So I bought new power tubes for my Marshall JTM600 head. It comes with 2 EL34's. I have a couple of JJ's in there now. I am replacing them with Groove Tubes. Can I do this myself or do I need to take it to a pro who can re-bias the amp as well? Does it even need to be rebiased? Help me.
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I'm not a professional or an amp tech but most older Marshall tube amps will benefit from a fresh bias. I know nothing about this amp though and I know no one on here that owns one. If the two tubes you bought are a 'matched set' I'm guessing you can pop them in there and everything will be fine. Just monitor them very closely for even brightness and tone. If the tone sounds real muddy or one of the tubes gets real bright then shut if off and take it to a tech. I also have a biasing blog if you want to learn more but it is for my amp.

Also, ask over on the Marshall forums. You'll want to determine if your amp is fixed or cathode bias and what the average plate voltage is for your amp. Good luck.

Last edited by 311ZOSOVHJH at Jun 15, 2010,
Looking at the schematic I can't see a trimpot to adjust the bias. It looks like a fixed non-adjustable bias. If so, just plug them in and play.
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