Crossline MV

[acheld]

OK, Bash posted about a new project using a crossline master volume.    This is entirely new to me, and as I reviewed Matchless Lightning https://el34world.com/charts/Schematics/files/Matchless/Matchless_lightning.pdf, it is an interesting beast.    Latole chimed in with a variant that appears safer to me, but the general idea appears to be unbalancing the drive signal to the output tubes.

How does this work?     What does it sound like?

[pdf64]

As the resistance of yhe master volume pot is reduced, each anode circuit loads the other. That reduced the signal voltage they can put out.
It works, doesn’t affect balance, sounds ok with 12AX7 LTP, not so nice with 12AT7.
The main drawback (with the 12AX7 LTP) is that before even 20dB signal reduction has been achieved, the balanced outputs flip into being common mode.
So the sound mutes.
So it operates ok for stages volume reduction but the sound mutes before there’s sufficient volume reduction in a domestic scenario.
Rob’s amendment of type 3, moving the control to downstream of the grid stoppers, may help with that.

[Ronquest]

The main drawback (with the 12AX7 LTP) is that before even 20dB signal reduction has been achieved, the balanced outputs flip into being common mode.
So the sound mutes.

Is this dependent on the power tubes or preceding stage?  I have not notice this on the few amps that I've used this type of MV.

[pdf64]

The main drawback (with the 12AX7 LTP) is that before even 20dB signal reduction has been achieved, the balanced outputs flip into being common mode.
So the sound mutes.

Is this dependent on the power tubes or preceding stage?  I have not notice this on the few amps that I've used this type of MV.

No, it’s just what it does, in its standard Trainwreck Pages implementation.
So have you been able to achieve more than 20dB signal reduction before it mutes? Or the muting isn’t due to the flip to common mode operation?

Here’s a MEF thread where this has been discussed, and where Ive posted test results and scope traces https://music-electronics-forum.com/forum/music-electronics/920070-master-volume-and-treble-loss

[sluckey]

I put the crossline in my Lightning. I hated it. It works but it was all or nothing. I had to turn and turn from zero until I had some volume then just a slight increase would result in too much volume. Very hard to dial it in. Maybe I had my pot wired backwards or should have used a linear pot. I never bonded with that amp for other reasons as well. Put it on the top shelf and never played it again. Later it became the donor for my AC-15. No regrets!   

[shooter]

2nd this ^^^
I stole from Steve when I built my version, the amp n sound was well liked, the guitarist said "this knob, we're not sure what it's for, it it like a mute?"
IIRC I went so far as to scope the 2 out of phase signals and add fixed R's to one side or the other of the pot to keep each side "balanced"
heard back from the new owner after about 6 weeks, I asked about the MV pot, said he put it on 7 and never touched it.


working schematic fwiw, many undocumented changes

[Latole]

I build the MV on few amps, it work very well.
Last mod fews days ago on a Traynor YBA-1 Bass Master

[Latole]

That's the double pots

[Ronquest]

No, it’s just what it does, in its standard Trainwreck Pages implementation.
So have you been able to achieve more than 20dB signal reduction before it mutes? Or the muting isn’t due to the flip to common mode operation?

Here’s a MEF thread where this has been discussed, and where Ive posted test results and scope traces https://music-electronics-forum.com/forum/music-electronics/920070-master-volume-and-treble-loss

The circuit worked, as far as, it gave the results that I wanted.  I never noticed any sudden muting, so there was never a question or any measuring of db reduction.  I know it's not the best PPIMV but it's simple.   Now I started reading that thread and will continue as it is an interesting subject and it may sway me from further use.  Always appreciate your support!

[kagliostro]

One other interesting PPIMV (from Rob Robinette)

if I were you I'll give a try

Franco

[Latole]

One other interesting PPIMV (from Rob Robinette)

if I were you I'll give a try

Franco

It is almost the same I talk about ; answer #6.

[pdf64]

One other interesting PPIMV (from Rob Robinette)

if I were you I'll give a try

Franco

I don’t like this part of Rob’s notes

The power tube grid leak resistance does not change when the
master volume is adjusted which keeps the bias voltage from changing.

as it implies there’s more than negligible V DC across the grid leak resistance. Which there isn’t, unless the valve is bad.

[kagliostro]

It is almost the same I talk about ; answer #6

Absolutely not

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About what say pdf64 there is one other variant in which the wiper  of the pots are connected to G1 of power tubes but this way some things must be changed, you must add 2.2M safety resistors

I think I have a schematic of this version, when I find it I'll post it

Franco

[Latole]

It is almost the same I talk about ; answer #6

Absolutely not

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About what say pdf64 there is one other variant in which the wiper  of the pots are connected to G1 of power tubes but this way some things must be changed, you must add 2.2M safety resistors

I think I have a schematic of this version, when I find it I'll post it

Franco

It should of course be understood that my schematic is only the main part.
Grill safety resistors must be added or modified depending on the amplifier used.

Kevin O'Connor on his book The Ultimate Tone Vol 1 page 5-9 wrote this is his preferred MV circuit.

And I build it few times

[kagliostro]

Ciao Latole

I don't doubt you had good result with the KOC version of PPIMV, only the version of Robinette is very different

In KOC (and many other version) there are Grid Leak resistors and dual ganged pots, in Robinette (Frondelli) version the Grid Leak resistors are swapped by the pots, more, as he explains the G1 of the power tubes in the KOC version are connected to the signal path via the wiper of the pots, in the Robinette the G1 are connected at the extremity of the pots and the signal path is from the coupling caps of the PI to the wiper of the pots, this way the value of Grid Leak resistors don't vary, if something is varying is the load on the PI, but this is all a different affair

Which will be better for the specific situation ..... as I told, it is to be tryed

Franco

[stratomaster]

On the Frondelli the resistor to ground (or bias voltage) as seen by the signal coming from the PI couldn't cap varies.  This modifies the corner frequency of the high pass that these two components create. As you turn down the sound will lose bottom end.  Depending on the cap value this may or may not be an issue. 

I've installed one of these on an amp with 0.022uF PI couplers. The loss of bottom end starting at around 2 o'clock when turning counter clockwise was very noticable.  At 12 o'clock the corner frequency is almost 300hz.

0.1uFs would probably not be an issue.

[PRR]

On the Frondelli the resistor to ground (or bias voltage) as seen by the signal coming from the PI couldn't cap varies.

You meant to type "PI coupling cap varies", I bet.

Yes, when turned well down the bass is cut a lot more than midrange. IMHO that may be the "wrong" sound, since your ears also loose bass at soft levels. (OTOH if the neighbors' complaint is "all that booming!" through the walls then bass-cut may be the best of a bad deal.)

[Leevi]

I put the crossline in my Lightning. I hated it. It works but it was all or nothing. I had to turn and turn from zero until I had some volume then just a slight increase would result in too much volume. Very hard to dial it in. Maybe I had my pot wired backwards or should have used a linear pot. I never bonded with that amp for other reasons as well. Put it on the top shelf and never played it again. Later it became the donor for my AC-15. No regrets!   

1MA crossline pot in Vox and Matchless circuits is too big. I normally use 250KA which works much better.


/Leevi

[kagliostro]

......... there is one other variant in which the wiper  of the pots are connected to G1 of power tubes but this way some things must be changed, you must add 2.2M safety resistors

I think I have a schematic of this version, when I find it I'll post it

Here it is (source Rob Robinette)



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BTW, there is a way to obtain a Volume control that isn't a Pre MV nor a PPIMV


(in the Rebel 20 this is used in junction with a PPIMV)

look to VR105 on the PI of the Egnater Rebel 20



In one of his books Merlin explain how to avoid unwanted scratch noise when rotating the pot connected that way when playing



Franco