I have a relatively new (year old) Marshall JCM2000 TSL122. It's a great, kick ass amp, but lately it has been making some noises intermittently; generally a low-pitched "hiss" and various low-volume pops. It seems to happen only when it is pretty hot, too. I took it to the place where I bought it and it wouldn't do it when I brought it there, so they thought I had lost my marbles. I am assuming it might have something to do with the output tubes, cuz they make sort of the same noise when I tap on a couple of those. I just want to see if someone has a second opinion before I shell out a wad of cash for a quartet of Svetlanas. I would hope that those wouldn't go out so quickly.....PLease reply if you have the slightest inkling of what I might be able to do...

If they make the same noise when you tap on them ,then you can be pretty sure it's a tube problem . Are they the original tubes ? Do you play hard at least 3 or 4 times a week ? If you've answered yes to both these questions then your probably due for a new set of power tubes and possibly pre-amp tubes 1 and 2 .

Ok, thanks....I assumed this was the problem, but I'm a cheap-ass and was hoping it might be something else....Now that I have ascertained the pronlem, does anybody know who has the best prices for a quartet of Svetlana EL-34's? And who makes the best 12ax7's?

Hey,
for my Marshall head I changed the preamp tubes to
the first in line is a MesaBoogie SPAX7A.
This tube is metal jacketed and selected
to be the quietest of all MB preamp tubes.
The next two in line are Groove Tube 12ax7's.
I was having hissing problems which went
away when I put the MB SPAX7A in.

Well, I changed out the power tubes with a matched quartet of svetlanas,and, well, the problem hasn't exactly gone away, though it does seem to be considerably louder. The static-y noise definitely increases as volume is increased also. I still haven't changed out my preamp tubes; I ordered some NOS sylvania-phillips 12ax7a's, bought they are a week on back-order. I assume that the original el-34's are still good, although they had decreased output. If the preamp tube change doesn't remedy the problem I suppose I will have to drop a load for whatever repair needs to be made.

WARNING!!
I do hope you biased your amp after changing the power tubes, if not take them out NOW!! and put the old ones back in. From what you said it sounds like a preamp tube, you can trace this down by taking the back off of the amp and gently tapping the tubes while the amp is on. Please use some common sense here, don't try this in the bathtub or with wet hands while grounded.

You MUST bias your Marshall if you change the power tubes, transformers can be expensive. Good Luck....MH

well, the amp was made for use with el-34's, so i assume it needs no re-biasing....I may be dumb, but I'm not a moron.....When I tapped some of the tubes (preamp and output), it did make the problem temporarily worse....Maybe the problem will be remedied when I recieve the preamp tubes (and I hope they sound good...I've never really used NOS tubes).

Mr. Hanky's right. You gotta re-bias even though you're still using EL-34's . All power tubes have slightly differing outputs.If your playing your amp hard and you've had it a year it's probably time for a tune up . They're very sensitive animals ... that's why they sound so f#%king good . There may be a little dust or dirt in a tube socket,a loose connection , etc.It's probably no big deal.

Hold on now! I did not intend to insult anyone, please do not take it that way.

I wanted you to tap on the tube to determine if it was bad, this sometimes happens with pre amp tubes.

You still have to bias your amp, period. If you change the power tubes YOU MUST RE-BIAS!.
I'm just trying to save you more headaches in the future. Some amps do not require re-biasing (Boogie, Top Hat,ect..) but I have never met a Marshall that did not need re-biasing.

I have to agree with Mr. Hanky and klasaine.
If you change power tubes, check the bias.
You'd be surprised, but you can buy a brand new tube amp and if you check the bias sometimes it is incorrect.
I sold my JCM-800 a couple of years ago, and now I have a Carvin MTS-3212, and was experiencing some strange noises recently. The solution in my case was to change the position of the first preamp tube with either of the following four tubes (all of them 12AX7). My problem did not involve the four power tubes.
Remember to be EXTREMELY CAREFUL when working with a tube amp.

O.k., I appologize if my reply was harsh, my bad. I have rebiased amps before for a different tube (5881->el-34), but never have for the same tube. How would I know what voltage to reset the amp to?

Since the last post is faily old, I assume the problem has been rectified. But to those browsing this topic, when replacing power tubes, the amp needs to be rebiased for the new tubes even if you're using the same type of tube (like changing old EL-34s for new EL-34s--could be different brands, different voltage ratings internally, from different countries or manufacturers)

The exceptions are fixed bias amps like Boogies, but you MUST use Boogie tubes when retubing, because their tubes are rated for optimum performance in Boogie amps.

Also, Groove Tubes and Rivera rate their tubes with a numeric system, so if you are retubing with the EXACT same GT or Rivera tube number (i.e. GT #7s), you won't need to rebias if the amp was properly biased in the first place.

That numeric scale that GT uses SLIDES over time . A #7 6L6 in March of '98 will be different than a #7 6L6 in June of '01 . Not greatly different but enough to possibly make a difference . My amp guy told me this and he built Groove Tube Soul-O 75's for years . He has also re - biased my fixed bias Boogie .22 several times .

OH, sorry about the absence of an update. They sent it away twice from guitar center and they couldn't seem to fix the problem either time, so they gave up on it. They gave me a brand new amp with a new warranty! Talk about customer service! The new amp has no percievable problems and seems to be a little louder and stronger. I talked to the store manager, and though he said that the amp tech wouldn't really go into detail as to the source of the problem, from what he said, the main transformer was nearly shot. I know those are expensive, but they went out of their way to give me a new amp! I know we all crack on guitar center as being some sort of uber-music-mall, but they surely hooked me up.

oh yeah, sorry about being a bitch concerning the power tubes, I was clearly wrong (though I didn't put in new ones, the tech did, and I assume he rebiased it to proper voltage). Thanks for all of the help anyhow.

One thing about large chain music stores is that they CAN afford to give you a new amp when a problem such as the one you have experienced does happen. Besides, if anyone does happen to have a problem with Marshall amps and is getting no satisfaction, try calling Marshall in England. They will get you help through their distributor in New York, Korg USA who will definetly take care of you. Marshall is a great company for taking care of their patrons.