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Amplified Parts
+  Ceriatone Forum
|-+  British Style
| |-+  JCM 800 2203, 2204, 2550, 2555
| | |-+  Biasing 2204
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Author Topic: Biasing 2204  (Read 1665 times)
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« on: August 25, 2013, 01:39:42 PM »

I recently picked up a used 2204 (2012 model).  I dropped in a pair of SED EL34s, and did a bias check (via Bias Rite); results were 31-33 mA.  I would have liked to bump it up to around 35mA per tube.

Reviewing the layouts, I cannot find a bias pot anywhere....does the amp have one, or is it just a resistor?  If a resistor, can someone tell me which resistor would need to be changed?  Suggestions for what resistance to use?

Thanks in advance.
« Reply #1 on: September 02, 2013, 07:38:00 PM »

I don't have a 2012 layout, here is the 2007 layout found on this forum... on here it is a 25k trim pot located on the turret-board, in series with a 56k resistor. They can be found behind the Presence control between the two 10uF bias caps.  NEVER use a metal screw driver to adjust a trim pot like this. Always use the non-conductive tools designed for this purpose (which look like a plastic screwdriver). The reason is you have to bias the amp with power applied, and you don't want to touch a metal screwdriver to the wrong part.

On the latest layout (directly below)...the bias has been moved to the 50k pot placed between the power tube (V4=EL34) and the PI tube (V3=EC833/12AX7). It is in series with the 47k resistor located right behind the middle control. THis pot is chassis mounted and can be turned from outside the chassis, avoiding exposure to high voltages. Nik is moving all the Marshall-esque amps to the same chassis/plates to streamline parts purchases.

You can probably find the older 2012 layout using the Wayback Machine on the internet archive...or just ask Nik.
« Last Edit: September 02, 2013, 07:46:55 PM by wyatt » Logged
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« Reply #2 on: August 07, 2014, 11:12:07 PM »

The information can be used in daily life very much. Beneficial to everyone

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