emphasize the body resonance when the strings are dampened)?

[Ricochet]

Well, I notice with acoustic guitars there's often a distinct high pitched ringing for a bit after loudly playing all the open strings and suddenly muting them. Muting the string segments between the nut and tuner posts stops it. Maybe your pickups are picking that up. Pickups sense relative motion between the strings and body the pickup's anchored to, so if the body's moving and the strings are still, the pickup's still energized.

[AeroG33k]

Thanks for the help ricochet! I didn't really 'fix' the problem, but I know what I've been doing differently and I'm back to the original smooth tone. For some reason, when I crank the amp and roll my volume down on the guitar, chords ring much smoother and the harsh trebble goes away.

[Ricochet]

Cool! Glad you got it sounding the way you want it to.