#1
Hey everyone!

Basically I've been having problems trying to get a decent sound out of my JCM 900.
Imo it sounds worse in ways that most practise amps sound better in.

Basically it can get a good crunch but lacks a quality sound.

I replaced the tubes with EHX EL34's 2yrs ago and during those 2yrs it wasn't played AT ALL for a straight 8 months (due to a blown T500 something valve output fuse).

It sounds thin as though the speakers are missing all depth. I'd describe it as sounding like paper.

Also I cannot get a sound remotely close to metal, partially due to the above problem but it also seems to lack bass, it has bass but it doesn't sound like proper bass, it's not full and it sounds cloudy. I'm not picky with my tone, this amp simply will not do metal period. My friend has an AVT that easily gets a brutal metal tone that my amp couldn't even dream off getting, it sounds like it has less gain although the gains not really the problem, it's the tone. It's hard to put the problem into words but the amp doesn't boom fullness at you, it's like it's been cut off like an mp3 or something, it sounds drained and weak. The quality of the sound is still decent however on the clean I swear it sounds cloudier than it used too.

Any ideas on what is causing this horrid tone?
I've owned the amp for 5yrs and I can't really remember how it sounded back then as I wasn't that good and had also never tried to play anything metal through it. Could the T500 be a clue on something that's not functioning correctly?
The channel select button on the head has also broke a while back and I've had to use the footswitch to swap channels.

Cheers!
#2
did u bias the amp wen u changed the tubes
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#4
The tubes were biased. I bought a matched pair of power tubes so I didn't need to rebias for the second pair (I put the second pair of power tubes in 6 months ago and the sound didn't change at all, I think it might've sounded a tiny bit better but it was so minuscule that it was probably all in my head. The preamp tubes were also replaced. I've ran it at half power the entire time. Save for maybe 3 times or so (I'd flick it back after I'd finished playing).

The speakers are celestion greenbacks (2x12 combo) which I've been told should make them amp sound better than most, it's a shame about that haha.

It's a 4502 Hi gain Dual reverb model.
Last edited by ILLaViTaR at Jul 26, 2010,
#6
that kinda sounds like how a jcm900 sounds.
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#7
Quote by 311ZOSOVHJH
when you switched to half power did you also reduce ohms in half?

could be an output transformer issue.....that is my guess at this point but i'm not an amp tech.


Ah crap I didn't touch the ohm button.

I had an output transformer replaced in it a couple of years back, but I think I have done it since then.

It's currently set on 8 ohms, running at half power. ach speaker is 16ohms and it's wired in parallel.
Last edited by ILLaViTaR at Jul 26, 2010,
#8
just cause you switched the same tubes in, doesn't mean you shouldn't rebias. you should always rebias no matter what.

and yeah it kind of almost sounds like the output transformer might be going again.

also try using an overdrive in front of it
#9
What volume are you testing all this at?

I was under the impression those amps had to be cranked pretty loud to get the full potential out of them.
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#10
Ah, the tech told me to get the matching pair because I wouldn't need to rebias. Guess I won't be going back to him.

Is there any way I can tell if it's the output transformer? So is this what's affecting my sound?

Cheers
#11
yeah just cause the pair is matched, it still needs the bias checked any time you change power tubes. especially with something like a marshall. the only amps you can go w/o biasing are mesa's and certain boutique amps. the tech is seriously an idiot, not your fault
#13
I think the half volume switch makes it sound bad, at least to me. For the lower headroom that you gain it just ****s up the tone.
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#14
Quote by 311ZOSOVHJH
^that's not really true

there are tons of amps that are cathode bias and non-adjustable fixed bias


TS - get your amp biased and if it still causes you problems consider another OT or just getting a new amp.



thats why i said except certain amps.......... a jcm 900 does not fall into the category by any means
#15
Quote by handbanana
thats why i said except certain amps..........

No. You said:

"the only amps you can go w/o biasing are mesa's and certain boutique amps"

Which is not true. A Peavey Valveking, for example, is not a boutique amp by any means.
#16
Not putting down headbanana, but I would listen to a guitar tech before someone on a forum. As long as you replace with matching tubes, most amps shouldn't need to be re-biased each time you switch the tubes.
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#17
i definitely wouldn't say "most amps" wouldn't need to be rebiased, just certain ones
i'd say more amps definitely need it than not
#18
Sounds like my Laney at low volumes lol. Really need to turn those up, unfortunately.
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#19
Do you clean out your amplifier regularly? Dust, dirt, and grime build up can affect the sound of your amplifier. Check all the solder joints and make sure there are no loose wires. An amp's sound can degrade over time without the proper maintenance.
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#20
Unless your tubes are redplating you don't need to rebias. Tonal benefits of rebiasing are debatable with some hearing more benefits than others. And it is possible to buy tubes that will line up with the old ones bias wise. That tech knows exactly what he's doing, at least in this regard.
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Last edited by mmolteratx at Jul 27, 2010,
#21
especially if your amp doesn't have a full wide range bias pot its only going to be adjusting it within a safe range anyway. If your bias was completely off you'd know it. It wouldn't sound thin it would sound eff'd up and completely broken.
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#22
JCM900s are really easy to bias, you can do it close enough without a bias probe. Adjust the bias trimpot until the voltage between R26 and R27 is 39V. Can't get much easier than that.
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#23
Quote by ILLaViTaR
Ah crap I didn't touch the ohm button.

I had an output transformer replaced in it a couple of years back, but I think I have done it since then.

It's currently set on 8 ohms, running at half power. ach speaker is 16ohms and it's wired in parallel.


Hey guys.

What's the correct ohms for my amp?

It's currently set to 8 ohms, and running at half power. Speakers are 16ohms each and are wired in parallel.

There's no information I can find regarding ohms and half power.
#24
Half power on a JCM900 means it is running the output as triodes instead of pentodes. The impedance remains the same. So two 16 ohm speakers in parallel makes 8 ohm so the 8 ohm setting on the amp is correct.
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#25
Quote by handbanana
i definitely wouldn't say "most amps" wouldn't need to be rebiased, just certain ones
i'd say more amps definitely need it than not

If the two tubes you put in are from the same matched quartett as the ones that were in the amp before (and it was biased correctly), a re-bias is not necessary. Because the new tubes are practically identical in their electrical properties as the old ones.

Checking the bias every couple of months or so is a good idea, because the bias can drift. This does not have anything to do with swapping tubes though.