Need help biasing My marshall jcm 2000 dsl 100

[joedell71]

Hello I'm new to this site and was wondering if any one could help me with a question regarding biasing my amp.It is a marshall JCM 200 DSL 100.I have an ocilliscope ,sine wave generator and variac I have also purchased new tubes for it(EL 34 power tubes and 12 AX 7 pre amp tubes)My question is what kind of resistor do I use for the dummy load 8ohm 100 watt or 4ohm 100watt.Like I said I am new to this forum and amp repair/service in general so if someone could help me I'd greatly appreciate it.Also if you could tell me about any gremlins or things I should watch out for when biasing this amp (I already know about discharging the filter caps to avoid electrocution)Thanks again in advance for any help

[Enzo]

Use any dummy load you'd like - the amp has jacks and switches on the rear panel to match up to most anything you have.

There are the three little test points on the rear, they make it very easy to set the current you'd like. Just allow the tubes 15-20 minutes to fully warm up. There is no high voltage on these test pins, and you don't even have to take the chassis out of the cabinet to access them.

In my experience, the little bit of crossover distortion in these amps completely disappears into the rest of the distortion. These guitar amps are far from clean. Crossover distortion might be a good way to set up a hifi amplifier for your home stereo, but I don't find it an effective way to set up a guitar amp.

[Amp Kat]

Joe is this a new DSL 2000 or older model ? Enzo I don't think the new 2000 DSL's have the bias pots and test points in the back as the older ones do ? They got wind to that gig and I have a good feeling it was a big part of the tube shorts we encounter with this amp. I have a newer 2000 hanging around but haven't popped it yet.

[pbdog]

Please tell me more about the tube shorts & the bias.

Thanks pb

[joedell71]

Hi Enzo
Thanks for the reply. I have a couple of questions that I hope you dont mind answering(if I'm bothering you tell me to go jump in the lake) First could you be more specific about the location of the test points on the back.Second If I dont have to take the the chassis out of the cabinet how do I adjust the bias pot.Third how should the sine wave on the oscilliscope look when it properly biased should there be a little hump on the leading and trailing edge of the waveformy is that what you mean by crossover distortion(I think thats what its called).Once again sorry if I'm being a pest and if these questions seem stupid but like I said I'm doing this for the first time and any help is good help.
Thanks
Joe

[joedell71]

Hi
I got the amp in 1999 does that qualify as an older model?
Thanks
Joe

[Enzo]

Look at the rear of the amp. Pull the metal grille so you can see inside. There are four power tubes in a row towards the right. The tube sockets are on a little bare metal sub-chassis that sticks up from the main chassis maybe half an inch. Right so far?

More or less between the middle two power tubes, is there not a hole in the metal subchassis along the rear edge, and three straight pins sticking out? And to either side of the three pins there will be a small trim pot. Each trim pot sets the bias for two of the tubes. Oddly enough the pin on the right along with the trimmer on the right sets the two LEFT tubes, and the trimmer and pin on the left end set the RIGHT two tubes.

The center pin is ground.

Does you amp not have these?

From the center ground pin, there is a 1 ohm resistor to eash of the end pins. Those end pins are conected to the cathodes of the power tubes. SO you get the classic 1 ohm resistor thing where each millivolt of voltage there represents 1 milliamp of current.

When you said you had a scope and signal gen and wanted to set bias, I just assumed you wanted to use the crossover distortion method. otherwise the scope and gen would serve no purppose to setting bias.

A little notch roughly at zero crossing sounds like crossover distortion. But getting a large sine wave through a Marshall amp clean is a tough trick in my book. As a rule I don't concern myself with what it looks like. I set the tubes to the same current specs that most guys use. Try it yourself - set the bias a good bit hotter than your target, and then a good bit cooler, and each way listen to the overall sound. That will give you an idea of how much the bias alters tone - or doesn't.

[Amp Kat]

You have the older one the new ones are 2007 models. If your looking at the amp from the back on the left side which is where the power tubes are there are three silver pins sticking out that look like a cable plugs into. These are your bias test points. The middle is ground,the left is one side and the right pin is the other. With your meter set to millivolts put the black lead on the middle pin and the red lead on the left pin and read the voltage. It should be around 80 mv's or .080 volts DC. There are two light blue square small trimmer pots set back in the chassis that are PR1 and PR2. The left pot sets the left side and the right pot sets the right side. Tweak the left pot to around 78 mv's then leave the black lead on the middle pin and put the red lead on the right pin and read that voltage which should be again around 80mv's. Use the right pot to tweak that voltage to 78mv's and your done. This is the safest way to bias this amp and let me caution you of two things. One is do not just turn those pots until it sounds good because that is why IMO so many of these amps fail and have tube shorts is because they are biased to hot. Any time you put pots on an amp for the end-user to tweak it is doomed for failure and no pun intended on any one just the way it is. Second is when doing scope method and shunt method bias procedures for the first time you are putting yourself in a dangerous situation and your dealing with voltages that could possibly kill you but in all reality will most likely just knock the everliving crap out of you but still not very good. However if you must endeavor into the journey heres a good link to try. http://www.aikenamps.com/CrossoverNotchBiasing.html

[joedell71]

Hi amp Kat

Thanks for the reply It was very helpful.Thanks for the concern about me frying myself by doing the scope method of biasing. One of the first things I found out when starting this endeavor was about the lethal voltages hiding inside these amps and how to discharge the capacitors so you dont get killed.The only reason I was going to bias with the scope is because I'm new to this so I didnt know my amp could be done with just the voltage meter. I looked up amp biasing on the net and bought a book and went and got the equipment needed.Will biasing the the two pots to 78mvs give me a good consistant sound?.Have you ever biased one of these amps if so how did it sound at 78mvs? I play mostly "metal" type music with some hard rock mixed in for good measure do you think the amp will sound good biased to 78mvs for these types of music I like a good amount of gain(Not over the top death/black metal type gain)Just good metal type chunkiness.Another question I have is these tubes have been in this amp since 1999 do you think I should replace them(Whats the life expectancy of tubes usually?)and one last question the gain cahnnelon this amp sounds weak and fuzzy when engaged not tight and defined is this a result of the tubes being old?. Sorry for all the questions But I'm new at amp biasing and service(for years I just played em until they blew up or I got something better)so I'm trying to gather all the info I can.

Thanks
Joe
P.S if I'm being a pain just tell me to "F" off

[Amp Kat]

Joe you are no way being a pain. That's what we do around here is help people fix their amps. A bias rite is another excellent tool for easy biasing but they are around $150 to $250 depending on the model and if you are going to bias amps a bunch it's a no-brainer in getting one however this amp only requires a meter that reads DC voltage to bias it. Yes I've biased many of these amps and at different milli volts. The plate voltage determines the bias because it is that multiplied by the tube current that gives you the desired wattage dissipation you want the tube set at. Biasing the amp real hot over 85% dissipation will give you possibly a better tone with compromise of destroying your tubes and possibly serious damage to your transformers. Not worth it so we bias it correctly and take what you get to achieve the maximum performance and reliable issues of your amp. As stated in Mr. Aikens text that scope biasing will take it out of crossover but may put it to hot and cause damage so the %70 dissipation rule is a good one but doesn't guarentee it will be out of crossover at the zero point of course so 70% really isn't written into stone. You could go maybe 80% dissipation and be just out of crossover and still in a decent range for your tubes not to fry. For getting a more metal tone I would suggest maybe a stomp box in the front of some sort and an EQ in the loop would help and maybe even a sonic maximizer but if the amp is biased correctly it will sound like what the amp is supposed to sound like so a few milliamps of bias change here or there won't have a tone effect on the amp even at full tilt using power tube distortion but large amounts may as I stated with compromise to your tubes if pushed to hot.

[Bruce / Mission Amps]

Joe you are no way being a pain. That's what we do around here is help people fix their amps. A bias rite is another excellent tool for easy biasing but they are around $150 to $250 depending on the model and if you are going to bias amps a bunch it's a no-brainer in getting one however this amp only requires a meter that reads DC voltage to bias it. ...

Developed here with Randy at Mission Amps, his Compu-Bias kills. I use one at home and at the shop.
I think by far it's the best of these biasing tools but much more.

http://www.compu-bias.com/

Try one of these:

http://www.compu-bias.com/Qvideo.shtml
or
http://www.compu-bias.com/Mvideo.shtml



If you have any questions about it, email him and tell him I sent you.

Randy@missionamps.com
or
Randy@linenberger.net

[Enzo]

And if you just use the stock test points, you will find that the difference between 78ma and 80ma and 76ma, and many of the rest of the ma family of fine readings, won't make a lot of difference. You want to be in the ball park, but it is not a critical setting.

[joedell71]

Hi Amp Kat
It's Me again with some more questions about biasing this amp.first off I went out and got a new voltage tester but I couldnt seem to find one that measured millivolts(except for one that costed about $280 which is way out of my spending abilities as I am a starving artist/musician/carpenter)So I got one that measures all the usual things and also has a setting for milliamps(I think thats what the Ma AC/DC setting means please correct me if I'm wrong)Can I measure at the test points with this tester.Second question When I turned on the amp and looked inside
only three of the tubes are glowing and to hot to touch (it has four EL34s one glowed briefly and then went cold and stopped glowing I don't really know to much about these kinds of things but this seems to signify a blown tube am I correct or is this just some weird feature of push pull operation
Thanks again for your help
Joe

[Enzo]

All four EL34s should run all the time.

If your meter has no millivolt scale, then set it for volts. 78 millivolts is the same thing as .078 volts. A millivolt is simply 1/1000 of a volt.

[joedell71]

Hi Amp Kat
I put the tubes in my DSL 100 and set the bias to 78mv on each side.I have a few questions.First was I supposed to keep every thing plugged in(I use a powerbrake type speaker attenuator to control the volume while still getting the tone) for this operation Because I did. Second the amp sounds a little fuzzy and lacks tightness is this a result of the rebias or did I do something wrong?.Third should I have Plugged in a dummy load resistor before I biased?And lastly I know that you said not to go past 78mv but will one or two mv hurt because it seems like just a bit more and it would clean up the tone.
Thanks
Joe