#1
I purchased a used JCM 900 4502 from ebay last spring and, being the idiot that I am, forgot to ask how old the tubes were. So now almost a year later I'm trying to get a smoother tone out of the amp. I am unable to crank it, so I use the lead gain for my distortion. Currently it is producing a somewhat sharp and staticy tone, and many of the lower notes (frets 1-12 on the heaviest 3 strings) sound rather muddy because of this. I don't have a lot of experience with tubes amps, and this is why I'm not sure what to do. Are JCMs normally this harsh? Are the tubes old? Do I replace the power tubes or the preamp tubes? Any help would be appreciated!
Jackson DKMG Dinky
Marshall JCM 900 50 watt Combo
MXR Carbon Copy Analog Delay
#2
It is probably time for new tubes. The 900 is not the smoothest amp out there but what you are describing is a bit unusual. Tubes can go whenever so you were not an idiot for not asking how old the tubes were.

What I would do, if you are fairly certain it is nothing else in your rig causing the problem, is to replace one or a few preamp tubes first and see if that clears it up. If not, move on to the power tubes.
#3
It's not a totally unbearable static, and I'm okay with having a bit of an edge to the tone, but when I look at videos on Youtube (even despite people using subpar microphones to record sound), I just can't get the same clarity of tone.

Do preamp tubes need to be biased? And if I purchase the same type of power tubes currently in the amp does it need to be biased then?
Jackson DKMG Dinky
Marshall JCM 900 50 watt Combo
MXR Carbon Copy Analog Delay
Last edited by Mike-Dude at Nov 1, 2009,
#4
Preamp tubes do not need to be biased.

Power tubes do...in most cases. You will want to buy matched sets but it is best to have their voltages matched up as closely as possible as they work in pairs.


On the preamp thing, if you feel like it you can take the cylinder cases off and tap on each preamp with the eraser end of a pencil (or similar) and see if you can hear anything noticeable through the speaker. Unplug the guitar and turn your amp up a bit.
Last edited by 311ZOSOVHJH at Nov 1, 2009,
#5
I will give it a try later this week. Thanks for the help!
Jackson DKMG Dinky
Marshall JCM 900 50 watt Combo
MXR Carbon Copy Analog Delay
#7
since this is a tube amp thread i'll ask some questions. two of my tubes burned out. basically ate the dirt. so i bought two and the guy told me to install em and then let my amp sit on standby for like an hour. well i did that and then a few hours later i let it warm up again and tried to play. i was on the gain channel for about 2 minutes and then a bright light flashed and my amp died. is it gone for good? i don't have life insurance and can't really afford a funeral :/
#8
afallenone - you really need to start your own thread.

Responses to yours could get confused with Mike-Dudes and the thread could get away from itself. But basically it sounds like to me that those two tubes were NOT biased to work together and you had a power tube red plate and yes you have damage so do not turn it on anymore.