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UNPLANNED DOWNTIME: 12th Oct 23:45

Valve Testing Parameters

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jaymenonjaymenon Frets: 761
in Amps
- MHOS
- Gain
- Transconductance

Many sellers provide only the MHOS value.

Is there a practical benefit to knowing the other two (and possibly other further) parameters please?
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Comments

  • RiftAmpsRiftAmps Frets: 2989
    edited September 2023 tFB Trader
    Unless you know the operating conditions of the test then those figures are somewhat redundant, so of little use to the purchaser.

    The gm (MHOS) result is arguably the most useful indication of the valve's performance, so often is the only result published.
    *I no longer offer replacement speaker baffles*
    Rift Amplification
    Handwired Guitar Amplifiers
    Brackley, Northamptonshire
    www.riftamps.co.uk

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  • ecc83ecc83 Frets: 1488
    I assume we are talking about used valves here? New, unused valves will have the parameters found in any data sheet.

    The figures for gm, Ra and mu are related such that mu=Ra.gm but as Mr R says, "depends on conditions". That is why valve data is delivered as curves of Ia v Va for a fixed grid voltage series and the ACTUAL working parameters derived from a 'load line'

    I can strongly recommend Merlin Blencowe's "Tube Preamps For guitar amplifiers" The math is '0' level at its worse. (not that I GOT any O levels!)

    The amplification factor 'mu' is fixed by the internal geometry of the valve and hardly changes with age and use. MOST of the claims for different valve 'tone' for the same type number are IMHO so much snake oil.

    Dave.
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