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deadmanwalking's picture
Mar 2004
10 years ago

Ok, I have read a bunch of sites telling me how to bias amps. The problem is, they don't make it easy. I haven't seen one site that goes through it with pictures. Here's one I found:

Here's the Marshall Service Bulletin on biasing the JCM2000 series amps:

Procedure:
1. Make sure the amp is connected to a load with the proper impedance selected.
2. Power up amp on Standby, and let the circuit stabilize for a couple of minutes.
3. Locate the male three pin molex connector (CON2) with the two mini pots (PR1 and PR2) (located on each side of the molex connector) on both ends found at the bottom of tube bay.
4. Connect DMM (set to read mV) with alligator leads, reference common lead to center pin (pin2) on molex connector CON2 and positive lead to pin1 on CON2.
5. Take amp off Standby with no signal, adjust mini-pot (PR1) closest to pin that the positive lead from your DMM is connected to and set it to mV voltage that is listed below.
6. Repeat steps 4 & 5 for pin3 mV and until pin1 & pin3 mV readings are the same.

Bias Settings:
DSL50 45 mV TSL 601&602 80 mV
DSL100 90 mV DSL 401 1.375V
TSL100/122 90mV DSL201 .675V

Here's a quick bias lesson:
What you want to do is measure the plate voltage (B+) from pin3 of one of the power tubes. It should measure between 450-500V. Now the max plate dissapation (Pa Max) for EL34s is about 25W. To calculate the correct bias, you want to:
(Pa Max/B+)(.7) = 38-45mA depending on the exact plate voltage of your amp. Check and adjust via the molex connector and pots from above.

This is all giberish to me. I'm a visual learner. Reading how to do it, doesn't help me at all. I know I could do it, if it was just explained better. One question I have is:

1. When biasing, do you bias the power tubes or preamp tubes?

Also, I know it may be trouble, but if ANYONE can show me pictures of the process, it is greatly appreciated. I know that's asking a lot, but if anyone is bored and wants to do something that'd be awesome.

Guitars:
Epiphone Les Paul Standard
Fender Squier Stratocaster

Amp:
Fender Bassmen Ten
Marshall JCM 2000 DSL 50
1960A Cab
Peavey Blazer 158 (Practice)

Pedals:
Boss MT-2
Big Muff Pi
DanElectro Hashbrown Flang
Boss TU-2
Zoom 505 (sounds like crap, but it was free)

deadmanwalking's picture
Mar 2004
10 years ago

anyone have anything?

Guitars:
Epiphone Les Paul Standard
Fender Squier Stratocaster

Amp:
Fender Bassmen Ten
Marshall JCM 2000 DSL 50
1960A Cab
Peavey Blazer 158 (Practice)

Pedals:
Boss MT-2
Big Muff Pi
DanElectro Hashbrown Flang
Boss TU-2
Zoom 505 (sounds like crap, but it was free)

pilotbarr's picture
Colorado
Apr 2004
10 years ago

You bias the power tubes. Just follow the instructions. You need a meter and some leads. The hardest thing will be getting your meter to read what you want it to. You need to set your meter to VOLTS. Clamp onto the point and turn the screw. Nice thing on JCMS you don't have to go inside. And if you don't have meter just bias to the least noise. They are the same place as correct voltage.

Now listen here, I'm gonna' tell you the thing of this gig.

deadmanwalking's picture
Mar 2004
10 years ago

Originally posted by pilotbarr
You bias the power tubes. Just follow the instructions. You need a meter and some leads. The hardest thing will be getting your meter to read what you want it to. You need to set your meter to VOLTS. Clamp onto the point and turn the screw. Nice thing on JCMS you don't have to go inside. And if you don't have meter just bias to the least noise. They are the same place as correct voltage.

When you say clamp onto the point, what point are you talking about?

Also, you said I don't have to open the back? So I don't have to unscrew the back plate and get to where the tubes are?

I'm sorry if I sound stupid. I'm just new at amp maintenance and I want to learn.

Guitars:
Epiphone Les Paul Standard
Fender Squier Stratocaster

Amp:
Fender Bassmen Ten
Marshall JCM 2000 DSL 50
1960A Cab
Peavey Blazer 158 (Practice)

Pedals:
Boss MT-2
Big Muff Pi
DanElectro Hashbrown Flang
Boss TU-2
Zoom 505 (sounds like crap, but it was free)

jondon1986's picture
Jupiter, 7366646217-69 Outerspace
Apr 2004
7 years ago

Originally posted by deadmanwalking
[B
Also, you said I don't have to open the back? So I don't have to unscrew the back plate and get to where the tubes are?

I dont think thats what he said, I think hes talking about on some amps you have to actually get inside the chassis, yers sounds like you dont.

pilotbarr's picture
Colorado
Apr 2004
10 years ago

You may have to remove the back cover, yes. I don't remember exactly where the test points are, but they are pretty convienient compared to most amps.

Now listen here, I'm gonna' tell you the thing of this gig.

eastwood6's picture
Mar 2004
4 years ago

Sorry to sound like a meanie, but if 1) you don't know which tubes you are biasing; and 2) you don't know that you have to remove the back of the amp to find the bias points, you really shouldn't be trying to bias a tube amp on your own right now.

Even though Marshall has made it relatively easy with the JCM 2000 line, the key term is "relative" - the process still requires an understanding of electronics, voltages, the ability to read a digital multimeter, and familiarity with tube amplifiers basics. Doing it wrong can harm your amp, or worse yet, you. There are lethal voltages to be found in an amp, even when off.

If you want to learn, I would recommend that you find a local tech familiar with Marshall amps and pay them to let you watch them bias it. Take notes, bring a digital camera, whatever you need, and see it done right once.

Would hate to see you damage your amp, or fry yourself. Good luck.

"It's never too late to be who you might have been."

George Eliot

English novelist (1819 - 1880)

deadmanwalking's picture
Mar 2004
10 years ago

Originally posted by eastwood6
Sorry to sound like a meanie, but if 1) you don't know which tubes you are biasing; and 2) you don't know that you have to remove the back of the amp to find the bias points, you really shouldn't be trying to bias a tube amp on your own right now.

Even though Marshall has made it relatively easy with the JCM 2000 line, the key term is "relative" - the process still requires an understanding of electronics, voltages, the ability to read a digital multimeter, and familiarity with tube amplifiers basics. Doing it wrong can harm your amp, or worse yet, you. There are lethal voltages to be found in an amp, even when off.

If you want to learn, I would recommend that you find a local tech familiar with Marshall amps and pay them to let you watch them bias it. Take notes, bring a digital camera, whatever you need, and see it done right once.

Would hate to see you damage your amp, or fry yourself. Good luck.

Thank you for your honesty. I'm gonna call a shop and see if they will let me do that. I'm not sure if they will, but I'll try it.

Guitars:
Epiphone Les Paul Standard
Fender Squier Stratocaster

Amp:
Fender Bassmen Ten
Marshall JCM 2000 DSL 50
1960A Cab
Peavey Blazer 158 (Practice)

Pedals:
Boss MT-2
Big Muff Pi
DanElectro Hashbrown Flang
Boss TU-2
Zoom 505 (sounds like crap, but it was free)

deadmanwalking's picture
Mar 2004
10 years ago

Well, I called around and hardly anyone around my city will bias amps. The store that I DID find that would do it, said they wouldn't let me watch. I don't know why. I might steal their secret or something. The Guitar Center in my city doesn't even have a technician. How stupid is that?

Anyway I've read a little more and I'm a more confident that I can do this. I understand it more now after reading.

One question I do have though is, what do I set my digital multimeter to when I'm checking? There is no mV settings on the multimeter. Do I set it just to AC volts or DC volts or something else? I would guess AC volts since the amp is plugged into the wall.

Guitars:
Epiphone Les Paul Standard
Fender Squier Stratocaster

Amp:
Fender Bassmen Ten
Marshall JCM 2000 DSL 50
1960A Cab
Peavey Blazer 158 (Practice)

Pedals:
Boss MT-2
Big Muff Pi
DanElectro Hashbrown Flang
Boss TU-2
Zoom 505 (sounds like crap, but it was free)

pilotbarr's picture
Colorado
Apr 2004
10 years ago

Set it to DC volts.

Now listen here, I'm gonna' tell you the thing of this gig.

deadmanwalking's picture
Mar 2004
10 years ago

Ok, I've taken a picture of where I think I am supposed to hook the leads to. Tell me if this is the correct place. And I see that there are two adjustment places on each side of the leads.

And also, if you guys would let me know which setting I should set my mulitmeter on. I know that it's supposed to be set to DC Volts. But there are a couple of settings. I have a photo.

I know I can do this if I just figure these two things out. Thanks for your help.

Guitars:
Epiphone Les Paul Standard
Fender Squier Stratocaster

Amp:
Fender Bassmen Ten
Marshall JCM 2000 DSL 50
1960A Cab
Peavey Blazer 158 (Practice)

Pedals:
Boss MT-2
Big Muff Pi
DanElectro Hashbrown Flang
Boss TU-2
Zoom 505 (sounds like crap, but it was free)