Master volume and treble loss

[Boy Howdy]

As we all know volume controls are very touchy down low on the dial. For a clean sound I'm running at about .5 (that's one-half of one) and it's a bit of a pain to get it just right. So I generally just set roughly where I want it and then use the master volume to control the actual volume.

I have a Ken Fisher type PPI master on the amp. mod101_ppimv_master_volume_mod.pdf (tubesandmore.com) The problem is that when I turn down the master, I lose treble. So then I have to do this dance of balancing the two volume controls to get the best sound, which defeats the whole purpose of using the master in the first place.

Can I install bypass caps on the master to recover the lost treble?

[pdf64]

Err it would be helpful to provide the specifics of the amp you’re asking about

[Helmholtz]

Yes you could.

But the actual place where treble is lost in the amp is the vol pot and the input capacitance of V1b at low vol settings.
Have you tried a bright cap of 47p to 150p across the vol pot?

Also I wonder why you use a dual 100k master pot. That low value loads down the PI output. I would use a dual 470k pot instead and try 100p to 330p bright caps if necessary. But as the MV is within the NFB loop, the bright effect will be small.

[vintagekiki]

No treble loss

[Helmholtz]

No treble loss

The OT's MV doesn't cause treble loss either (except maybe for some Fletcher- Munson effect at low volume). Don't forget pentode input capacitance is very small.

And cross-phase MVs cause bass loss at low settings.

[pdf64]

No treble loss

Did you see the thread where Jon Snell raised the issue of a common mode positive bias caused by the type 3 / crossline master vol? https://music-electronics-forum.com/...olume-question

[Boy Howdy]

As we all know volume controls are very touchy down low on the dial. For a clean sound I'm running at about .5 (that's one-half of one) and it's a bit of a pain to get it just right. So I generally just set roughly where I want it and then use the master volume to control the actual volume.

I have a Ken Fisher type PPI master on the amp. mod101_ppimv_master_volume_mod.pdf (tubesandmore.com) The problem is that when I turn down the master, I lose treble. So then I have to do this dance of balancing the two volume controls to get the best sound, which defeats the whole purpose of using the master in the first place.

Can I install bypass caps on the master to recover the lost treble?

Yeah, I always put a bright cap on the volume control, can't live without it. This time I actually installed a 470K resistor in series with the cap to get it just right. That worked out really well. I get what I want there, as long as I don't turn down the master too low.


The dual 100K is just what the recipe called for. I didn't give a second, or even a first, thought. . . . Gulp.

[Boy Howdy]

No treble loss

I actually have used that type of master on another amp I have. I wanted to compare the two types. I can't remember what I concluded about the differences, but the single pot version does work well, is cheaper and sounds good. I may just take out the dual master and go with the one pot master.

[Boy Howdy]

Err it would be helpful to provide the specifics of the amp you’re asking about

It's the amp in the schematic I posted, slightly modified.

[Boy Howdy]

Did you see the thread where Jon Snell raised the issue of a common mode positive bias caused by the type 3 / crossline master vol? https://music-electronics-forum.com/...olume-question

I'm not terribly technical - I take it that this type of master is not a great idea?

Here's a little spur of the moment thing I recorded of a Garnet PA I turned into Marshall that uses that Matchless type MV. The guitar is a Strat, the volume is at speech level. GARNET REBEL PA 90 BLUES by gene downs | Free Listening on SoundCloud

[J M Fahey]

Loved it.

[vintagekiki]

The OT's MV doesn't cause treble loss either (except for some Fletcher- Munson effect at low volume). Don't forget pentode input capacitance is very small.

And cross-phase MVs cause bass loss at low settings.

Unfortunately, I do not share your opinion. Treble loss occur at resistive attenuators, so it is necessary to perform capacitive compensation (e.g. instrument probes).
At amps the resistive attenuator is a volume control.
To compensate treble loss in the process of volume control, in the assembly volume control, manufacturers install capacitors which raise treble (bright/ treble boost).
At Fender old generation it's bright sw (120pF), Marshall (JMP, JCM) it's fixed treble boost (.005u), Vox AC30 it's fixed treble boost (100p), etc.

About Fletcher-Munson curves
The Fletcher-Munson curves (not effect) is some story from the Hi-Fi domain which indicate the ear’s sensitivity to different frequencies at various levels. That would not be copy / paste, whoever is interested can be informed here:

https://en.wikipedia.org/wiki/Equal-loudness_contour

https://www.teachmeaudio.com/recording/sound-reproduction/fletcher-munson-curves

About CLMV
MV 1MA pot simply mixes the two mirror image audio streams from the phase inverter together. The two streams will cancel each other out.
Matchless and in some new Vox korgusa amp series are used CLMV

Because of its simplicity, I use the CLMV schematics for years when the user requests to install an MV.
It has performed well at old generation Cender, Marshall, Vox amp.
Bass loss at low volume settings was not significantly noticed.

https://music-electronics-forum.com/forum/amplification/guitar-amps/theory-design/12307-cross-line-master-volume

https://schems.com/bmampscom/matchless/

http://bee.mif.pg.gda.pl/ciasteczkowypotwor/%23SM_scena/VOX/Vox_AC30CC2_AC30CC2X%20(2005)%20SM.pdf

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

[Helmholtz]

Treble loss occur at resistive attenuators, so it is necessary to perform capacitive compensation (e.g. instrument probes).

A resistor or resistive voltage divider cannot produce treble loss by itself. Rather treble loss is caused by a low pass filter which requires a series resistance and a shunt capacitance to ground.
As the input capacitance of pentodes/tetrodes is very small, a dual pot MV itself will not produce any treble loss in the audio range.
But there is some possibility, that long (shielded) wires to and from the MV have enough shunt capacitance to actually cause some audible treble loss.

A bright cap especially makes sense when a vol pot feeds a high gain triode stage, as there will be considerable Miller capacitance, typically >100pF.

For example, with 100k series resistance and 200pF shunt capacitance the -3dB frequency is 8kHz.
The input capacitance of an EL34 is only 16pF.

About CLMV
MV 1MA pot simply mixes the two mirror image audio streams from the phase inverter together. The two streams will cancel each other out.

If you mean signal currents by "audio streams", your interpretation is wrong. The 2 PI output currents do not cancel with a CLMV, rather they add/double and force excessive PI signal currents at low resistance/ CLMV settings. In fact a CLMV acts like loading/ shorting the plate outputs of a push-pull amp.
Don't forget that PI plate voltages are opposite phase. So when one plate goes positive, the other goes negative. A load connected between the PI outputs sees doubled signal voltage. Consequently a CLMV causes increased PI currents. No cancellation effect at all.

[vintagekiki]

CLMV Question

If signal at point A as at the picture, what will be the signal at points ?

[Helmholtz]

CLMV Question

If signal at point A as at the picture, what will be the signal at points ?

Well, you surely know that shorting kills voltage but may cause high currents.
So signal voltage between PI outputs will be zero. But you may still have considerable common mode (in-phase) voltages wrt ground as Chuck H demonstrated in the other thread.
PI triodes will work at their limit to supply short circuit currents.

Equal common mode grid signals cause plate dissipation but no output.