Marshall JTM 30 bias mod question

[forevermarshall]

There have been a few different posts on how to adjust the bias on this amp(doing a bias mod). Some have replaced the 9v zener diode with an 11v or 15v and others have added a 220K pot between ZD103 and R132. I think the pot method sounds the best, but exactly what pot should I get? What quality/brand? Any thoughts/opinions?

Here is the schematic: http://www.drtube.com/schematics/mar...jtm3-61-02.pdf

[Mick Bailey]

This is the kind Marshall use in the DSL (See the spec sheet for rating) - note this is 22k, not 220k. Just illustrating the type;

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As there's no real current draw in this situation I'd go for a 3/8" 25 turn pot to make setting easier. Unless you also decrease the bias voltage (making it more negative) the best you'll get is what is already set by the zener string. As these amps typically run hot I guess your intention is to set the bias cooler. I'd go for a 15v zener to replace the 9v, with a pot to give the adjustment.