Amp GAS here also

Discussion in 'Ampage Area' started by MrWookiee, Apr 29, 2021.

  1. wabash slim

    wabash slim Gretschified

    Feb 10, 2010
    lafayette in
    RMS is the base line everyone should use. Back in the '60s and '70s, cheap stereo gear was sold using the ludicrously inflated "music power" and "peak music power" wattage figures. They have nothing to do with reality. Music power is twice RMS, and pek music power is twice that. RMS is based on Ohm's Law. Music power and peak music power is based on salesman speak---i.e., marketing. Caveat Emptor, my friends!
    HCSterg has it right. I'd also suggest that the class of amp makes a difference---A, AB, D, etc.
    How does a class A Vox AC30 keep up apparent SPL with a Fender Twin class AB at 85 watts? Solid state amps can't really compare with tubes at equal output levels, and they still haven't caught up in terms of tone. A simple tube amp like a Fender tweed oozes tone naturally, where the highly involved circuitry of a modelling solid state amp can't get close. They're getting better, but still have a ways to go.
    Rmccamey, Waxhead and hcsterg like this.
  2. Henry

    Henry I Bleed Orange

    Apr 9, 2014
    Tone king gremlin is a bit over your budget but you may be able to negotiate it down or buy used. I have their Imperial MKII, a great amp for home and band.
  3. hcsterg

    hcsterg Friend of Fred

    Feb 13, 2012
    Well once again, it is not a good idea to go against preconceived ideas, is it ? :oops: But the VOX AC30 is not a class A operation amplifier, but rather a class AB1... o_O

    Push-pull cathode bias operation doesn't imply a class A operation, and in fact, it is most of the time an AB1 operation, more or less close to class A, depending on the voltages applied, the mode of biasing (cathode or fixed).

    According to * RCA, General electric, Tungsram manuals (among others) :

    * Pure class A operation states that the average plate current stays constant, the tube being modulated or not. It can be cathode or fixed bias.

    * Class AB operation is divided in 2 categories :
    AB1 = average plate current varies but the the input grid absorbs no current. Fixed or cathode bias.
    AB2 = average plate current varie neatly and some input grid current is allowed. Fixed bias only.

    * Class B operation means a bias at plate current cut-off in absence of modulation, and maximum plate current vs. tube life expectancy with modulation applied. Input grid current is allowed, and requires a low impedance driver, even able to give a driving power. Fixed bias only.

    In class B operation, the power stage must be symetrical in order to cancel unacceptable distortion due to tube cut-off, which never occur in true class A, but partially occurs in class AB.

    AB operation means that, in fact :
    - at low modulation, the tubes works nearly in class A.
    - at high modulation, the tubes works closer to class B.

    * Class C is overbiased operation used for RF transmitters, never used in AF amplification.

    Efficiency (common values, supply power vs. usable power, heater not included) :
    - Class A = 25-30%
    - Class AB1 = 40-45%
    - Class AB2 = 50-55%
    - Class B = 60-70%
    - Class C = 80-85%

    Class D, E, etc... have been developed for high-efficiency solid-state and digital designs.

    Most of the guitar amps and HiFi amps using tubes in push-pull operates in class AB1 operation.

    A single-ended amp operates necessarily in class A since it has not the required symetry of the push-pull to cancel cut-off distortion. That said, some plate current variation exists in class A, due to non-linearities at high modulation, and hence increasing distortion. Think a circa 5-10% plate current variation, commonly.

    The main advantage of a cathode bias operation power tube, single-ended or push-pull, is that the maximum output power is more smoothly limited than in fixed bias, where no serial resistance comes to limit the plate current by varying the bias by input grid - cathode voltage drop, like it is in cathode bias.

    It is often called "Soft Clipping" vs. "Hard Clipping" when you look at the waveforms on an oscilloscope screen, as shown below :


    That's why cathode bias operation amps tend to compress and enter earlier, more smoothly in overdrive, with less harshness than fixed bias ones. This is partially - but not the sole one - the explanation why the Tweed Fender amps like 5E3, Vox AC15 and 30 cathode bias amps are so tone-revered when cranked.

    Of course, there are - as usual - exceptions to the rule... ;):)

    Hoping that most of you will understand my explanations... :oops:o_O:confused:

    Last edited: May 15, 2021
    MrWookiee likes this.
  4. wabash slim

    wabash slim Gretschified

    Feb 10, 2010
    lafayette in
    I learn something every day.
    hcsterg and Henry like this.
  5. Henry

    Henry I Bleed Orange

    Apr 9, 2014
    So true. Although in this case I learned that there are some things I don't seem to be able to learn.:rolleyes:
    hcsterg, wabash slim and MrWookiee like this.
  6. MrWookiee

    MrWookiee Synchromatic

    Jun 17, 2020
    SoCal, USA
    I'm very, very tempted by this offer based on several hours of videos I've watched, but am trying hard to maintain a disciplined approach.
  7. hcsterg

    hcsterg Friend of Fred

    Feb 13, 2012
    Many people here consider the Katana good... :)

    I never played one to tell anything about them, unfortunately... :oops:

    But Roland/Boss products are premium quality, that's a fact - an moreover with simple, efficient and well-designed ergonomics, considering the Cube series, among others. :cool:

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