#1
So I posted about half a year ago or so about a problem I had with my IRT120. It made a distinct 'blat' kind of muddled sound on lower notes, and I rectified it by changing a preamp tube in V1 I believe. Now at the time, every tube I had (including the one I replaced with) made an audible sound when I tapped it. It was more a dull 'thump/clunk' sound, but it was on all of them so I assumed none of them were actually microphonic.

Over the last few days, I've noticed I've had to turn my channel volume up slightly higher than usual, although the distortion side of things seemed fine. Well now my original problem is back, and I seem to have lost a lot of the liveliness in my tone. My clean doesn't sound so clean anymore, and my gain channel produces very low gain distortion when I strum the strings ever so gently, whereas before, it still kept quite a bit of crunch to it.

So naturally I've popped the cover off the front, and the first thing I notice is that the shield on V3 (preamp) is a LOT hotter than the other tube sockets. So I take the shields all off, and I notice that the top part of V3 is more of a washed out grey colour (kind of blotchy, almost like rust, but without the orange) while the rest all are black at the top. Now the tubes in V1, V3, and V4 are TAD's. I think I changed V1 with a new TAD I got, so V3 and V4 are the originals. The tube in V2 is a Groove Tubes gold pin one.

I just did the microphonic test by tapping them, but none sounded particularly weird.

So basically, I'm rambling here and I don't quite know where to troubleshoot, but the current problem I have is a slight decrease in volume (have to turn it from 2 to 3 now to get a similar level of volume) and I've noticed a decrease in gain/clarity.

Any advice to give? I'm still quite new to all this tube amp stuff!
Gibson SG (Goddess - Violet Burst / Standard - Alpine White)
Laney Ironheart 120W and Celestion V30's

(Front) Polytune 2 noir > Cry Baby Wah 105Q Bass > Octave Multiplexer > BAT Pharaoh Fuzz > MXR Line Driver
(Loop) ISP Decimator G String II > Hardwire Supernatural
Voodoo Labs PP2
#2
Tubes have no particularly defined life. They can go bad in six weeks or six years. Absent the tube testers that used to adorn every drug store, about the only way to find out is to swap them out for a tube that's known good.
#3
That stuff gets hot. It's how the wizard that invented tubes got his mana.

If it's not really really bad your filter caps are fine, but it's a newer amp right? That's irrelevant then.

Get a JJ off the net or something and roll it through your amp. Alternatively I hear of people using the eraser side of a pencil to tap them and see if they're going bad.

Don't worry about the transformer those go out or don't; there is no middle ground.
"If you're looking for me,
you better check under the sea,
because that's where you'll find me..."
Last edited by DeathByDestroyr at Dec 15, 2015,
#4
Quote by TheLazer
So I take the shields all off, and I notice that the top part of V3 is more of a washed out grey colour (kind of blotchy, almost like rust, but without the orange)

Does it look anything like this?
If so, that's what happens when a tube loses vacuum. Nothing to do but replace it, might just be bad luck.
Attachments:
rectifiers-top2.jpg
#5
Nah doesn't look like that. They're all black at the top, but this one (while black) has an almost faded look about it. Kind of white a little bit, but mainly grey.

Also, isn't my V3 the phase inverter slot? Does that get worked harder or something? Curious why that tube in particular was so hot, I'd have assumed it'd be V1 that would
Gibson SG (Goddess - Violet Burst / Standard - Alpine White)
Laney Ironheart 120W and Celestion V30's

(Front) Polytune 2 noir > Cry Baby Wah 105Q Bass > Octave Multiplexer > BAT Pharaoh Fuzz > MXR Line Driver
(Loop) ISP Decimator G String II > Hardwire Supernatural
Voodoo Labs PP2
#6
Quote by TheLazer
Nah doesn't look like that. They're all black at the top, but this one (while black) has an almost faded look about it. Kind of white a little bit, but mainly grey.

Also, isn't my V3 the phase inverter slot? Does that get worked harder or something? Curious why that tube in particular was so hot, I'd have assumed it'd be V1 that would

Preamp tubes typically run well below maximum dissipation. If the tube really was noticably hotter than the other ones, take the amp to a tech. Maybe a bias- or grid-leak-resistor is faulty or something.