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    pdf64
    Supporting Member

  • pdf64
    replied
    Originally posted by vintagekiki View Post
    The resistors that you specify serve to separate the "tone control" (VR9/ C23 for AC30CC or R21/ C19 for TB18C1) from the volume control....
    That could well be one reason for their inclusion. But it may be telling that the designer has chosen to put the tone control before the resistors, and the master volume after them. That way around, the issues noted with regular Type 3 operation are resolved, whereas the other way, they are not.

    Originally posted by vintagekiki View Post
    ...I did not notice any convincing difference with and without these resistors.
    Even when attempting to set the master volume to achieve LTP overdrive at domestic friendly sound pressure levels? As without the resistors, that does not seem to be feasible, as the LTP loses differential outputs before even -20dB can be set.

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  • vintagekiki
    Uneducated

  • vintagekiki
    replied
    Originally posted by pdf64 View Post
    See R31 and R32 of the AC30CC and R47 and R40 of the TB18C1. These 10k resistors in series with the LTP outputs but prior to the master volume act eliminate the common mode effect that occurs (as described above) at low settings of a regular Type 3 master volume (ie the arrangement suggested in post #4 etc).
    They act also provide a somewhat improved range of control, eg it becomes feasible to achieve signal reduction of -20dB and more.
    The resistors that you specify serve to separate the "tone control" (VR9/ C23 for AC30CC or R21/ C19 for TB18C1) from the volume control.
    Personally, I did not notice any convincing difference with and without these resistors.

    Leave a comment:

  • pdf64
    Supporting Member

  • pdf64
    replied
    See R31 and R32 of the AC30CC and R47 and R40 of the TB18C1. These 10k resistors in series with the LTP outputs but prior to the master volume act eliminate the common mode effect that occurs (as described above) at low settings of a regular Type 3 master volume (ie the arrangement suggested in post #4 etc).
    They act also provide a somewhat improved range of control, eg it becomes feasible to achieve signal reduction of -20dB and more.


    Leave a comment:

  • Helmholtz
    Old Timer

  • Helmholtz
    replied
    Originally posted by pdf64 View Post
    2kHz doesn't seem right, please could you confirm, and, if you're happy it's correct, explain your rationale for reaching that result?
    You're right. 2kHz is nonsense. Seems my brain was too tired last night. Just saw 400Hz , assumed 0.1 caps and multiplied with 5, without carefully reading the rest.

    If we assume a differential source impedance of 88k, 0.1 caps should give 36Hz and 0.02 caps should give 180Hz as -3dB frequencies.
    The exact MV pot setting doesn't change much as long as its resistance is low compared to 88k.
    Helmholtz
    Old Timer
    Last edited by Helmholtz; 01-09-2021, 02:23 PM.

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  • pdf64
    Supporting Member

  • pdf64
    replied
    Originally posted by Helmholtz View Post
    ...88k should roughly correspond to twice the loaded source impedance of an ECC83 PI triode.
    Considering that other amps use 22nF coupling caps, moving the -3dB frequency to around 2kHz,..
    2kHz doesn't seem right, please could you confirm, and, if you're happy it's correct, explain your rationale for reaching that result?

    As I see it, it's a series circuit, comprising the source impedance of one LTP output, its 0.1uF coupling cap, the added 10nF, the master vol track resistance (probably about 10k, I can verify if desired?), then the 10nF, 0.1uF and source impedance of the other LTP output.
    So a total effective series capacitance of 4.545nF and a -3dB freq of 400Hz gave the 88k total effective resistive impedance I mentioned above, which equates to about 40k source impedance for each LTP output.

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  • Helmholtz
    Old Timer

  • Helmholtz
    replied
    Originally posted by pdf64 View Post
    I think that the LTP anode impedances, being in series with the high pass filter, act to prevent the filter freq moving up too high.
    That certainly makes sense.


    I changed the target to -12dB, and found that the -3dB freq was now about 400Hz, indicating that the total series resistive impedance was about 88k
    88k should roughly correspond to twice the loaded source impedance of an ECC83 PI triode.
    Considering that other amps use 22nF coupling caps, moving the -3dB frequency to around 2kHz, I would consider this effect significant, especially at low volumes.

    Thanks for sharing your results.

    Leave a comment:

  • pdf64
    Supporting Member

  • pdf64
    replied
    Originally posted by vintagekiki View Post
    The story about here debated is about Cross line master volume (CLMV)
    Please see post # 37
    I have already provided scope traces of the signal you are enquiring about.
    I'm sorry, I know this must be frustrating, but I can't understand why you keep asking for something that's already been done in post #34.
    Please try to explain what further information you want?

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  • pdf64
    Supporting Member

  • pdf64
    replied
    Originally posted by Helmholtz View Post
    ...And cross-phase MVs cause bass loss at low settings.
    The effect is not as significant as might it might first appear, I think that the LTP anode impedances, being in series with the high pass filter, act to prevent the filter freq moving up too high.
    To asses this, I added a 10nF cap in series with each 0.1uF coupling cap from the LTP anodes. The resulting -3dB freq was about 160Hz (100k grid leak resistors on the power valves). I then tried to reduce the LTP output voltage by 20dB but was surprised by the poor range of control, in that to achieve -20dB the control was almost at minimum (Alpha USA 15% nominal audio taper), such that the LTP output was no longer in normal differential mode, rather it was in transition to the common mode condition.
    I changed the target to -12dB, and found that the -3dB freq was now about 400Hz, indicating that the total series resistive impedance was about 88k

    Leave a comment:

  • Helmholtz
    Old Timer

  • Helmholtz
    replied
    Originally posted by vintagekiki View Post
    Who mentioned the measurements.
    Well, I did.
    Again: Please show your own measurements.

    You will agree there is no point in conducting a vain dialogue.
    No, a dialoque based on facts/evidence is never vain.
    So please show your own (not internet) facts.

    Leave a comment:

  • vintagekiki
    Uneducated

  • vintagekiki
    replied
    Originally posted by pdf64 View Post
    I don't understand, what parameter are you enquiring about?
    The Type 3 master vol pot is in a real amp, it is being tried, that is what the measurements and scope traces are in regard of.
    The 1M type 3 control pot has been in the amp for several years, however after initially fitting it, I didn't use it much, so then removed the wiring to it. So the redundant pot has been sitting there for a few years. I wired it back up yesterday for the purposes of the tests I've been reporting,
    The story about here debated is about Cross line master volume (CLMV)
    Please see post # 37

    Leave a comment:

  • vintagekiki
    Uneducated

  • vintagekiki
    replied
    Originally posted by Helmholtz View Post
    That's not an answer to my questions and I could return your statement to you. The truth is not in opinions but in real measurement results.
    Who mentioned the measurements.
    Why get into a story when the answer is known in advance.
    Originally posted by Helmholtz View Post
    I don't like this CLMV so I don't use it.
    You will agree there is no point in conducting a vain dialogue.


    Leave a comment:

  • pdf64
    Supporting Member

  • pdf64
    replied
    Originally posted by vintagekiki View Post
    ...The question is what do you get if g1 power tubes mutually shorted. (CLVM potentiometer at min).
    No current or voltage values are required, a logical conclusion is required.
    I don't understand, what parameter are you enquiring about?

    [/QUOTE]...By the way the oscillogram you specify is for type 3 master vol and "Both EL34 cathode current full overdrive min master vol.png"
    An honest question. Have you ever practically tried CLVM? It only takes 2 wires ... ...[/QUOTE]
    The Type 3 master vol pot is in a real amp, it is being tried, that is what the measurements and scope traces are in regard of.
    The 1M type 3 control pot has been in the amp for several years, however after initially fitting it, I didn't use it much, so then removed the wiring to it. So the redundant pot has been sitting there for a few years. I wired it back up yesterday for the purposes of the tests I've been reporting,


    Leave a comment:

  • Helmholtz
    Old Timer

  • Helmholtz
    replied
    Originally posted by vintagekiki View Post
    It is illusory to comment on something where you have expressed an opinion in advance.
    ]
    That's not an answer to my questions and I could return your statement to you. The truth is not in opinions but in real measurement results.

    I have
    Well, I have too.
    Helmholtz
    Old Timer
    Last edited by Helmholtz; 01-08-2021, 08:26 PM.

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  • vintagekiki
    Uneducated

  • vintagekiki
    replied
    Originally posted by Helmholtz View Post

    Actually only one.

    So, why don't you show your own measurements?
    Or don't you have a scope?
    It is illusory to comment on something where you have expressed an opinion in advance.
    Originally posted by Helmholtz View Post
    I don't like this CLMV so I don't use it.
    I have
    Click image for larger version

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  • vintagekiki
    Uneducated

  • vintagekiki
    replied
    Originally posted by pdf64 View Post
    The pic I posted is a screenshot of my scope traces taken from the amp mentioned, my scope being a USB picoscope. It's real, not a simulation
    With EL34 now in circuit and the type 3 master vol turned to min, their (dc average) cathode current rises about 50%, eg from 32mA to 45, when an input signal is applied, as per the scope trace pic previously posted.
    So the common mode effect does seem to be, in this case, real.
    I'm pretty sure the finding is reproducible.
    Here are traces of the signals at the EL43 1 ohm cathode resistors (I can't find my grounding clip for the red probe).
    The communication problem during machine translation arises due to different language syntaxs. sorry.
    The question is what do you get if g1 power tubes mutually shorted. (CLVM potentiometer at min).
    No current or voltage values are required, a logical conclusion is required.

    By the way the oscillogram you specify is for type 3 master vol and "Both EL34 cathode current full overdrive min master vol.png"
    An honest question. Have you ever practically tried CLVM? It only takes 2 wires ... ...

    Leave a comment:

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