Biasing a JCM800: Tell me if this is right.

HolyDirt

HolyDirt

New member
Hey.
I have to bias my Marshall JCM 800 2203, and I've decided to do it myself (I have experience working in amplifiers, so don't worry aobut that..). Tell me if this is how it's done... I disconnect the ground thats attached to pins 1 and 8 on each power tube, and replace it with a 1 ohm resistor the leads to the ground. I turn the amp on and leave it in standby, adjust the current across the 1 ohm resistor to around 35-40 mA (because these are EL34's) using the bias trim pot, put the tubes in, let them warm up, then take the amp off standby and fine adjust the current across the resistor til it sounds its best (while staying in the 35-40mA range). Right? Am I missing something? Thanks.
 
Re: Biasing a JCM800: Tell me if this is right.

I thought the tubes were supposed to be already in the sockets before you plug it in, anyways thats what my amp tech did when he biased my new tubes.
 
Re: Biasing a JCM800: Tell me if this is right.

One thing you may want to buy is a bias tool, which is a two sided socket that goes in between a tube and it's socket. It automatically breaks the circuit in the proper spot, and has 2 leads that plug into your mA meter.

Most EL-34 amps run around 35-38mA, and 6L6's are around 31-33mA.
I think I bought my bias tool on Ebay for around $15-$20. It's a good thing for every tube amp owner to have.
 
Re: Biasing a JCM800: Tell me if this is right.

Gearjoneser said:
One thing you may want to buy is a bias tool, which is a two sided socket that goes in between a tube and it's socket. It automatically breaks the circuit in the proper spot, and has 2 leads that plug into your mA meter.

Most EL-34 amps run around 35-38mA, and 6L6's are around 31-33mA.
I think I bought my bias tool on Ebay for around $15-$20. It's a good thing for every tube amp owner to have.
Click to expand...
Alright, will do, but I'd really like to know how to do it both ways... Is the procedure I mentioned correct?
 
Re: Biasing a JCM800: Tell me if this is right.

You might want to search the net for that tutorial. I'm not sure, and wouldn't want to say yes.....then turn you into Yosemite Sam.
 
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