Changing tubes in Pro Junior

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by rjs6700, Aug 27, 2008.

  1. rjs6700

    rjs6700 Member

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    Is it as easy as pulling them out and putting new ones in?

    Getting a new cabinet for 12in speaker and thought I'd put new JJ's in at the same time.
     
  2. Riscchip

    Riscchip Supporting Member

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    I believe a Pro Jr. is fixed bias, so you will need to have the bias tested and potentially swap the bias resistor to change the power tubes.

    The preamp tubes you can swap out without any adjustment.

    Since you're talking JJs, you could always try contacting Eurotubes and see if he knows the factory bias range of your amp. He could then sell you a set would work with the stock bias setting. This isn't ideal, though, because things may have changed since the amp was new, or if the amp was previously owned, someone else may have changed the bias resistor already.

    Alternately--and I may be wrong here--I think Fender uses pre-rated tubes like GTs these days? If you went with those, you may be able to find a set that Fender guarantees to fit the stock bias setting on your amp. But they wouldn't be JJs, unless Fender is using those now.
     
  3. NortheastHick

    NortheastHick Supporting Member

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    Huh? Pro Jr. Is simply plug'n'chug..
     
  4. SatelliteAmps

    SatelliteAmps Member

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    Even if a company guarantees matched or pre rated tubes, they should be checked every time you change them by a tech or with some sort of meter. Too many times have tubes shown up that aren't what they were supposed to be.
     
  5. Riscchip

    Riscchip Supporting Member

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  6. SatelliteAmps

    SatelliteAmps Member

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  7. brad347

    brad347 Member

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    nope.

    It is non-adjustable fixed bias, but it should have a bias adjust. IN fact, I have modified mine so that it does, and can be properly biased.

    It's very warmly biased from the factory, at that, with most EL84s.

    You could definitely burn up tubes in there if you don't at least check it.
     
  8. rjs6700

    rjs6700 Member

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    I emailed Eurotubes.com for clarification (very quick response by the way).

    They say the tubes are plug and play with no bias adjustment needed.
     
  9. brad347

    brad347 Member

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    Sometimes people confuse "no bias adjustment needed" with "no bias adjustment possible with the amp in stock form."

    This is one of those times.
     
  10. deeval

    deeval Supporting Member

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    I have had mine for 10 years and still never biased and have changed several tubes over the years,never burned or red plated,I,m sure it would be good to check bias,but they seem to work fine.:phones
     

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