master volume problem

[worth]

I've installed this type - 3 master volume in a stock Deluxe Reverb. Although it sounds fine , the master never goes to zero... in other words , there is ALWAYS quite a bit of audible signal when the pot should be turned off. I've tried 3 different Alpha pots , no difference. Can anyone see why this happening ?

[jjasilli]

Of the 4 types of Trainwreck MV's, Type-3 is the only one which does not show a ground connection for the pot.  Is this an error?

[sluckey]

That MV is called the crossline MV. It's wired correctly. It works by shorting the two output tube grids together when the pot is turned to zero. Since you are now applying the same signal to each grid the common mode rejection properties of the output tubes/OT should cancel the signal completely....... on the drawing board.  

In the real world the power amp probably isn't perfectly balanced. One side will usually amplify more than the other one and pass some signal even though the grids are seeing the same signal. Therefore, you can never turn the volume completely down if there is any imbalance.

Sears used a switch to short the two grids together in their 1484 Twin Twelve. They called it Standby. Worked pretty good most of the time.

Edit... I should also say that this mv depends on the PI outputs being well balanced also. For example, if the PI puts out two equal amplitude signals that are 180 degrees out of phase and the mv is set to zero ohms, the two signals will cancel perfectly, leaving zero signal to apply to the PA. But if the PI really outputs a 20v signal to one grid but a 15v opposite phase signal to the other grid and the mv is set to zero ohms, the two signals don't perfectly cancel. After the two signals fight it out there will still be a 5v signal left over. This 5v signal appears on both grids of the PA. Now if the PA circuit is unbalanced, some of that signal will get to the speaker.

[kagliostro]

What about to try this ? 

Kagliostro

[dynaman1]

I just used the crossline MV in a Spitfire build and it's worked well for me.

[FYL]

Crossline is OK with cathode biased amps, doesn't work so well in fixed bias models.

[The_Gaz]

Do you know the reason for this? I've read this same experience, but no explanation - I'm really curious!

[worth]

What if I "balance" the PI by using two 100Ks' instead of 100K/82K ?

[worth]

The reason for my original question is this... I installed this MV , maybe four years ago. I KNOW it was right then , ( I'm too proud to give this back to a customer if the MV doesn't do what it's supposed to).... and the customer swears that there was never a master volume issue until very recently.. could uneven ouput tube wear , ( imbalance ) over a few years , produce this "bleed through" ?

[sluckey]

Swapping tubes is easy.