Microphonic Dsl40cr

[harry345]

I got a Dsl40cr from GC about a week ago now and was instantly amazed by the tone on that thing. Hooked up my pedals, a MXR 10 band eq and a Full Tone OCD, and it was amazing. This is now my second Dsl40cr in under 3 weeks (the first one had a bad tube, so I swapped the amp) but now this one is picking up me tapping on my guitar, pedals, cable, even my strap rubbing against me, and it feeds back like crazy. Tried different guitars, cables and my old practice amp and none of them pick up the noises or feed back. I’ve tried going guitar straight into the amp and there is still noise, but only on the Ultra Gain channel. I’ve lowered the gain and volumes all the way and there is still noise. But not on the clean channel. I suspect it may be a bad preamp tube, but I’m by no means an amp expert. Any ideas?

 

[Micky]

Swap in a known good spare preamp tube one at a time to find the culprit.

 

[Mitchell Pearrow]

Do exactly what Micky said, I will bet you find the problem, and don’t be in a hurry, do one at a time!

 

[tschrama]

-If your guitar cable has suddenly become microphonic, that indicates higher than usual grid leak current. It causes the varying capacitance of the cable (cause by you tapping on it), to push more current through the reference resistor, causing a small AC voltage, which gets amplified etc etc

-Room acoustics play a major role in feedbacking... as well as overoll gain.

-static electricity (different clothes, less humit air) might be influencing the sound you hear when moving your guitar strap.

 

[harry345]

Thanks for all the suggestions. This is my first tube amp and I don’t have any spare tubes, and I don’t want to have to buy new tubes for a $750 amp. I’ve heard a lot about tapping the preamp tubes with chopsticks to check them. Good idea?

 

[harry345]

-If your guitar cable has suddenly become microphonic, that indicates higher than usual grid leak current. It causes the varying capacitance of the cable (cause by you tapping on it), to push more current through the reference resistor, causing a small AC voltage, which gets amplified etc etc

-Room acoustics play a major role in feedbacking... as well as overoll gain.

-static electricity (different clothes, less humit air) might be influencing the sound you hear when moving your guitar strap.

About the grid leak, can that be adjusted or anything? (Sorry I suck at amps.)

 

[KraftyBob]

As an owner of a tube amp you're going to want to have spares. Yes, the tone is awesome but they are higher maintenance than a SS amp. Just the nature of the beast. Preamp tubes are pretty inexpensive unless you start going nuts. It's a good idea to have spares as you never know when you'll need one.

The chopstick test is not real definitive in my opinion. For me it's not always obvious when tapping on a bad tube so I don't think it's the most reliable method.

Go over to the Workbench thread and look for the pinned post by MartyStrat54 regarding tubes. There's a wealth of information about tubes in that one thread - it's a great read.

 

[Jethro Rocker]

Likely a microphonic pre tube. For $15 as other members mentioned, you need spares anyways. Try turning the amp up with no source, no guitar or cable. If you tap the top of the amp it may well ring jnto feedback.
That's a microphonic tube. Likely V2 but start at 1 and change them 1 at a time.

 

[Dmann]

Tubes are consumable and having spare tubes is akin to having spare strings for a guitar. Price of amp has zero bearing on tube life, they can go at any time, it's just the reality of what they are.

Also, problems with a tube amp are almost always tube related.

Additionally Tubes on new Marshall amps are warranted for 3 months, so take the amp back to where you bought it, and they will check and fix the amp, replacing tubes if required.

Pro tip: Always have a full set of spare tubes.

 

[harry345]

I’ll test the tubes tomorrow and see which is the culprit. Thanks for the feedback and tips. Also wondering, would I have to replace all four preamp tubes or just the one?

 

[KraftyBob]

Preamp tubes can be replaced one at a time and you don’t bias them. It’s the power tubes that have to be replaced in pairs and then biased.

Report back and let us know what you find.

 

[Gene Ballzz]

Get one new preamp tube. Start at one end of the row of them, by removing one original tube and replacing it with the new one. If the problem goes away, you found the culprit. If the problem doesn't go away, put the original tube back into that socket and try your "new" tube in the next socket. Repeat this process until "hopefully" the offending culprit is discovered!
Easy Peasy & Welcome To The Forum!
Gene