How does Vox half-power switch work?

Discussion in 'Shock Brother's DIY Amps' started by HaroldBKNY, Aug 6, 2019.

  1. HaroldBKNY

    HaroldBKNY TDPRI Member

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    The AC15HW has a 15-7.5 power switch. Initially I assumed it was a pentode-triode switch, but in examining the schematic, that's not the case:

    [​IMG]

    It looks like in half-power mode, it puts a 270ohm resistor (R200 in the diagram) in series with the cathode bypass resistor and cap.

    Why does adding this resistor cut the power in half? I thought that adding cathode-bypass resistors just changes the bias, not changes the power output.
     
  2. Dacious

    Dacious Poster Extraordinaire

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    The cathode bias resistor controls the power flowing through the power tubes in terms of current. Think of it in crude terms like an accelerator pedal. The full power setting is like you have the pedal mashed to the floor and are achieving around 15 push-pull watts.

    In the half power setting you are lifting your foot off the pedal. So regardless of whether you turn your gain and master volume fully up, your amp cannot develop maximum power potential, because the bigger cathode resistor is acting like a limiter on current to the tubes.

    Power (watts) = potential (volts) X current (amps). Restrict the current and you restrict the power.

    Some other amps like my Marshall Mini Jubilee the low power setting is actually lower voltage taps on the transformer. Same result.
     
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  3. HaroldBKNY

    HaroldBKNY TDPRI Member

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    Cool, thanks! My next question, then, is why don't I see people talking about this ever as a potential way of halving power? Most people on forums seem to prefer a pentode-triode switch when designing an amp. Is that generally better or more effective? Or, I guess this method may only work in cathode-biased amps?

    Also, does this method change the distortion characteristics of the amp? Does it lower the headroom at which the tubes distort along with the reduced signal, or does it just halve the power of the signal, but leave the headroom-cutoff the same (meaning less power tube distortion overall)?
     
    Last edited: Aug 6, 2019
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  4. sds1

    sds1 Tele-Afflicted

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    Interesting thread. @HaroldBKNY have you tried this switch for yourself?

    I've only heard that pentodes running in triode mode don't sound too good.
     
  5. beninma

    beninma Friend of Leo's Silver Supporter

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    Great thread.. I wonder if the Orange half power switch is similar to the Vox one.

    They've done a pretty good job hiding their recent amp schematics/circuits and getting mad at people who post them online.

    I'd love to see how their headroom/bedroom switches work in tandem with the full/half switch to get the 15/7.5/1/0.5 or 30/15/2/1 options. Some kind of attenuator, but I'd guess it's a simple one.
     
  6. HaroldBKNY

    HaroldBKNY TDPRI Member

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    Yeah, it's on my Vox AC15HW amp. It sounds to me like it really does just drop the volume and keep the sound almost exactly the same (though it's not like I specifically know whether the power tubes are distorting the exact same way, or just the preamp and PI tubes that precede it).

    The only problem is that the volume drop is so minuscule that it doesn't really matter. It's a drop in a bucket compared to even a small turn of an attenuator knob or master volume knob. However, if I were to swap out the resistor for one that's a greater value to limit the current even more, it could be made useful. What resistor would I have to put in for it to be a 1/10th power switch?
     
    Last edited: Aug 6, 2019
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  7. Dacious

    Dacious Poster Extraordinaire

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    This particular design will only work with a cathode biased amp. Marshall on my Mini Jube despite cathode bias supplies less power to the power section. Mesa on it's 5:25 amps runs them on one tube for 5 watts.

    Interestingly Fender with the EC Champ uses a big resistor switched in to the speaker out as an attenuator. I think that's how the incremental power setting on the AC10 and AC4 works, too.

    Half power will only drop about 3db - that's about the smallest increment your hearing can discern. To really perceptibly drop power you need to quarter it or more. Db is exponential.

    On Fender's Evil Twin and also the Fender 75 Jahns designed 80+ amps the 15/25 watt setting was a separate set of taps. It does noticeably impact tone. Partly because of presumable preamp power rail changes. At least Vox' setup should have minimal impact there.
     
  8. gusfinley

    gusfinley Tele-Holic

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    The AC4TV uses an attenuator network between the Power tube and the speaker.

    The AC4C1 series uses a potentiometer to drop signal prior to the Power Stage

    The AC10C1 uses a potentiometer across the out-of-phase signals entering the power tubes. Decreasing the resistance allows the out-of-phase signals variably to cancel eachother out, reducing the signal going to the power tubes. (Much like the classic 'Cut' Control, but not frequency dependant)
     
  9. tubeswell

    tubeswell Friend of Leo's

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    On the ‘15W’ setting, the cathode resistor is fully bypassed, so the full bandwidth of frequencies is boosted as much as possible, and its running at a hotter bias, so there is more tube current overall and tubes are working harder.

    On the ‘7.5W’ setting, the cathode resistance is significantly increased, making the bias a lot cooler (decreasing available tube current and power) AND a substantial part of the cathode resistance is not bypassed, resulting in a large amount of cathode current feedback, which reduces the gain of the output stage.
     
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  10. D'tar

    D'tar Friend of Leo's

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    IMHO. If you bias the output any cooler you will end up with some ugly non musical crossover distortion. Just speculating but this may be why you don't see this a lot and are now seeing built in attenuators at the output secondary, however, these add cost, component count and use valuable chassis space but may be more useful than halving your bias. Building a quality attenuator that could be used with any of your amps might be a more fruitful endeavor.

    This one grabs my attention and may be a winter project. Who knows?

    https://www.tdpri.com/threads/reactive-loadbox-attenuator.806311/
     
  11. HaroldBKNY

    HaroldBKNY TDPRI Member

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    That makes sense. I really honestly don't need this switch, cause I have a great attenuator (Weber minimass 50). Just thought it could be good to actually make the switch useful. Maybe I'll change it to something else one day.
     
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