Marshall Amp Forum - The Workbench

Has my JMP been modded?

Mikeyg — Fri, 03 Sep 2010 05:45:09 GMT

Should've posted here first ...

http://www.marshallforum.com/lets-ta...en-modded.html

I've done some research, and the polarity switch does come stock to some shipping markets ... the white switch may have been to replace the original bad switch.

I have other gut shots if need be.

Short bottle Sovetec 5881's in a JCM 900!!

Ascension — Fri, 03 Sep 2010 04:16:05 GMT

I dropped a set of Mesa labeled Sovetec 5881's into the JCM 900 MK III 2501 combo I have and WOW! These ROCK in this amp!! I normally loath the tone with Sovetec power tubes but these things are warm tight fat and SWEET!
:headbanger::headbanger::naughty:

Help! My DSL's died on me!

Fat Pirate — Fri, 03 Sep 2010 01:57:05 GMT

Hey all. Can anyone shed some light onto this for me?

My JCM200 DSL401 just sort of powered off by itself while I was playing. Everything was plugged in fine, but the electricity just stopped running through it. Upon closer inspection I noticed a faint but definite burning smell coming from the back of the amp.

I'm afraid to take anything apart in case I mess things up. I know very little about the hardware of tube amps beyond letting the thing warm up and cool down before and after using it. I just play guitar!

I'd really appreciate any help you could give me with this.

Adding LED's to a footswich

musicman — Thu, 02 Sep 2010 23:28:44 GMT

Hi,

Hope someone can help...

I've just bought a Laney Lionheart L20 Head.

2 channels - clean & drive.

Reverb.

It comes with a 2 button footswitch without LED's.

I would like to add LED's to show when the drive channel is active & when the reverb is on.

I have some LED's and DPDT switches but can't figure the wiring.

With the clean channel engaged there is about 4v dc between the tip and sleeve, with the drive channel engaged there is 0v.

There is 0v between ring and sleeve no matter whether the reverb is on or off.

Is it possible to use the 4v to light the LED when the drive channel is active ?

Is it possible to use the 4v for the reverb LED.

If not I'll have to add a 9v battery.

Does anyone have a simple circuit please.

Thanks.

Newbie Changing Tubes JCM 900

Patrol42 — Thu, 02 Sep 2010 07:21:16 GMT

So it's going to be my first tube change because it's my first amp. I plan on using the resistor to ground method to adjust it. But I have a few questions

Do I have to do this for all the valves if they are all matched? I assume once you have one valve to where you want it, they'll all be to be at the same idle, right?

I am probably going with Electro harmonix 6l6s. If you guys recommend anything that is around EH tubes price range, that'd be great. Reliability is my concern, and they seem pretty sturdy.

I assume to discharge the caps in the amp all you have to do is take pin 1 to ground from the first tube 12ax7, right?

MG50dfx cooling fan woes

DayTripper — Wed, 01 Sep 2010 19:00:55 GMT

I have a MG50dfx and the cooling fan is very noisy and sounds like it's gonna crap out. I've searched all over the internet for a new fan and come up with nothing. I might resort to modifying and installing a generic electronics cooling fan but would really like to stick with a Marshall fan so I can sell the amp if needed. Anybody with ideas will be greatly appreciated.

Signal path

ynot — Wed, 01 Sep 2010 18:24:04 GMT

I didn't know where to post this,I hope this is OK.I'm not having amp problems & don't need help fixing/modding anything,I'm just curious & figured this would be the place to ask.Where does the signal path go & more specifically,in what order in a plexi type amp?For example,where is the EQ,presence,volumes located in the chain?Since there is no master,how does this work internally,is the power stage hard-wired to be wide open & the volume/"loudness" actually a preamp gain knob,or is the preamp fixed & the volume/"loudness" actually power amp gain?I'm guessing the EQ & presence are in different locations related to pre/power stages as well,where are they located?

For such a seemingly simple amp,it still leaves me with a few questions regarding gain staging.I apologize if this is covered somewhere in the forums already (& I'm sure it probably is) but I just couldn't find it.Thanks.

What the hell is a ground loop?

carnada — Wed, 01 Sep 2010 01:35:09 GMT

I hear a lot about ground loops and humm and buzz and ground loop.

What the hell is thiss!??

Good amp tech in Inland Empire?

Salsg — Wed, 01 Sep 2010 00:01:09 GMT

I need an amp tech to look at my 6100.

Anyone know one? The Amp Doctor is a no go, I've dealt with those idiots before and I wouldn't trust them at all.

One guitar, 3 amps, HTF???

Dead Jackson — Tue, 31 Aug 2010 23:56:14 GMT

Ok, this may sound completely stupid here, (uh, I'm a drummer,) but how do you wire the output of one guitar into three amps? I have seen and used the simple A/B boxes, that select either or both channels, but they don't have three outs, I have used a "Y" cable, I can surely make another, however, what would be the "correct" way to do it with regards to loss, impedance, and what-have-you. I know Pete Townsend, Johnny Ramone and others did it, I know in the case of Ampeg SVTs, (my personal favorite bass rigs,) there is an interesting little pair of jacks on the back that says "external amp in/out" which simply parallels the preamps of however many amps you plug together. You can daisy chain many of them together and which ever preamp you plug the bass into, that one controls the rest. Another fine little trick, SVTs have 2 channels, 2 separate volume controls, 2 separate tone stacks, and 4 inputs. You can jumper from one channel to the other and use both preamps and tone stacks to really get your tone together so to speak, and then pipe that out to all the rest of the amps.... REALLY handy for reggae festivals where we have an SVT rig on each side of the drum riser, and it don't mater which one the dude plugs into. Tada!
So, how do we do it with our beloved Marshalls without modding the amps? Is it just as simple as "Y-ing" the input cable?

And don't just make up some crap from supposition either, I want to hear from you guitar gods on here that actually do this, and I don't mean you A/B box dudes, to easy. I want that guy who runs 4 f***in' double stacks whether he's in the stadium opening for Judas Priest, or in the Coconut Cheaser in Hollywood for the A&R guy who isn't coming anyway! That's the guy who will know how to go to 11 times four! "That's 44 louder, init???" Nigel Tufnell, where are you?

Good amp techs in Michigan

DarthxTroy — Tue, 31 Aug 2010 23:49:07 GMT

Anybody know of any in the metro detroit area? I don't want to take my children to just anyone!

Marshall Valvestate 8100 - sound problem (bad tube?)

pitimor — Tue, 31 Aug 2010 20:33:10 GMT

Hi. I've got problem with my amp and I want to ask for help here because I'm an equipment noob (I just play lol) and a lot of experienced people seem to post here. So I've got Marshall Valvestate 8100 with one 4x12 cab, bought it rather cheaply from a friend. I've got one issue though. The sound on overdrive channel is just weak. For example, see this movie:

You Tube

. This guy has the same amp and a good powerful sound here. When my amp, on OD2 channel, cranked up in both gain and volume, sounds AC/DCish at best. Just like a classic overdrive, not a metal tone like in the video. I already thought about pickups, but I don't think it's the case because my Ibanez V8 has rather higher output than Gibson 498T in the video (though I might be wrong, as I said I'm an equipment noob). I suspect the tube - in this amp there's only one in preamp, but I heard that failing tubes crackle. And when I play with the volume, the crackling is very high and it doesn't happen on clean chanell which doesn't use tube. So can anyone confirm if that's the tube or some other thing? Thanks a lot in advance.

What the hell does JAN mean?

Dead Jackson — Tue, 31 Aug 2010 08:48:40 GMT

I've been looking at tube suppliers, and I got the NOS, (new old stock) thing, however, I have not been able to figure out what JAN means. Anybody? Also, while we're at it, in regards to NOS tubes which can get REAL expenesive quick, seems there are many different "builds" depending upon era of manufacture and purpose. Of course it stands to reason that tube elements made differently will sound different, yeah? So, who has the experience here to delineate the differences between, say, 6550A, 6550 H, the military specs, the stapled plates, the welded plates, the mica insulators, the plastic insulators, the round holes, the "other" holes, and what the hell is a "getter"? Go boys, GO!!!

Mod Safety

Joe2000 — Tue, 31 Aug 2010 07:12:53 GMT

I want to do the bright cap mod on my 2204. Is it safe enough to go in and just snip? Do I have to do anything else?
Thanks

6100 Very Noisy...help

Salsg — Tue, 31 Aug 2010 03:22:29 GMT

My 6100 seems to be acting up. There is a hum going on just turning it on, and it is much louder than any other of my amps.
When I switch over to the green and especially the red, it is very noisy, a lot noisier than my VM, even at max detail on HDR. This is even without a cable plugged into the amp. It is a gainy type of noise but so much it sounds like I am running a bunch of distortion pedals all on at once.
I've swapped out tubes, including the power tubes and all the preamp tubes.
What else could it be?

Power or output transformer? Caps?

I need it for a gig this weekend, we're playing in a small place, and I wanted to use one amp, hence the reason I got a 6100, all the tones I need in one amp.
Otherwise I will need to take my VM and my Fender Twin, I do need squeaky clean for several songs.