Discussion in 'Amps and Cabs' started by LARRY GERSHON, May 9, 2019.


    LARRY GERSHON Silver Supporting Member

    Jul 6, 2006
    can I use a 5v4 in place of a 5ar4? What will I get or loose?
  2. hogy

    hogy Member

    Jan 4, 2002
    5V4 draws slightly more heater current than GZ34 (2A vs. 1.9A). Not enough to worry about, really, so yes, a swap should be safe.

    You will get slightly lower B+, so the amp may feel a bit softer.
    Timbre Wolf likes this.
  3. PurpleJesus

    PurpleJesus Internet Moderator/Super Hero Staff Member Supporting Member

    Jul 11, 2014
    Hogy, would you be willing to give us a Cliff's Notes version of what exactly a rectifier does? I know it's a AC to DC conversion, but I read about heater current a lot, various power levels, and sag characteristics "feel" between the different versions of rectifiers.

    So, what's what?
    mabinogeon likes this.
  4. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

    Oct 14, 2005
    Berkeley, CA
    FWIW - I ran a Vox AC30CC with a 5V4G subbed for the GZ34, and had years of trouble-free playing. As Hogy mentioned, the "softer" feel of lowered B+ is apparent, with a little less dynamic punch.
  5. Baxtercat

    Baxtercat Member

    May 11, 2009
    just west of Monterey, CA
    Make sure it's not loose.
    Tube should be in there securely, or some pins may lose contact.
  6. teemuk

    teemuk Member

    Apr 21, 2009

    Ignore for a moment that discussion on these sites is about rectifying with solid-state diodes. It’s rectifying nevertheless and vacuum tube diodes work following similar principle.

    A rectifier is a device that only passes current into one direction. Therefore rectifying can be used to convert AC (alternating current) into DC (direct current). In half wave rectifying (of power supply) the diode(s) conduct current only during one of the half waves of the sinusoidal AC mains waveform. In full wave rectifying diode(s) conduct current at both half waves, however, the current is “steered” into one direction only, therefore generating essentially a DC waveform. (It’s effectively “unipolar”, either "positive" or "negative" current flow but not both).

    The full wave rectified waveform is basically like a sinusoidal signal with one of the half waves “folded”. It’s “pulsating” but nevertheless DC. Filter capacitors of the power supply store a charge when the rectified current increases and in turn supply current during periods when the rectified current decreases. This filtering produces the “steady” DC voltage one needs to power typical electronic circuits.

    In practice there will be some “rippling” as well so its not steady DC like that you'd get from a battery power supply.

    A vacuum tube will need heating of the cathode element (either directly or indirectly) in order to work. This is done via “filament”. Heating up the filament will naturally require current and a vacuum tube diode will therefore also draw additional “filament current”, which it needs to work. Different vacuum tube diodes will draw different amounts of such current.
    Because solid-state devices do not work by "boiling off electrons" they do not require filaments and they do not therefore draw filament current either.

    In comparison to solid-state diodes vacuum tube diodes also have significant internal resistance. Internal resistance of a semiconductor diode in forward conduction is in the region of few ohms. Internal resistance of a typical vacuum tube diode is several dozens of ohms, perhaps even a few hundred ohms. This resistance will, as usual, oppose current flow, which in turn limits the rectified voltage swing during periods of higher current draw. In effect, the rectified voltage will drop, or in other terms “sag”.

    Solid-state diodes introduce a similar current-opposing effect but when employed in high impedance tube circuits the effect is negligible due to very low internal resistance of the SS diode. In low impedance solid-state circuits, however, even the solid-state diode will introduce significant amounts of “sag”.
    A SS diode can be made to introduce “sag” in high impedance circuit very easily: Just introduce some external series resistance to it.

    Sagging of the voltage(s) that powers an amplifier circuit will most distinctly cause decreasing of headroom of the amplifier, as the amplifier will “clip into rails” more easily when the supply rail voltage is decreases. Essentially “sagging” limits the maximum “clean” or “linear” output voltage swing of the amplifier. Being a dynamic effect it can be quite “musical”.

    In addition, sagging will typically elevate the "ripple voltage" as charging of the filter capacitor is hindered when current draw is opposed. The ripple in the supply voltage will modulate the amplifier's output signal during clipping. This modulation generates offtune "ghost notes".

    Note that I use the term "amplifier" in this context rather freely, and it can refer to a complete amplifier system, just a single gain stage that amplifies, or everything in between.
    Last edited: May 9, 2019

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