Biasing Jcm 800

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populartsl
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Biasing Jcm 800

Post by populartsl »

Whats the easiest way to bias the output tubes on a ceriatone jcm 800.

Thanks Ben
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s2
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Post by s2 »

Do you have a bias head for your DMM or similar?

Most JCM800s I've come across are happy by just dialing in 35mA. These amps seem to be made for that simple bias current.

If you don't have a bias head, you will have to open the chassis. IIRC, there is a 1 Ohm resistor on each tube socket. You can set your DMM to DC Voltage, connect the black lead to ground, and the red lead to the pin on the one of the sockets that has the resistor (not the side that is grounded). Even though you are reading voltage, it translates directly to current.

Just don't shock yourself, K?
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populartsl
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Post by populartsl »

Thanks S2

On the tube socket pin 8 has the 1ohm resistor to ground. I take thats where i take my reading. The 1 ohm resistor is actually reading 1.2 ohms would that make much difference.

Thanks Ben
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Post by Bluesmechanic »

populartsl wrote:Thanks S2

On the tube socket pin 8 has the 1ohm resistor to ground. I take thats where i take my reading. The 1 ohm resistor is actually reading 1.2 ohms would that make much difference.

Thanks Ben
Touch your probes together and see what you read. You will probably get some kind of reading other than "0". You will be close enough even if you don't.
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floridajack
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biasing

Post by floridajack »

thats why i love my old simpson 260 analog meter - you can "0" it out by touching the leads together and twisting the zero adjust knob. i have used very long leads and still achieved accurate results. (i know i can't get those 4 decimal place readings, but it has a r x 10,000 scale - how much closer can you get!!) i have a fluke dmm and i think the simpson is more accurate (and soooo old school cool sitting on my bench.)

fj
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populartsl
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Post by populartsl »

I'm getting 38.2 on one tube and 33.8 on the other i presume thats because the tubes aren't matched would i be right.

Cheers Ben
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Post by joepopp »

Ben:

I bought a Dual Head Bias tester on ebay and it allows me to flip between the two tubes very quickly.

Dual Head Bias Tester

Make sure the amp is warmed up well. Do you have any pics of the JCM? Mine rules!!!

Joe
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populartsl
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Post by populartsl »

No not yet just finished making it from one of nik's kits although i'm going to have a look at an original on thursday may buy it he also got two jmp mark 2 with master volume as well. Hes selling them cus he moving abroad.
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Post by Bluesmechanic »

populartsl wrote:I'm getting 38.2 on one tube and 33.8 on the other i presume thats because the tubes aren't matched would i be right.

Cheers Ben
You can find out by swapping the tubes. If the readings follow the tubes, you are correct. :hmm:
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