#1
Recently I got my JCM900 back from the tech, and now it's got this annoying hum on channel B, which only occurs when the volume on my guitar is up. There's no effects running through it, and I've checked with 2 guitars and 2 cables, and it was still there.

Before I sent it off for repairs, there was no noise whatsoever (I could turn it up to like 7 and still have no noise from it) It's really bugging the crap out of me, and I was wondering if it might be anything really simple I can just pretty much poke and fix it.

I'll be calling the tech on Monday to ask them why the hell it's humming like this when it wasn't before.

I don't think it's a microphonic preamp tube, cause I've checked them all, other than the phase inverter.


Edit: Just demonstrated the first reason you should always have a cab that your head fits on properly.... Almost knocked the head onto the floor, one of the feet slipped off the side of the cab and it all kinda fell to the side. no tubes damaged in the process thank god.
My Gear
Guitars
Cort VX-2V
Basses
Stagg BC300
Amps
Marshall JCM900 4100 DR
Marshall 1965A
Hartke HA3500 Combo
Last edited by Jesstaa at Jun 27, 2009,
#2
How new are the tubes? A microphonic preamp tube often causes a lot of hum.
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#3
This is like the first time I've used the preamp tubes other than the 10 minutes I used them after I got them and before I sent it off for repairs, and it was fine then, and as I said, the only tube I haven't checked was the Phase Inverter, and it'd better not be that one, it's the only balanced tube I've got left. I'll check it tomorrow.

Only way I can think it would've become microphonic (Unless the tech ****ed with it) would be during the car ride.
My Gear
Guitars
Cort VX-2V
Basses
Stagg BC300
Amps
Marshall JCM900 4100 DR
Marshall 1965A
Hartke HA3500 Combo
#4
I just checked the PI tube, it was still humming when I switched it out.
My Gear
Guitars
Cort VX-2V
Basses
Stagg BC300
Amps
Marshall JCM900 4100 DR
Marshall 1965A
Hartke HA3500 Combo
#5
i have the same problem with my 4100. but i found that its only when you have the guitar volume turned up all the way. If i roll of even just a little the hum ceases. Is yours sort of a really low hum? not like a loud tube amp hum but like a really low frequency feed back?

also on my clean channel i recently came across a problem, where the volume is incredibly loud when turned down very low. Like, i can have my guitar half way up and the master volume and gain on 1 each and its as loud as it should be with them both at about 5

any ideas?
#6
it's probably how he routed some cable inside. one inch difference can be huge. Talk to him about it...not hard to fix. You open it up, plug it in and turn it on, and then move cables around till it goes away (with something non-conductive, hence this being called the chopstick method).
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#7
Yeah, it's a kinda low hum. I'll check the cables sometime later, maybe when I get some sort of rubber poking device so I don't fry myself with the 475+Volts running through the amp.

With that volume issue, you might wanna try changing the volume and gain pots in your amp...

If you replace them, you might as well replace all of them tbh, make sure you use really high quality ones, so they last.
My Gear
Guitars
Cort VX-2V
Basses
Stagg BC300
Amps
Marshall JCM900 4100 DR
Marshall 1965A
Hartke HA3500 Combo
Last edited by Jesstaa at Jun 28, 2009,
#8
Bleh, it's getting worse... I fear it may be a little more than just a wire sitting in the wrong place. Once I get the money I'll have to replace the caps and transformers ASAP. Anyone know what the best choice would be for caps? I've already decided upon Mercury transformers.

Edit: I'll try the chopstick method, I just gotta find something very non conductive sitting around, don't know if I have any rubber poking devices. Would a marker do the job? Lol.
My Gear
Guitars
Cort VX-2V
Basses
Stagg BC300
Amps
Marshall JCM900 4100 DR
Marshall 1965A
Hartke HA3500 Combo
Last edited by Jesstaa at Jun 30, 2009,
#9
Meh, tried the chopstick method, poked at everything that could be poked at, still no solution to the noise.
My Gear
Guitars
Cort VX-2V
Basses
Stagg BC300
Amps
Marshall JCM900 4100 DR
Marshall 1965A
Hartke HA3500 Combo