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I just bought a bias probe. I want to install new tubes in my Marshall 3203. It takes a pair of EL34's. What do I set the bias at? Is this Marshall a fixed bias or is there a trim pot? If it is fixed, does anybody know what resistor I need to change or what I have to do?
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Re: Marshall bias
I'm not the worlds best schematic reader, but it looks to me like it's not adjustable.
http://img.photobucket.com/albums/08...2550_ver_2.gif -
Re: Marshall bias
Here is a link to Wbers site to run the calculation for bias values.
http://www.webervst.com/tubes/calcbias.htm
Just put in the information, ie: what tube, Plate Voltage...hit calculate and there it is. I used it last night for a quad of new issue Mullards in my XTC.
Generally EL34's are biased at about 35Ma +/- 5Ma
I don't know the answer to your aother question.Comment
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Re: Marshall bias
You have to find out the plate voltage of the amp before you do anything. Once you find out the plate voltage there is a formula you use to find out where to set the bias. Most of the older jcm800 are at about 450v. Some a tad higher some lower. You will need a voltage meter and just put the red lead to the 4th prong of the tube socket and rest the black lead on the metal chasis. This will give you your plate voltage.
There is a trim pot on the under side of the chassis. Make sure you adjust the one that says bias next to it.Comment
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Re: Marshall bias
I think that is the wrong schematic Chuck. There are specs for this; http://www.drtube.com/schematics/marshall/3203pwr.gif
That is just for the power amp section..theres another for the preamp. http://www.drtube.com/schematics/marshall/3203pre.gif
The guy who biased mine would know for sure as I bought one from him. Most have a trim pot for on one of the boards but this could be fixed. Most Marshalls running EL34's run around 400v-450v.Comment
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Re: Marshall bias
It's the same output section, though your's are better.
And I don't see a bias pot.Comment
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