JTM45 Bias problem

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67plexi
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JTM45 Bias problem

Post by 67plexi » Thu Jun 22, 2017 4:28 am

Recently I acquired original JTM45 in very good condition. Sound fantastic but bias was only about 17ma/413V( bias resistor installed was 68K with no bias pot) so I added 25K bias pot with 57K resistor instead of 67k. Then I tried to increase bias, it started to make some kind of noise sound(Beeh~~) when bias pot is set over 22 ma.
My weber bias rite also shows strange reading(looks like minus symbol-???) at this point.
Just below that point, it's gone and works fine....

What can be the reason? maybe Bad Power tube( it looks to be 90's chinese kt66)? Diode? or Bias Cap? or resistor......
Any advice would be appreciated.

Thanks

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Freebird
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Re: JTM45 Bias problem

Post by Freebird » Thu Jun 22, 2017 3:14 pm

Hi 67plexi

That can be many things. How did you wire the additional bias pot with resistor? Some picture would be helpfull.

Best regards
Daniel

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