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Marshall 18w vs Vox AC15 power section

Discussion in 'Glowing Bottle Tube Amp Forum' started by Philbert_S, Nov 28, 2020.

  1. Philbert_S

    Philbert_S TDPRI Member

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    Hey All,

    Question for you amp builders. Both of the above run EL84’s in the power section. Are the power amps similar?

    Cheers!
     
  2. LightningPhil

    LightningPhil Tele-Meister

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    I’ve meddled with ac30 circuits and built something similar to the Marshall 18w. Nope, not very similar. The Marshall is perhaps a little similar to to the normal channel, but with a little less complexity, different filtering and the option of paralleling the first tube’s triodes. Certainly nothing like the top boost circuit.
     
  3. Dacious

    Dacious Poster Extraordinaire

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    Yes. But the preamps are not. The non-topboost older AC15s used an EF86 in the V1 position. The other difference in Blues vs Greenbacks.
     
  4. 3-Chord-Genius

    3-Chord-Genius Poster Extraordinaire

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    Right off the bat, two diametrically opposed responses.
     
  5. mkdaws32

    mkdaws32 Tele-Afflicted Gold Supporter

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    Not an expert here, but Isn’t the AC15 a single ended power amp? Whereas the Marshall 18 watt is push- pull? The OP was asking about power section differences, not preamp.
     
  6. Dacious

    Dacious Poster Extraordinaire

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    Not realty - we both said the difference is in the preamp, which it is. Both power sections are cathode biased EL84s

    Even the early AC15 without topboost sounds different to the 18w because of the EF86 pentode preamp.

    Even if you cross connect the channels in both amps they're different sounding.
     
  7. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    They're very similar, but not identical. The AC15 uses a choke in the power supply, where the 18-watt just relies on more capacitance. But otherwise, it's 300-ish plate volts, ~130Ω cathode resistor for the pair, into an 8k load, just like the EL84 datasheets recommend. (And neither uses negative feedback.)

    I highly recommend studying the schematics at www.drtube.com:
    https://drtube.com/en/library/schematics/65-vox-schemas
    https://drtube.com/library/schematics/69-marshall-schemas#JMP18W


    P.S. Some folks in the 18-watt community like to do builds with both a Vox-ish and a Plexi-ish preamp channel. Although I already have more amps than necessary, that's something I want to build someday! :cool:
     
    Last edited: Nov 28, 2020
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  8. Dacious

    Dacious Poster Extraordinaire

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    No. They're both push-pull class AB1 cathode biased EL84s.
     
  9. mkdaws32

    mkdaws32 Tele-Afflicted Gold Supporter

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    There you go! That’s why I’m not the expert ;). Thanks for the correction!
     
  10. dan40

    dan40 Friend of Leo's

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    Single ended amps such as the Fender Champ are single ended class A, cathode biased circuits. Many folks (including the manufacturers) wrongly label the Vox as a class A circuit because they are cathode biased. This may be where the confusion starts. As Dacious mentioned, both amps are cathode biased but they are definitely class AB/push pull.
     
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  11. mkdaws32

    mkdaws32 Tele-Afflicted Gold Supporter

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    Very cool. I thought the AC15 and AC30 were both dual single ended power amps with the power tubes summed for more power. I know a little bit about tube amp construction, but you learn something every day.
     
    dan40 likes this.
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