questions regarding my new marshall amp

i just brought a second hand marshall jcm900 4100 head and cab off ebay, and was aware at the time of purchace that there were problems with it. i was laso aware that the valves (tubes) hadnt been changed for nearly five years which i know is a long time. when i collected the amp and got it home and started to strum away, i realised the the b string and top E string sounded kinda out of tune, no matter what fret i played. i was just wondering if this was a sign that it was time for a change of valves (tubes) :smiley:
also, the rewverb unit from the amp was removed by the previous owner, i was also wondering if it is possible to buy spring treverb units on their own, does anybody have any URLs they care to share with me? :smiley:

also any other information regarding the maintainance and general care of valves (tube) amps (as this is my first one) would be grand!!!

long-winded i know but if anybody could provide assisstance in one of those areas, i wsill be more than grateful

thank you

I have a jcm 9000. I don’t think that tubes cause notes to sound out of tune. Tone would certainly be affected, unless of course you normally have a bunch of static in your playing…:wink:

I’m going to replace mine whenever the god$ smile upon me…maybe a nifty Vox…

[spinal tap] These go to eleven! [/spinal tap]

A google search on “marshall amplifier parts spring reverb” will get you a lot of online parts hits. I didn’t see a reverb unit for your amp listed on the first page of hits but a couple of the sites said to call for parts availability. I’ve seen spring reverb units occasionally in places that sell electronic surplus parts, but that’s kind of hit and miss.

You shouldn’t have to replace the tubes every five years. They should last a lot longer than that. If you see any tubes with a bit of a purple glow then they are on their way out. Other than that I probably wouldn’t replace any until they give out, unless you absolutely need this amp for stage work and need to do a lot of preventative maintenance on it to make sure it always works.

I’m thinking your tuning problem has something to do with the part behind the guitar :wink:

the guitar is fine through my other marshall, sounds perfect. as far as the tubes appear at the moment, they are glowing a beautiful orangey red, and are lovely and warm. i cant wait till the cold weather kicks in soon so i can sit next to my amp and warm myself up :smiley:

In a related note: if you’re just practicing at home, you can pull one or more of the power tubes to reduce the volume without changing the tone too much. I would guess you end up with tubes that aren’t 100% matched for wear but that the tubes last longer overall.

another question which iforgot to include in the opening post was, the presence knob doesnt seem to make any difference at all to the sound, will this only work with a reverb unit inside?
what does this knob actually do?

The prescence knob adjusts the high freguency response on the feedback circuit. Some of the amp’s output is fed back to an earlier part of the amp that is about 180 degrees out of phase with the output. This helps flatten out the amp’s frequency response. The feedback knob adjusts how much of the very high end gets fed back, and basically it adjusts the “muddiness” of the sound.

The notes sounding out of tune might be due to a bad filter capacitor. This would allow a little 50Hz hum (from the wall outlet) to interfere with the sounds from the quitar. Then again, a bad coupling capacitor might cause problems that could match your description.

You might consider having a tech go through the amp. My guess is that you would be surprised by how much better it sounds afterwards.

Here are some good resources on tube amps: