JCM 2555sl questions...

[GibsonSlash]

jcmjmp, I'm thinking of bring the amp to get retubed, checked, and biased, will the amp tech normally know this thing? or the have to be let known on how to, because it's a Jubilee?

 

[Jack71]

Steelhorse - The effect loop on the jubilee is not buffered. When nothing is plugged into the effect send/return, it is the jack's normally closed connection that's letting the signal through from V2b to V3a. The jack itself has a built-in switch, hence the normally closed term I've been using. When nothing is plugged in, the jack's switch is closed letting signal through. That is considered the normal state, which is why its called a normally closed switch on the jack.

With full access to the switch's internals and a cable inserted, you can now spray contact cleaner right on the switch's contacts because they are now open (because a jack is plugged in. Cleaning those contact points are very important on the jubilee. It is the #1 source of intermittent behaviour and volume drops and swells.

Does this also apply to the 2555sl? I know it's a replica of the 100 watt Jubilee but is it also the identical fx loop?

 

[jcmjmp]

2555sl is the same as a 2555 jubilee

 

[smogfalls]

hey guys, lots of useful info here. I also own a JCM2555SL and was wondering if anyone else had noticed that when you turn up the bass pot, the bass sort of increases subtley and then after about 8 it really jumps and the amount of bass pretty much doubles! It's not a problem or anything, I'm just wondering if thats normal behaviour?
Thanks alot,
Xan

P.S: I use Winged C 34's for the Power Section and a Gold Tung Sol in V1, and JJ's in V2 & V3... this combination seems to seriously kick ass!

 

[jcmjmp]

Maybe the bass pot is bad. Change it and make sure you get a linear taper pot. If it doesn't make a difference, change it back to the original. Did you try cleaning it?

 

[smogfalls]

thanks for the advice but this is a totally brand new pot that I literally installed 10 minutes before posting that. The original pot also did a similar thing... the bass suddenly jumps up at the end of the pot's range. It can make it hard to dial in the precise amount of bass as moving it even very slightly seems to really change alot once the control is above about 7 or 8.
The pot should be the correct replacement... it's a Dual Gang 1Meg Log pot and I certainly soldered it up correctly. I got it directly from Marshall themselves and they said it was a Logarithmic that I needed rather than Linear. Do you think this is wrong and maybe a Linear pot would work better?
Thanks again,
Xan

 

[jcmjmp]

thanks for the advice but this is a totally brand new pot that I literally installed 10 minutes before posting that. The original pot also did a similar thing... the bass suddenly jumps up at the end of the pot's range. It can make it hard to dial in the precise amount of bass as moving it even very slightly seems to really change alot once the control is above about 7 or 8.
The pot should be the correct replacement... it's a Dual Gang 1Meg Log pot and I certainly soldered it up correctly. I got it directly from Marshall themselves and they said it was a Logarithmic that I needed rather than Linear. Do you think this is wrong and maybe a Linear pot would work better?
Thanks again,
Xan

I suggested linear taper since you find the current taper weird.

The bass pot doesn't do much until you reach the 1/2 way mark. You could also try a resistor in parralel.

 

[Primo]

I've been using my JCM Slash for several years now....hundreds of gigs....the head has even fell off my cab twice (both of which were insane gigs on very week stages). I've had to change one output jack (to cabinet) after one of said instances, but all works fine.

Well, until a few months ago. The amp has been retubed and biased by Sonny Landreth's guitar tech, so I know that he did a good job, but I sometimes get volume drops. He could find nothing wrong with the amp.

Based on this thread, I'm thinking it's the dirty effects loop. I run a Boss EQ with 2 db boost through the effects loop. Once, a stupid engineer ran a line out of protools right into the front end of my amp (to fatten/rerecord an already tracked guitar part). When I noticed the amp was suckin bigtime, I unplugged the cable fast.

So, if the amp acts up with the effects unit unplugged, then it's the dirty connectors?

Thanks for the help... I'm brand new to the forum.

Rock on,

Primo

 

[jcmjmp]

So, if the amp acts up with the effects unit unplugged, then it's the dirty connectors?

Yes.

 

[siav]

Steelhorse - The effect loop on the jubilee is not buffered. When nothing is plugged into the effect send/return, it is the jack's normally closed connection that's letting the signal through from V2b to V3a. The jack itself has a built-in switch, hence the normally closed term I've been using. When nothing is plugged in, the jack's switch is closed letting signal through. That is considered the normal state, which is why its called a normally closed switch on the jack.

With full access to the switch's internals and a cable inserted, you can now spray contact cleaner right on the switch's contacts because they are now open (because a jack is plugged in. Cleaning those contact points are very important on the jubilee. It is the #1 source of intermittent behaviour and volume drops and swells.

Hi,
I had a couple of problems of volume drop with my 2555sl, I have changed the old power tubes and the technician told me to plug a short cable in the Sent and return plugs when not used...

 

[Primo]

Thanks for the answer guys.... I'll clean the connectors from within if it starts up again, and otherwise jump the send and return.

Replaced power tubes thinking it was that, but that didn't work.

Thought my transformers were dying, but now have hope.


Rock on,

Primo