#1
My Marshall JCM 2000 TSL 100 has been making a horrible screeching sound when I try to turn the volume past 2. This happens only on the lead and crunch channels. This is the first tube amp I've owned and still kind of new to it, so I'm not very knowledgeable about how they work, but I'm thinking its a microphonic tube, especially since their over 15 years old. I do not get the screech when the guitar is unplugged or if its volume dial is turned all the way down (I'm using a fender strat). I'm running the head with a jcm 900 lead 1960 4x12 cab.

I tapped both the pre amp and power tubes with the amp on and didn't get any sound coming from the speakers, but I'm not 100% sure of how a broken tube sounds and had a hard time hearing them apart.

Can anyone with more knowledge help me out? Also, how much would it cost to fix? I know how to replace tubes but if it's more than that, would it be best for me to just take it to a shop since I really wouldn't know what I would be doing.

Oh, and sometimes when switching between the lead and clean channel, there's a loud popping sound, I have a suspicion that that's not right.
Last edited by jcon1123 at Oct 18, 2015,
#3
No, the only way to stop it is to turn down the volume on either the guitar or amp.
#4
Have you tried stepping away from the amp?
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#5
Yes, At first I thought that was the problem, since my room is so small. I went out into the hallway and into a whole other area of the house. It was still really bad. The chord ended up getting pulled out of the socket because I went too far.
Last edited by jcon1123 at Oct 18, 2015,
#7
It sounds like a tube going bad. You can do a tap test on the preamp tubes to see if they are going microphonic and if none of them are bad it might be the power tubes. My Laney was doing more or less the same thing and it was a blown screen resistor and a bad power tube. Got the resistors replaced and I swapped out the tubes and rebiased and I was good to go.
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#8
it sounds like you have a bad preamp tube. since you bought it used, I'd highly suggest replacing them all. You can just buy 1-2 12ax7's and roll them until you find the culprit

I use a mix of New mullard 12ax7 in V1 and JJ ECC83 in V2-3 and a Sovtek 12ax7LPS in my JCM2000 DSL100. I use SED EL34, but they are really expensive, JJ E34L (not JJEL34) are a good power tube for Marshall amps.
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#9
well, I did the tap test on the preamp tubes, and nothing came out of the speakers, come to think of it, the power tubes sounded slightly different from each other, but as I said, it was hard for me to hear any significant difference. Also, what is a screen resistor?
#10
Quote by Robbgnarly
it sounds like you have a bad preamp tube. since you bought it used, I'd highly suggest replacing them all. You can just buy 1-2 12ax7's and roll them until you find the culprit

I use a mix of New mullard 12ax7 in V1 and JJ ECC83 in V2-3 and a Sovtek 12ax7LPS in my JCM2000 DSL100. I use SED EL34, but they are really expensive, JJ E34L (not JJEL34) are a good power tube for Marshall amps.


I think I'll try that first. what about the power tubes, should I replace them too?
#11
I have a Marshall 6101, which is a 3 channel amp, same as the TSL. I get the same popping noise on mine, again only occasionally. I've been told it's a static build up in the channel changing relay that seems to be peculiar to the older 3 channel Marshalls, but is nothing to worry about. If you turn the amp to stand-by & toggle between the 3 channels about 6 or 7 times, it should get rid of the popping for a while. Pain in the arse I know!
On the screeching problem, try with a different guitar, maybe with humbuckers, to see if it's your guitar causing the problem, not the amp. Also, try less gain & see if that helps. If not, buy one pre amp tube & swap it with each of the current ones in turn, see if you can find the problem tube. Don't forget to turn the amp FULLY off before swapping them, otherwise it'll be your lights going out permanently!!
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Last edited by anguskilminster at Oct 18, 2015,
#12
It's an internal component on your power section between the voltage supply and the screen grid on your tubes. I'd take it to a tech, get a full retube, bias and service which would pick up issues like screen resistors.
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#13
already tried turning down the gain, and switching guitars has only given a different but equally horrid screech. Since money's real tight Right now, I'm probably going to go with switching some of the preamp tubes first and see if that helps.
Last edited by jcon1123 at Oct 18, 2015,
#14
Quote by jcon1123
well, I did the tap test on the preamp tubes, and nothing came out of the speakers, come to think of it, the power tubes sounded slightly different from each other, but as I said, it was hard for me to hear any significant difference. Also, what is a screen resistor?

it is a component on the PCB.
try swapping the preamp first. the tubes just get pulled straight out and pushed straight in
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
#15
I'd replace all of the tubes
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
#16
I'm now noticing a faint high pitch sound when playing at under volume 2.
Last edited by jcon1123 at Oct 18, 2015,
#17
Quote by jcon1123
I'm now noticing a faint high pitch sound when playing at under volume 2.

get some tubes
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
#19
Have you tried tapping on the power tubes? They go microphonic too.
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#20
yeah but they didn't make any sound. im gonna go ahead and replace them all though.
#21
It's an ancient bump, I know, but after I found out the problem, I forgot about this thread. It turned out that the problem actually was my guitar, It was really bad feedback caused by low pickups and high action on my guitar.

I went to get my tubes tested and they were just fine, they were actually top of the line and costed around 200 for a pair, the guy even said that if he would of known, he would have kept them for himself lol.

Anyway, A good friend of mine fixed my guitar up for me, lowered the action and raised the pickups, and it's much better now. That was maybe a few days after my last post, and I'd thought I'd bring this thread closure since I was not able to find the exact same problem I had online.