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Classic 30 Class ?

Discussion in 'Amp Tech Center' started by Jammin'John, Jun 30, 2017.

  1. Jammin'John

    Jammin'John Friend of Leo's

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    Hi guys. Is the Peavey a class A amp ? A/B ?
    Is it fixed bias or cathode bias ?
    What is a good plate voltage and current to run the EL84's at ?

    thanks,
    JJ
     
  2. clintj

    clintj Friend of Leo's

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    Looking at the original schematic online, it's a Class AB push/pull, fixed bias, with no bias adjustment provided. The board would need to be modified to add an adjustment pot.
     
  3. Jammin'John

    Jammin'John Friend of Leo's

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    Thank you clintj.
    I wonder how they expect a person to get the tubes to run at their best with no adjustment available.
    I guess they don't care.

    JJ
     
  4. Ringo

    Ringo Poster Extraordinaire

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    Most folks are not capable of adjusting bias, they just plug in and play their amps.
    And there are lots of amps that do not have adjustable bias, Mesa amps are made that way.
    The plus is you just buy some matching tubes and install them.
    There is some info here for the C50 which is pretty much the same amp w different transformers to get 50 watts.

    http://blueguitar.org/new/articles/blue_gtr/amps/peavey/c50_bias.pdf
     
  5. Jammin'John

    Jammin'John Friend of Leo's

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    I wonder what tubes will run correctly in this amp. My bias Rite doesn't have the adapter for the EL84's.
    Perhaps my friend would buy the adapters ? Probably not.
    What is the desired plate current and voltage for this amp ?

    JJ
     
  6. alnicopu

    alnicopu Friend of Leo's

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    You would have to measure the voltages then do the math from there.
     
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    What alnicopu said. Different tubes may draw different amounts of current, so there isn't a "right" tube or "right" amount of current.

    Just because there isn't an adjustment pot doesn't mean it's not adjustable. You may have to change a resistor, or you may want to add an adjustable pot.

    Think of all the zillions of amps out there that have been working fine for decades with no service and people not worrying about tube matching or bias or really anything except playing.

    If you want to get technical, pop in some tubes, take some measurements, and do some math. You can always put some tubes in and watch them in dark environment to see if they redplate. If they are good after a half hour or so, you're fine.
     
  8. clintj

    clintj Friend of Leo's

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    They built the bias circuit to -14V, which should be sufficient for the majority of tubes you can buy. If you're buying from a good dealer, ask in advance if they recommend a specific set of power tubes.

    There's a few ways to figure bias, and they involve measurements, math, and sometimes a little fudge factor to correct for screen current. If you want to know how and why, and how you can figure your numbers without a probe, read this page:

    https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

    Read the warning at the top of that page, then re-read it. Carelessness, inattention, and complacency can be lethal. Even using a bias probe hooks into the high voltage supply to the power tubes, which is why you usually get told to take the amp to a tech if you're even the slightest bit uneasy or inexperienced with electronics.
     
  9. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

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    Nope. That just means they are matched to each other, not that they are properly biased. that often changes based on each tube brand/model (if they make multiple ones( installed.

    Voltage is always "as measured". The schematic may show originally specified voltages, but they rarely are the same later on. the bias is adjusted per the plate voltage that's measured. What current depends on how you want the amp to sound *and* the safe range for the tubes based on the measured plate voltage.

    As you can see, it always circles back to the *actual* plate voltage.

    The "-14V" is a very rough setting used during construction and after major work - not for tube biasing. The current can ned up much too cold or hot using that as the only bias guideline.
     
  10. Ringo

    Ringo Poster Extraordinaire

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    There are easily many thousands of folks who have and do use matched sets of tubes in amps that have fixed bias, Mesa and others, I'll agree that doesn't mean the amp sounds as good as it could but it is simple and safe, it's much more work to mod an amp that has fixed bias to one that has adjustable bias and not necessary IMO and apparently also not in the opinion of those who design and build those type of amplifiers.
     
  11. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Totally true. But...if you get tubes that draw too much current from the amp they are gonna redplate. The amp ***should*** be designed to take a majority of tubes on the market, but you can never be certain without actual measurements.

    I would say the good news is that once you find a set that works you can keep buying that set without re-checking the bias. Yeah, it can drift some over time, but that small difference shouldn't amount to much.
     
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