Jcm 900 Mk iii 2500 new purchase

JCM Range, 800s, 900s, 2000/DSL, 2000/TSL

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Mrmaxelcat
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Jcm 900 Mk iii 2500 new purchase

Postby Mrmaxelcat » 28 Apr 2017, 05:56

I just picked up a 900 mk iii 50 w second hand. I'd had my eye out for one for a while. It's a great amp though one thing I noticed is that the volume seems tame on the master until about 4 whereas my 800 is ear splitting at this setting. Anyone here own one or has experience, what volume do you think would be band practice level? For a rock band that is. I know this is pretty subjective
I don't use the loop though I notice it's set on high level??
Any help or advice would be greatly appreciated.

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Re: Jcm 900 Mk iii 2500 new purchase

Postby Anitoli » 28 Apr 2017, 06:39

The taper of the volume pot can have allot to do with how loud it sounds at a given number on the dial. Some amps come on very loud very fast with a small amount of pot travel but then don't really give too much more as you keep turning it up while others you need to really turn it up to get it to crank. Does the amp have channel volumes? These can also affect the overall loudness on the amp if they are too low. Post a pic of the fronts control panel, i would like to see the layout and that would tell me more. Also have you tried just turning it up higher?

The loop level shouldn't matter if theres nothing in it. That is there to buffer effect pedals verses rack units differing impedance levels.

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Re: Jcm 900 Mk iii 2500 new purchase

Postby Roger » 28 Apr 2017, 07:07

Nice amp! congrats.
I also have an MK3 2500. As you say the master volume level is subjective. I think there are many factors that influence. Guitar. Place. Or how strong the drummer is. I don't play in any band now but sometimes I play with a drummer that will crush the battery very hard. I had to play the master volume at 8 with my LP standard.(He is grazy). If you do not use the loop connect a cable jack between the send and the return. More punch. Can you post a photo of the front panel?. Do you know what year it is? If you do not know, give us the serial number. Lately I only played with this amp.
:whatever. SORRY for english level
MARSHALL:TSL100/6100LM/JCM900 MKIII 50w/VM2466/JVM1H/
Lead12 5005(moded)
Lead 1960A/1965A/Vintage 1936/Lead 1936
Blackheart BH15H(moded) 7W-15W
Madamp J5MK1(moded 5W-10W)
Arial 4X12 60W
Peavey XRD680
:Cheers

Mrmaxelcat
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Re: Jcm 900 Mk iii 2500 new purchase

Postby Mrmaxelcat » 28 Apr 2017, 09:41

Hi, The serial number begins with Y so I'd say it's a 1990.
Here is a photo of the front panel.
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Mrmaxelcat
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Re: Jcm 900 Mk iii 2500 new purchase

Postby Mrmaxelcat » 28 Apr 2017, 09:56

I'll have to try cranking it next time I have band practice. I also played in a band with a drummer who used to like to beat the hell out of his kit, I had to run my VM 2266 at 7 master vol. just for band practice sometimes. He was also crazy. I've read about people putting a jump lead in the loop to link it, does this add more gain ? what I don't need is more gain, I prefer to keep the gain lower on the amp and use a tube screamer to push it, that way I can clean up the sound using the guitar volume.
Also what about the low / half power switch? What are the opinions on this, does it really let you get a nice driven power stage at lower volume or is there a compromise with the tone?
Any thoughts?

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Re: Jcm 900 Mk iii 2500 new purchase

Postby Roger » 28 Apr 2017, 10:21

The jump lead in the loop to link it doesn't add more gain. A little more presence/volume/punch. With the jump lead in the loop to link it adjust the loop pot at your preference. In low power output mode the amp has a little more gain.
:whatever. SORRY for english level
MARSHALL:TSL100/6100LM/JCM900 MKIII 50w/VM2466/JVM1H/
Lead12 5005(moded)
Lead 1960A/1965A/Vintage 1936/Lead 1936
Blackheart BH15H(moded) 7W-15W
Madamp J5MK1(moded 5W-10W)
Arial 4X12 60W
Peavey XRD680
:Cheers

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Re: Jcm 900 Mk iii 2500 new purchase

Postby Mrmaxelcat » 28 Apr 2017, 13:25

Ok thanks, I'll have to try the loop mod.
Next thing I need to do is change the tubes for some mullards and bias it. Also the tolex has some blue paint patches that I need to try cleaning off.
Cheers

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Re: Jcm 900 Mk iii 2500 new purchase

Postby Anitoli » 28 Apr 2017, 18:10

The low/half power switch changes the tubes power configuration. On half power the screen grid is not tied to the B+ supply rail but tied to the same point as the anode. This reduces the power somewhat but prolly not a full 25 watt reduction. It does make it sound different. Try it both ways and see what you like better. Just remember to put the amp on standby before changing the switch.

http://drtube.com/schematics/marshall/c ... 0-iss1.pdf

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Re: Jcm 900 Mk iii 2500 new purchase

Postby Roger » 29 Apr 2017, 01:53

Mrmaxelcat wrote:Ok thanks, I'll have to try the loop mod.
Next thing I need to do is change the tubes for some mullards and bias it. Also the tolex has some blue paint patches that I need to try cleaning off.
Cheers


I adjust the Bias at 42 mA at 428V on the plate. It's hot. If you want less gain on the power stage you can adjust it with Lower amperage. For example at 30 mA or 32 mA. I use this graphics.
http://www.tedweber.com/webervst/tubes1/calcbias.htm
:whatever. SORRY for english level
MARSHALL:TSL100/6100LM/JCM900 MKIII 50w/VM2466/JVM1H/
Lead12 5005(moded)
Lead 1960A/1965A/Vintage 1936/Lead 1936
Blackheart BH15H(moded) 7W-15W
Madamp J5MK1(moded 5W-10W)
Arial 4X12 60W
Peavey XRD680
:Cheers

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Re: Jcm 900 Mk iii 2500 new purchase

Postby Anitoli » 29 Apr 2017, 04:44

Multiply the cathode current by the plate voltage to get dissipation ( watts). For example, 460v x .035Ma = 16.5 W. It is that easy to find the wattage.

Mrmaxelcat
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Re: Jcm 900 Mk iii 2500 new purchase

Postby Mrmaxelcat » 01 May 2017, 11:23

Thanks guys. I'll get round to biasing the amp when I change the valves. I tried the jump lead in the effects loop and set the dial on the back to noon. It did make it louder, but I can't make up my mind if it made it sound better? Being louder will give the impression of a more present punchy sound but then so would turning up the master volume.
I haven't been able to open the amp up by cranking it as yet, hopefully soon.

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