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Thread: How does this Marshall Hi/Lo power switch work?

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    Supporting Member Randall's Avatar
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    How does this Marshall Hi/Lo power switch work?

    Marshall JCM 900 SLX 100W 2100.

    http://www.thetubestore.com/lib/thet...-Schematic.pdf

    How does this Ho/Lo switch work? I see in one position the screens see plate voltage through 1K 3W resistors, but I'm not getting what is going on the the other position. Customer believes one setting is 100W and the other is 50W, I don't think is the case, and I need a solid stance to present to him.

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    It's weird, because it WAS working fine.....

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    Supporting Member The Dude's Avatar
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    It's a triode/pentode switch. In one position, the 1K screen resistors are fed from the B+ (plate node) of the PS. In the other position, the resistors are fed from another PS node.

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    Lifetime Member Enzo's Avatar
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    In one position, the screens are connected to the screen supply in the B+ string. In the other position, the screens are connected to the tube plates, making the tube a triode instead of a pentode. That substantially reduces output, but I leave it to wiser people to determine the amount of reduction.

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    Supporting Member The Dude's Avatar
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    I don't think it would be quite half power. Even if so, it wouldn't equate to half volume. 2 different things.

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    Supporting Member Jazz P Bass's Avatar
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    Measure the output to see exactly what the difference in output power is.

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    I retubed a 4100 recently, same deal, Power was half in triode mode, biggest change was the sound not volume.
    IMHO that mode eats some type of tubes faster.

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    ...biggest change was the sound not volume.
    +1
    It's not so much a "power switch" as a "tone switch".
    In triode mode the sound becomes "rounder" but to me it sounds dull. I prefer the pentode tone.

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    Lifetime Member Enzo's Avatar
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    So many people equate power with loudness. If we cut the power output in half it only reduces the loudness by 3 decibels, not much. SO yes the result is more a matter of tone and feel.

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    There is only a 3db reduction in volume. Not really effective. The output transformer then isn't driven enough, which equates to tonal differences.
    First prize would be to get a smaller amp. Second prize, install a ppimv.

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    I suspect that the most significant contributor to the tonal difference between triode and pentode modes is due to the lower gain and output impedance of triode mode.

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    In the half power position then output tubes are in triode mode as guys have said above.
    With the tubes in triode mode you get a VERY large increase in Miller Capacitance at the output tube grids (coz the screen grid is no longer effective in shielding the anode from the grid) and lose top end (the phase splitter can't drive that capacitance as well).
    It is similar in effect as if you had wired about 470pF from the output tube grids to 0V.
    Often described as a "darker" tone - in fact that is the usual criticism of triode mode in Git Amps.
    Cheers,
    Ian

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    Bent Member Chuck H's Avatar
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    According to my J.L.Hood book the ideal OT primary impedance is considerably lower when pentodes are wired for triode operation. Assuming this is true (as I must because it's J.L.Hood) some of the power reduction (and tonal change/detriment?) may be due to mismatch from an ideal load on the power tubes. And I have to wonder what difference idealizing the load might make both for power and tone. Especially considering the popularity of triode operated pentodes in hi fi amps.?. Just an observation that I can't reconcile with my tech limitations.

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    Quote Originally Posted by Gingertube View Post
    In the half power position then output tubes are in triode mode as guys have said above.
    With the tubes in triode mode you get a VERY large increase in Miller Capacitance at the output tube grids (coz the screen grid is no longer effective in shielding the anode from the grid) and lose top end (the phase splitter can't drive that capacitance as well).
    It is similar in effect as if you had wired about 470pF from the output tube grids to 0V.
    Often described as a "darker" tone - in fact that is the usual criticism of triode mode in Git Amps.
    Cheers,
    Ian
    Thanks for the explanation Ian. That triode "darker tone" - some of my customers love it. Some, not so much.

    I'm sure the hi fi amps that use triode have OT's made to match impedance. Whether there's some treble pre-emphasis (or feedback compensation) to make up for the Miller effect treble loss, that's optional and I'm sure what makes the difference one brand of amp to another. Here's another place for the smart custom amp builder to use his/her brain & ears.

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    Miller effect or lack of treble boost over voice coil inductance (present in pentode current drive, less present in 'mixed' triode drive)?

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    Some of my Hifi builds use switchable pentode/triode, though the OTs are for pentode use. I use a local feedback loop to help linearize operation. There isn't that much tonal difference overall - quite subtle, though there's a slight increase in volume in pentode mode. I think the main difference is that hifi amps don't intentionally distort so a guitar amp emphasizes the different characteristics a lot more.

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    Concerning the OT primary Marshall are using their "regular" type OTs so nothing special about that.
    In one of my 100W amps I wired only two of the tubes to be switchable in Pen/Tri mode and there's a clear audible difference in tone at low and higher levels.

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    I guess other factors in triode connected screen operation is that there is no screen sag compression and the PI has to swing harder for equivalent(ish) volumes/output so there is more PI dirt in the tone.

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    Quote Originally Posted by Leo_Gnardo View Post
    Thanks for the explanation Ian. That triode "darker tone" - some of my customers love it. Some, not so much.

    I'm sure the hi fi amps that use triode have OT's made to match impedance. Whether there's some treble pre-emphasis (or feedback compensation) to make up for the Miller effect treble loss, that's optional and I'm sure what makes the difference one brand of amp to another. Here's another place for the smart custom amp builder to use his/her brain & ears.
    For HiFi there are usually several things going on.
    1st - they usually have beefier phase splitters or dedicated drivers after the phase splitter to better drive that increased Miller Capacitance at the output tube grids. Thats why HiFi guys like Concertina (Cathodyne) Splitters - they have very low output impedance.
    2nd - The output impedance of the output tubes themselves is much lower in triode mode. That means better drive of the primary inductance of the OT (so the bass roll off [fromed by teh outtube output impedance and the primary inductance] will be lower) AND better drive of both the leakage inductance (primary to secondary) and interwinding capacitance (promary) of the OT so the TWO High frequency roll offs will both be moved higher. So Triode Mode gives better Low and High frequency response from the OT.

    Cheers
    Ian

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    Last edited by Gingertube; 11-15-2017 at 06:41 AM.

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    I think dissipation in the tubes is about the same in the two modes. Compared to the pentode mode, the triode mode can draw less peak current, but the voltage drop across the tube is higher when this peak current occurs and so V*I would be similar. I think you would have to measure or model to really know.

    It is what happens when the screen is not kept at a high enough voltage to keep accelerating the electrons to the plate.

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    Senior Member Pedro Vecino's Avatar
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    Dissipation is the same in that amplifier. The main problem of the triode mode is the dependence of the symmetry on the iddle currents of the tubes. It is much more susceptible to generating hum. In fact, a difference of 4 or 5 mA. (negligible in penthode mode) presents a hum in triode mode more than obvious.

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    Senior Member Malcolm Irving's Avatar
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    At the usual operating point, the plate current in a pentode is not very sensitive to plate voltage (in other words the plate characteristics are almost flat). A hum voltage on the supply to the plate will therefore not create much hum current in the speaker.
    The plate current in the pentode is more sensitive to screen voltage, but that is invariably controlled by a further power supply smoothing stage
    In triode mode, the plate current is sensitive to plate voltage and hum voltage on the plate supply will create greater hum current in the speaker.

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