#1
Hey guys, I've had a new Marshall DSL40C for a couple weeks now, and today when I turned the amp on, I started to hear a soft thumping noise, similar to the one you get when you lightly tap a microphone with one finger. It started out slow, but its pace picked up after a while and it became very rapid. I checked my guitar, switched pickups, nothing, switched out cables, still no change, but the odd thing was that it only occurred on my amp's clean channel. The sound got faster and faster until after a minute or two, it just stopped. Nothing has happened since, but i was wondering if i should be worried at all. Thanks
#2
It does that even with the guitar unplugged?

Does it do it on the gain channel if the gain is turned up?

Maybe a bad preamp tube. I'd swap the preamp tubes around starting with V1. In other words, switch V1 and V2. Then V1 and V3. V1 often runs the clean channel on an amp. It will be the preamp tube furthest away from the power tubes and closest to the insert jack. Take the aluminum shield off and pull the tube strait out. Note how the pins align. May work better if you flip the amp upside down.


This may help:
http://vid545.photobucket.com/albums/hh384/buckethead_311/Amp%20stuff/Microphonicpreamptube.mp4


Here is my old Valveking upside down:
Last edited by 311ZOSOVHJH at Nov 4, 2014,
#3
I never got a chance to unplug the guitar, and no, on the gain channel there was absolutely no problem. I would switch back and forth between gain and clean, and the sound was only heard on the clean channel. But like I said, it only lasted for a minute or two. I haven't had any problem since.
#5
Alright guys, turned on my amp today and the same thing happened, only it hasn't gone away. I unplugged my guitar and I could still hear the sound, again only on the clean channel. I'm really hesitant to open up the back of my amp, I feel I might just make things worse. If I did switch the preamp tubes, what would I be looking for? Please help!
#6
You'll be looking for ECC83 tubes. Apparently there's 4 total, 3 in the pre-amp and 1 in the power amp section. I have no idea if that's accurate, though that's what the Marshall website says. MF says 4 12AX7's, so you'll need to check and see what the tubes in your amp have printed on them. They should say ECC83 or 12AX7. Sometimes they say something else because some tubes are interchangeable. However, these are likely to be factory tubes. I would expect them to be what Marshall advertises them to be.

In terms of replacing them, go through thetubestore.com or tubedepot.com and pick whatever ECC83 tubes are in your budget and have a description that meets what sound you think you want. First do the pencil tap trick. If you hear any sound coming through the amp when you tap a tube, it's likely bad. V1 tubes seem to always resonate through the speaker. I don't know if that's a universal occurrence or not. If it does, take the tube out and put it in V2 or V3 and tap it again.

You can't go wrong with tubes that cost $10-$20 a piece. I don't believe in spending anymore than that (especially on preamp tubes) because I don't think there's that much of a difference in tubes. Plus, they can break, so spending $50 on a single preamp tube doesn't sound like a good idea to me. You also don't need matched preamp tubes, only power tubes. You can choose to do the 24 hour burn-in option. It will make the tubes sound more like they should. Brand new, unlit tubes will sound different than broken-in ones. The choice is yours.

Don't go through MF or GC because they do not test the tubes prior to shipping. I received a batch of bad Tung-Sol tubes from MF and didn't know that they were the cause of my problems until long after the return date.

Best of luck.
Last edited by nick.culliton at Nov 5, 2014,
#7
for a new/newer amp i couldn't think of a bad cap or other component.

it almost has to be a tube issue if its only one channel. 12AX7's, are dual triode, so they can do two things, whether it be a gain stage or for reverb or an effects loop, etc.

but if you have a bad tube, you should look it up and see what tube affect the clean channel's gain stage.
#8
Ok, although I definitely won't be doing it myself. I might just take it to Guitar Center or something and have them find the problem tube and fix it
#9
It's not that hard to do. You'd be better off doing it yourself. GC will take 1 to 2 weeks to get it done and charge you labor + materials to "fix" it. They will likely put a whole new set of tubes in your amp which will cost you ~$150. Plus, we don't know if it's truly a tube problem.

These amps are made to be serviceable. Don't be afraid to take off the back. Try the eraser tapping trick on the preamp tubes. You will save lots of time and money doing that. It could be only one tube causing the problem.
#12
You forgot to let the accessory mouse out? He's probably starving by now.

Check with Bob at Eurotubes.com; he can probably tell you instantly what the likely cause of the problem is. His ECC83 and 12AX7s are excellent (will run you $11-16) -- mostly JJs. Replacing a tube is something every tube amp owner really needs to know how to do himself; something like changing strings on the guitar. Everyone in the US knew how to change out a tube in the 50's and 60's, because anything electronic ran on them. They also knew to take the tube into the local drug store and use the tube tester to see if they needed a new tube (conveniently located in sorted shelves immediately below the tube tester).
#13
Quote by moskeedog
Ok, if I did tap the tubes, what would I be looking for?


You're looking for a tapping sound that comes through the amp. It sounds like tapping on a glass light bulb (which is essentially what a tue is anyway). The sound should be cold and flat, and it could resonate. If it resonates, that means that the tube has a leak. Check youtube for some videos demonstrating what a good tube sounds like vs. a bad tube when tapped.

And like I said before, the V1 tube (typically the tube furthest away from the power tubes) may resonate when tapped. To effectively check this one, swap it out into a different slot.

You may need to turn up the volume on your amp to hear the taps, though they are quite audible. I'd say 9 o'clock should be plenty loud enough. You do not and should not have your guitar plugged in while you do this. Make sure you don't have any onboard effects on or reverb turned up.

Try to tap lightly on the tube so that you don't hear the tapping of pencil against the tube, but the sound through the amp. A bad tube will give a much louder sound than a good tube.
Last edited by nick.culliton at Nov 6, 2014,