Master volume and treble loss

[Helmholtz]

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.

[pdf64]

...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

[pdf64]

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?

[Helmholtz]

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.

[pdf64]

...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.

[Helmholtz]

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 0.1µ caps and multiplied with 5, without carefully reading the rest.

If we assume a differential source impedance of 88k, 0.1µ caps should give 36Hz and 0.02µ 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.

[pdf64]

...Look at practical solutions
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/%23SM_scena/VOX/Vox_AC30CC2_AC30CC2X%20(2005)%20SM.pdf
page 6

http://dealers.korgusa.com/svcfiles/TB18C1.pdf
page 3

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.

[vintagekiki]

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.

[pdf64]

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.

...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.