6072A valves.... less gain but more 'current output'? What does that mean?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by 5F6-A, Jul 15, 2019.

  1. 5F6-A

    5F6-A Member

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    I have a GE 6072A that I'd like to use with an American voiced 1x8" valve combo that I will receive soon. What can I expect vs the stock Chinese 12AX7B? What do valve geeks mean by greater current output? Will it be louder? Thanks!
     
  2. Tron Pesto

    Tron Pesto Member

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    6072 is the "military spec" designation for a 12AY7, which is a "medium mu" dual triode. "Mu", all else being equal, basically means gain. A 12AX7 has a nominal* gain of 100 while a 6072/12AY7 has a nominal gain of 40 I believe.

    Dropped into the same circuit, the main effect is the gain of the tube. In simple terms, a 12AX7 will be noticeably louder than the 6072/12AY7.

    The other differences between the tubes are their plate resistances, transconductance, and as you mention their plate current handling capabilities. However, those differences typically have little effect on "loudness". They have greater effect in how the tubes can "keep up" with the demands of certain situations in the circuit, such as providing enough current to drive (or efficiently match the impedance of) a reverb transformer or provide sufficient signal and plate load characteristics in a phase inverter circuit.

    *nominal, as in what is listed on a datasheet given "ideal" conditions - which really don't exist in most circuits. Nevertheless given the same set of parameters in a circuit a 12AX7 will provide about twice the gain as a 12AY7.
     
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  3. DonP

    DonP Member

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    No, it will not be louder unless the current preamp tube is not providing enough gain for full output (weak or bad or wrong tube). Loudness is provided by the output (tubes / transistors).

    Your tube is a preamp tube. As stated above, the nominal gain is ~40. If you stick it in a 6072/12AY7 circuit, it will sound about the same. If you stick in a 12AX7 circuit, it will have less gain (than ~100), and possibly less loudness.
     
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  4. 5F6-A

    5F6-A Member

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    So basically, it will quiet down the amp. Right? Will it increase headroom?
     
  5. Tron Pesto

    Tron Pesto Member

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    Yes, it will quiet down the amp. In a sense, it will increase headroom in that you will have to dial it up higher before it begins to breakup - the thing is, it may beging to break up at the same "volume" - it's just that you'll have to turn it up higher to reach that volume.

    Nevertheless, the gain structure will change - with the amp on 10, it will be "cleaner" and less compressed at maybe a lower volume compared to the 12AX7 (remember, at some point, the amp doesn't get louder as you push the volume knob - it will get more compressed and less linear without necessarily getting louder in dbs).
     
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  6. 5F6-A

    5F6-A Member

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    I'm having second thoughts about it... I fear the amp might sound a bit lacking in oomph with the 6072A. I guess I won't know until I try it in the amp. The amp is designed with an 12AX7 valve in mind....
     
  7. Tron Pesto

    Tron Pesto Member

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    It won't hurt the amp to try - 5751 is another way (mu at about 70 I think) to go. You can pretty much drop in any 12A?7 type tube in the first preamp slot in an amp designed for a 12AX7.
     
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  8. HotBluePlates

    HotBluePlates Member

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    Adding to what others said:

    They mean, "I didn't understand the forum-babble I repeated."

    Most preamp triodes with lower gain than a 12AX7 also have lower internal plate resistance. In theory, they could pass more current than a 12AX7 (which has higher internal plate resistance).

    Except... Ohm's Law. Current = Voltage / Resistance, and preamp triodes usually have a load resistor. Maximum possible current for any tube in the socket is limited by supply voltage & load resistance.

    For example, if the supply voltage is 250v and the plate load resistor is 100kΩ, it is impossible for any tube put in the socket to flow more than 250v / 100kΩ = 2.5mA (and generally they will flow somewhat less in normal operation). This assumes the tube is able to reduce its internal plate resistance to 0Ω (can't happen).

    If the tube circuit was altered to take advantage of the capabilities of the lower-gain tube, then possibly it could pass more current. However, this is usually immaterial, as the preamp most often uses voltage amplifier stages, and current capability isn't a factor.

    This might be different with a phase inverter, which could need to deliver current into the output tube grids when they're overdriven. However, even then this theory will fall on its face because the output tubes will likely draw more grid current than any typical dual-triode can deliver.

    The 12AY7/6072 has an amplification factor around 44, while the 12AX7 is up around 100. As a first-guess, this means the 12AY7/6072 will provide ~44% of the gain of a 12AX7 stage.

    Being more complete, the 12AX7 doesn't provide a "gain of 100 times" because the internal plate resistance interacts with the load resistor; the net effect is that 12AY7 stage gain is usually something closer to 62 times the input voltage. The 12AY7's lower internal plate resistance interacts with the load resistor to deliver a higher-% of the amplification factor as actual gain; drop it into a "12AX7 socket" with a plate load of 100kΩ and a cathode resistor of 1.5kΩ, and gain is 28 (page 2 of this data sheet). That's about 45% of the gain of a 12AX7 in the same socket.​
     
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  9. Johndandry

    Johndandry Supporting Member

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    Is that voltage gain?
     
  10. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    This chart may be useful as you contemplate the excellent information above

    Note that the 6072A is essentially a 12AY7 type, so check out that 12AY7 line. The 12AY7 has similar transconductance (gm) to the 12AX7, but the dynamic plate resistance (rp) of the 12AY7 is less than half that of the 12AX7, rendering the amplification factor (mu) to be less than half. It does have a greater capacity for dissipating current at the plates, when compared with 12AX7, but really, that just means it is less likely to burn out if it is used to replace a 12AT7 in a BF Fender reverb driver position (when compared to a 12AX7 used there - a poor choice for that position).

    [​IMG]
     
    Last edited: Jul 15, 2019
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  11. HotBluePlates

    HotBluePlates Member

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    Oh... And separately, you should know that in most cases, vacuum tubes are voltage-controlled devices. That is, a voltage signal applied to the grid controls tube/plate current. This is true for most (if not all) preamp gain stages, and most guitar amp output sections.

    There is a special case for a certain type of output stage where the output tube grids draw current from the phase inverter/driver stage. However, the amplifiers that use such grid-current output stages usually have a tube like a 6V6, 6L6, EL34, etc driving a small transformer, which then drives the actual output tubes.​
     
  12. HotBluePlates

    HotBluePlates Member

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    If you're referring to my post, all preamp gain stages are "voltage gain" because they're voltage amplifier stages.

    There is an important (and non-obvious) detail to consider...

    Tubes such as the 12AV7, 6SN7, 12AU7, and 12BH7 in the chart show very low internal plate resistance (rp) and quite high transconductance (Gm). Note that low rp and high Gm occur at high plate currents, and the plate current listed must be flowing for the listed rp & Gm to occur.

    Said a different way, they are "show off" data sheet conditions; if you run the tube at lower idle plate current, the rp will be higher and the Gm lower. Really, you have to evaluate the tube's characteristics at the intended operating point, with the intended supply voltage, plate, and cathode resistors to know where Gm & rp will actually land.

    For example, check out the graph on page 3 of the 12AU7 data sheet, and then notice that in many guitar amps tube idle current is less than 2mA. If you run the 12AU7 at 2mA with 150v plate-to-cathode, rp rises from 7.7kΩ to 15kΩ, and Gm falls to ~0.94mA/volt.​
     
    Last edited: Jul 16, 2019
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  13. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    Interesting points - I was not aware of those extreme changes in operating conditions. Thanks!
     
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  14. HotBluePlates

    HotBluePlates Member

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    If the "plate characteristics" graph of a given triode had parallel straight lines, then transconductance, internal plate resistance and amplification factor would be constant values, regardless of plate current (distortion would also be zero, until the tube bumped into cutoff or saturation). The value for each of those 3 characteristics can be graphically found from the plate characteristic curves on a data sheet, and the bend in the lines (and the fact they're not always equidistant) points out where/how the characteristics change.

    However, the data sheets below each have a graph depicting the changing value of the characteristics at several different plate voltages. Note that Mu (amplification factor) is the product of Gm and rp, and is therefore the most constant (the other two characteristics change in opposite directions as plate current changes), but even Mu changes with a change of plate current (more in some tubes than others).

    12AX7
    12AT7
    12AY7
    12AU7
    6SN7
     
    Last edited: Jul 15, 2019
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  15. 5F6-A

    5F6-A Member

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    Great information. This will keep me busy for a while! Thanks!
     

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