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Thread: Two tone stacks parallel after cathode follower

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    Two tone stacks parallel after cathode follower

    I guess it's possible to connect parallel e.g. two classic Marshall tone stacks after cathode follower and use relay switching only on outputs of stacks (tone stacks for crunch and lead channel) and it will work, but:
    - isn't it too high load for cathode follower ?
    - won't affect changes first stack tone controls frequency response / sound on output second tone stack ?
    - doesn't two stacks parallel change operational point of follower and so signal harmonics / shape of overdrive signal instead of usually only one tone stack ?

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    You could use a double pole relay to switch both the input and outputs to isolate the front and back end of the stacks from each other. This way there will be no interaction of the two. I hope I understood your intention when suggesting this. Also, the cathode follower will drive a wide variety of stacks, so I wouldn't worry too much about the difference in them unless they have a very different input impedance.

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    Turn it up so that everything is louder than everything else.

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    Checkout the Marshall JVM410. Series resistor between the treble viper and volume for each channel, then channel mute and a series mix resistor.

    No need to switch on the input side of the tone stack.

    EDIT: Schematics here: http://forum.metroamp.com/viewtopic.php?f=25&t=21932

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    Last edited by d95err; 10-13-2015 at 07:31 PM.

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    Of course, double pole relay is standard circuit to disable any interaction between two tone stacks, but I would like to know if possible to do it only with one pole relay, two stacks parallel, without some major difference in sound and circuit operation.

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    Thanks, I have this schematic, it's clear.

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    Old Timer J M Fahey's Avatar
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    Short answer: the cathode follower *easily* drives 2 regular Marshall tone stacks, just switch between stack outputs or if you wish, between following volume controls if you have them there.

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    Juan Manuel Fahey

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    But there may be still significant interaction between the two stacks. It would be wise (and not too difficult) to check and possibly optimize Your intended setup with LTSpice.

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    Quote Originally Posted by bea View Post
    But there may be still significant interaction between the two stacks. It would be wise (and not too difficult) to check and possibly optimize Your intended setup with LTSpice.
    I have to second this. Cathode followers have low impedance when they operate linearly, but halving the impedance of the load decreases the linear voltage swing by a factor of two. (You only have so much current available)

    If a stage of gain operating into a cathode follower is operating linearly, then the low impedance of the CF (order of 1/gm) keeps interaction low when driving two stacks. But if you drive two stacks instead of one when one is intended, it would reach its linear limit sooner, and then you have interaction since you lose the low impedance.

    Also, if you over drive the stage of gain driving the cathode follower, then the follower does not necessarily operate linearly, and then it does not have a low impedance output and you lose the isolation between the stacks.

    I think it is better to switch at the input unless you can be sure neither of these things happens.

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    And each tone stack sees the impedances of the other one. The system is hardly overseeable.

    An alternative to a switch in front of the stacks might be one or two JFET source followers after the cathode follower. With a common input resistance somehow mimicking the load of one single tone stack. Then You should be able to safely switch with a single switch.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Another strategy is to swap the CF cathode resistor for a transistor or MOSFET based constant current source of around 2 to 3mA. Very inexpensive and drives capacitive loads very well.

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    Quote Originally Posted by bea View Post
    And each tone stack sees the impedances of the other one. The system is hardly overseeable.

    An alternative to a switch in front of the stacks might be one or two JFET source followers after the cathode follower. With a common input resistance somehow mimicking the load of one single tone stack. Then You should be able to safely switch with a single switch.
    The two tone stacks do not see each other as long as the impedance of the cathode flower is low. You do not want to use solid state with such a large voltage swing

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    Quote Originally Posted by nickb View Post
    Another strategy is to swap the CF cathode resistor for a transistor or MOSFET based constant current source of around 2 to 3mA. Very inexpensive and drives capacitive loads very well.

    The current source is a high impedance; it cannot help here.

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    Old Timer J M Fahey's Avatar
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    Agree with last two and not much with earlier ones, but learnt not to argue against Deeply Established Beliefs

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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Agree with last two and not much with earlier ones, but learnt not to argue against Deeply Established Beliefs
    OK, but I would argue for the difference between a low dynamic impedance and the ability to drive a low impedance load to near the power supply rail. The extreme example is an op amp follower, which might have a few micro ohms output impedance at 100 Hz, but lack the ability to drive a 500 ohm resistor to near the power supply rails. This is not a DEB, but rather a DET.

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    If you watch the output of the common 12AX7 cathode follower when the signal is a big square wave you can see where positive to negative transitions are slewing as fast as the 100K can pull down. The tube cuts off until the 100K can pull the tone stack low enough to catch up with the cathode follower's input. This changes as you adjust Bass and Mid pots so there will be some interaction between two tone stacks if signals are large. One way around this is to use a cathode follower with at stronger current source. Perhaps a 12AU7 with 22K load.

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    If Marshall can make it work with 3 tone stacks plus the clean channel volume control in parallel from a regular 12AX7 cathode follower, it obviously works with two tone stacks.

    The key is the series resistor (39k) between the treble viper and volume to avoid interaction with the channel mute switching.

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    I was simply skeptical that the impedance of a 12AX7 as a cathode follower would be really low enough. Therefore my suggestion to spice before building.

    Another possibility to consider might be to check wether an ECC832 could be used instead (i.e. if the pinout matches so that the 12AU7 system of the tube would be in the position of the cathode follower.)
    Or if an ECC81 which combines high and low output impedance would work better.
    But always: spice before taking the iron.

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    I think that the waveform at the output of an overdriven dc cathode follower will be affected by adding more tone stacks to load it, in the same way that it is significantly affected by adding the first tone stack (compared to its unloaded output); the cathode follower's ability to sink/source current with large signals being the issue, rather than its small signal output impedance.
    But it will very likely sound fine and not be a problem, as per the Marshall 410.
    Whereas a different tube type for the cathode follower may be more likely to result in a difference that the user may notice.

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    But the Marshall 410 was just an example. The question was generic.

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    We would never want to do anything in a guitar amp that makes it perform in a non-linear fashion; that never works out well. ;p


    To paraphrase my main man Jim Croce, "If it sounds good do it. If it sounds really good, do it twice."

    (None of this is aimed at you PDF64, just seemed like a good place as any to interject).

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    Although a 12AX7 CF would probably drive two Marshall tone stack simultaneously without problems, I would use Fender tone stacks (but tweaked for a similar frequency response), i.e. higher impedance.

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    Why not use two CFs, one for each tonestack?

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    What would You do with the remaining triode?

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Seems we are trying to discuss in a data vacuum so I've got some info to add fuel to the fire

    I've done three things and I'm making the assumption that only one half of 12AX7 is available.


    1. Determine the input impedance of a Marshall tone stack. I'm calling this nominally 50K ohms - see the top chart below.
    2. Charted the output impedance of a 12AX7 with plate to cathode voltage of 150V vs plate current. On the second chart below the output impedance is 420 ohms with a plate current of 1.5mA and 350 ohms with 2.5mA. Gm goes up with plate current.
    3. Set up four sample cathode followers. All are fed with 140Vpp from with a source impedance of 50K. All are loaded with two tone stacks represented as 25K in series with 1uf ( for DC blocking). The supply is 300V and the source offset is 150V. See the bottom chart.
      1. Cathode follower with 100K cathode resistor (1.5mA plate current)
      2. Cathode follower with a 1.5mA constant current sink (CCS)
      3. Cathode follower with 60k i.e. 2.5mA plate current to try to lower output impedance and increase drive current
      4. Cathode follower with a 2.5mA constant current sink


    Conclusions:
    1. The 100k case only manages approx 20 Vpp out. This might be enough, it depends on the levels in your particular design. If your signal is small enough this is the way to go. Some compression.
    2. The 1.5mA CCS does a little better at 28 Vpp (at the expense of complexity). This happens because the high AC impedance of the CCS means all the signal current flows through the load. Least compression.
    3. The 60K case gets you to 72 Vpp but there is significant compression due to grid current.
    4. The 2.5mA CCS has 112 Vpp out, with moderate compression.
    5. The CF output impedance is so low that interaction between tone stacks will be minimal. I can't see that being an issue in practice.


    Click image for larger version. 

Name:	CF_ToneStack.PNG 
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ID:	36133

    'course this is just a simulation and is therefore subject to the setup, interpretation, models used, my mistakes and other random effects. Just my opinion on compression. Some might argue more is better

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    Quote Originally Posted by bea View Post
    What would You do with the remaining triode?
    Sorry, I don't understand the question. It was just an idea. Are you thinking I'd have a spare triode because I'd have to add a 12Ax7 for the extra CF? In that case I'd use the existing 12AX7 for a parallel gain stage and the added 12AX7 for the two CFs.

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    Quote Originally Posted by nickb View Post

    1. The CF output impedance is so low that interaction between tone stacks will be minimal. I can't see that being an issue in practice.

    If the CF runs out of current for any reason then it no longer has low impedance, and there is interaction. Since a tone stack stores energy, the interaction can take the form of mutually altered frequency responses.

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