Master Volume

[NannerPuddin]

I am getting a bit ahead of myself, since I have not yet built my amp.  I am building a BF Vibroverb clone, and I am concerned that it will be too loud some of the time.   What I am thinking is that I could put a Master Volume on the back with a pull knob that would activate it, so it isn't always a Master Volume, hopefully eliminating any effects on tone.  Now, I am still in the learning stages, so do any of you guys have ideas on what would be a good circuit for something like this?  I know there are probably several ways to construct a Master Volume and some probably would sound better than other.

Any help woulod be appreciated!!!         Thanks,  Doug

The forum members added to this thread to have a reference in ARCHIVES regarding master volumes.  The thread has been edited from it's original format.      With respect, Tubenit

[jhadhar65]

The MV most commonly seen is just a passive pot inserted prior to the phase inverter coupling cap.  It will change tone no matter what you do, but how much is too much is subjective.

There are other options, such as a master volume that goes after the phase inverter and this can be either a dual concentric pot or a cross line single case pot.

As for the switching option, that may or may not be necessary.  Thoughtful installation can make full up the same as no master.

I'd recommend a book or two that details MV options and installation.  Try Doug's site and look into some educational material that may cover the subject.  You'll need input from others here as to whether these references cover MV's or not.  I haven't read/viewed them.  I can recommend Kevin O'Connor's 'The Ultimate Tone', which does discuss MV's and has lots of example schematics to see.

[TubeGeek]

I would get the amp built and then experiment with all the different pre and post phase inverter designs out there.  This way you'll learn a ton and you'll be able to pick the master volume circuit you prefer in that amp.

Another option to consider is power scaling where you can control the amount of B+ voltage in the circuit which will lower the loudness while maintaining the tone as much as possible.

[The Radium King]

master volume. you'll bump into a variety -

pre-phase inverter master volume - this is just a volume control at the output of the preamp (much like the volume control that follows the tone stack in the middle of your preamp). it allows you to drive your preamp hard and get a bit of tube saturation, then attenuate the signal before it gets to the phase inverter and keep things quiet. typically a 1 meg pot, it affects tone by providing a 1 meg path to ground even when maxed; test and see if you find any noticeable effect on tone. a switch that lifts it from ground will take it out of the circuit. make sure you put the pot after the coupling cap, otherwise you'll get dc on it (scratchy).

post-phase inverter 'cross-line' master volume - this is a pot that connects the two outputs of the phase inverter. this mv relies on the fact that the phase inverter outputs are 'inverted' from each other and, when summed, cancel each other out. at '10', there is a 1 meg pot between the two phases and the circuit has no noticeable effect on tone. as the pot is dialed down more cancellation occurs and less signal reaches the power tube grids. this allows you to drive the preamp and phase inverter hard while reducing power amp output. you can use a switch to take the pot completely out of the circuit, or or use a no-load pot to disconnect the circuit when the pot is maxed (or make a no-load by opening up a pot and scratching off the carbon at one end). some folks use 250 k pots (they find they get more useable range from the pot); try and see. make sure you put the pot after the coupling caps, otherwise you'll get dc on it (scratchy).

post-phase inverter master volume - this uses a two-gang pot after the phase inverter and operates like a standard volume control on each phase inverter output, attenuating the phase inverter output as you turn it down. again, you get to drive the preamp and phase inverter while controlling signal to the power amp. 1 meg pots are typical, again after the phase inverter coupling caps. again again, check to see if the 1 meg pots affect tone, otherwise you can use a switch to lift them from ground. you can also use two single gang pots and control the outputs of the phase inverter independently, allowing you to play faux-SE tricks, etc.

look around and you can find actual schematics all over, but that is the theory behind the various options.

[Fresh_Start]

NOTE:  this method is NOT recommended by some.

There's another approach that replaces the 220K resistors between the bias supply and the power tube grids with dual-ganged 250K pots, using the bias supply as an AC ground.  It avoids the multiple coupling caps problem; however, there's a potential problem if the wiper ever loses continuity with the bias supply - disaster for the power tubes.  KOC thinks that is very bad.  You could add 2.2meg resistors between the bias supply and the wiper to ensure continuity without changing the net resistance much.  The other downside is that you have the negative bias voltage on those dual-ganged pots.  It's not a lot of voltage or current but I guess it could be a safety issue.

Schematic attached - sorry that it doesn't include any component values.  I think that the idea is to make everything "look" just like the PPIMV isn't there when it's at "11".

HTH,

Chip

[Fresh_Start]

The more convential approach discussed above as "#3" in RadiumKing's post is shown here, a la' Bruce Collins.

Thought it would be useful to get these both in one post since a search for PPIMV turns up a number of dead links.

Hope this helps,

Chip

[HotBluePlates]

HBP's explanation of why there are coupling caps before and after the PPIMV master volume.

The multiple 0.1uf caps troubled me also - it does seem like duplicating high pass filters.

But also notice that Bruce Collins' schematic has the pairs of 0.1uF caps.

You *are* right though. If you had an amp you knew for certain sounded perfect with the stock phase-inverter input cap values, adding the master volume and the extra 2 caps will make the existing caps look smaller and shave the bass.

Don't think in terms of 2 hi-pass filters (although that is perfectly valid), think in terms of 2 series caps. What happens to the effective value of a pair of series caps? Like parallel resistors, the total value is smaller than the smallest of the caps. You could say caps in series act like resistors in parallel.

So with a smaller effective value, the turn-over point of the hi-pass filter moves higher, and bass is reduced.

You want to make sure your "perfect stock value" caps don't suffer a tonal change when you add the master volume and the extra blocking caps? Then make the added caps 10 times larger than the existing values. It will result in a negligible lower of the total effective value from the stock existing value.

Switching to resistor values for easy mental math, imagine a 100 ohm resistor is a "perfect stock value". You need to add a resistor that will effectively be in parallel, but don't want to change the total effective resistance much. Use a 1k resistor (10x bigger).

(100 * 1,000)/(100 + 1,000) = 100,000/1,100 = 90.91 ohms

So 10 times bigger part changes the resulting value less than 10%, and your part's tolerance is probably wider than that.

[macula56]

here's an interesting take on it. has anyone checked this out? scroll down about 5 or 6 replies then you will see it.
http://forum.metroamp.com/viewtopic.php?t=4390&postdays=0&postorder=asc&start=645

Fresh Starts response to this post:

That's basically the same as the circuit shown in post #12, except the 2.2M resistors are added to make each 250K pot "look" as close to 220K as possible.  Plus those resistors guarantee bias voltage to the power tubes even if one of the pots fails.     Actually, those extra resistors could be used to compensate for variances between the pot values.  At least you could get close to 220K on each side when the MV is full up.

This is also called the LAR-MAR PPMIV.  

[Animatic]

My current SE has a 12AX7 and stage 2 has a pot instead of a cathode resister.
This is feeding the output tubes, so same placement as a PI cathode.
I think it effects the distortion as Panhead said,
but also how much drive the output tubes see and it's type.

So while it DOES affect the over all volume,
it is more of gain to change distortion point of
what drives the output tubes.

[panhead]

This is in regards to using a pot in the cathode section of a phase invertor to control distortion (and to some extent volume)


Do you think it helps control volume enough that it would be worth mentioning in a thread on master volumes?

It attenuates the volume, but not all the way. I would say about halfway if that much, so it's not a true master volume. But no one turns their master volume to zero anyway. With that said, I think it should be mentioned with the above "disclaimer". It's a five minute mod that can be easily reversed. And when the pot (wired as a variable resistor) is wide open it's like it's not there anyway.

NOTE:  Plexi50 uses a 25k pot instead of a 220k pot and reports that as a good fit for the tone he wants.  Panhead reports using a 220k pot

[The Radium King]

on an slpi you can replace the 1M grid leak resistor with a 1M pot (bring the grid to the wiper) and it makes a pretty good MV (a little different from a pre-pi mv in that the grid leak resistor in an slpi is tied-in with the cathode).

on most opto-isolotor vibrato amps (ie, most blackface-style circuits) you can disconnect the roach and wire the intensity pot as a pre-pi mv (the hoffman layout uses a 47k resistor to simulate this intensity pot - same diff). replacing with intensity pot with a larger pot also helps - the 50k pot hanging off the plate of the third gain stage drastically reduces tha gain it can produce.

on a cross-line post pi mv you can add a cap and turn it into a treble cut instead of an mv (if you are using caps to keep bias/plate dcv out of the pot, the caps are typically sized to pass audio and only stop dc; use a much small cap and you will only pass treble, which results in only treble cancelling). use an inductor to only pass/cancel bass. use both for a notch filter.

some carvins use a rotary switch on the plates of the pi to pull in different sized plate resistors for different levels of gain (search out an x-series schematic).

in the blackface fender circuit you also have attenuation occuring at the reverb dropping resistor (the 3.3M dropping resistor forms a voltage divider with the resistors on the reverb return circuit - 220k in parallel with 390k on my 65 twin ri - about 140k for a 3.3M/140k ratio). you can increase gain by revising this ratio (relative strength of reverb will be affected, however).

[sluckey]

And do you have a schematic?

Here's a variation using a switch and fixed resistors. Just use a 50k pot instead. Search for power dampening. Some said it was the 'cat's meow'. I did it to my 18Watter and thought it sounded kinda strangled, choked, or something like that.

http://www.el34world.com/Forum/yabb2/YaBB.pl?num=1173272024/19#19

[tubenit]

An example of a cross line master volume that I use on my Carolina Blues Special.  I use one of Hoffmans's 1M pots with a spst built in so I can completely remove the PPMIV crossline from the circuit.

[tubenit]

An example of a pre phase invertor master volume such you might find on a Marshall amp (& others). The example is from a Plexi 25W reverb (schematic & layout in SCH library)

There is some change in tone (as well as volume) using this master volume.  Some highs are lost with the volume turned down, IMO.  

You can add a 250- 500p cap across the master volume if you want to help retain highs. You could use a mini-toggle spst to cut that cap in or out.

From BluesBear (Dave):

"Has anyone experimented with the Marshal MV two post up."

That's the one I usually use, mainly because when turned full up, it's that same as having no master volume in the circuit.
Dave  

From Radium King:

regarding the pre-pi mv performance at low volumes, here's a fix from the tone lizard page ...

"The last modification I'll discuss here in the preamp is at the Master Volume control. If the control seems to make your tone thin out at low settings, even after all of the previous modifications, we can do the following. Insert a 100K (Brown-Black-Yellow), 1/2 watt resistor between the middle wiper and the wire going out to the phase inverter circuit. Cover the resistor with a piece of heat shrink tubing. I have also been known to dab a spot of hot glue or silicone on the back of the Master Volume control, and affix this extra resistor firmly in place. This may seem like overkill, and it might be. But I do it anyway."

[tubenit]

Another type of "master volume" control that could be considered are the multiple gain stage controls that one would see in a D'Lite amp.

Essentially you have a volume, three gain pots and a master volume.
The schematic/layout can be seen here.

http://www.el34world.com/Forum/yabb2/YaBB.pl?num=1193532616

With respect, Tubenit

[Fresh_Start]

Here's an attempt to draw a schematic of the Metroamps style PPIMV.  It's a duplicate of the circuit in my first post above, except it includes typical Blackface component values and the 2.2M resistors from the layout presented in the Metroamps thread (and above).

One question: is it dangerous to have the bias voltage on the dual-ganged pot?  Please think about both a user and a tech who might not expect DC voltage, albeit negative, fairly low in absolute terms, and not much current.

Thanks in advance for your thoughts.

Chip

Sluckey's response:  
No. However, if there are any shielded cables connecting to the pots, I would not connect the shields to the bias. I'd take shields to ground.

[Animatic]

Has anyone experimented with the Marshal MV two post up.

I was thinking if that was a dual ganged pot,
and while one pot dropped level
the other added back just a touch of bright back into it.
Even if it got to just some brights, when total off,
it almost never goes totally off.

Or maybe just a consentric pot that can dial back in some highs
to taste for any level once chosen.

Ultimately the main one will control total level,
but maybe it can have some RC network
feeding back some highs.
Just a thought.

EDIT: Drawing from his idea ,  using a spdt with center off
and cap values of 180p & 500p would also give some options
for tone and brightness.

Animatic's follow up response:

"As a switchable it quickly gives two possiblities.
And a place to start with pot hook up condiderations.

As a switchable I think a stepable  resister master volume  
style control with parallel stepable caps picked to match each
level of level reduction.

Say 10% down and 500p
25% down and more 270p
50% and even more 150p etc
all on a rotary gang switch.

Here is an example I just found.
Think one ring for resisters,
and the second for caps or an RC pair depending.


Then you can tune in pretty precise amounts
of level and preset compensation.
 

[Megachunk]

Ma head! It's spinnin! There's so many!

Out of all these master volumes, (I did one Sluckey posted and it's great! Acts as a Hot Plate attenuating -type MV). If one wanted to achieve a MV sound like a JCM800, (gainy, chunky-like) which one of the versions above would one use?

Thanks Dudes!

Chunk

[tubenit]

From this thread:  http://el34world.com/Forum/index.php?board=13.0

http://www.londonpower.com/all-kits/london-power-master-volume

[Papa Jim]

Here's one I have used that works really good. Like one in the post above but with opposite rotation of the pot (CW = less headroom), and more self explaining pot connections. This one works really great. It also shows both the NFB Tail resistor and the  NFB resistor too. Use your existing amps resistors and cap values except use (10K grid stoppers).

[Willabe]

heres one I have used that works really good. Kinda looks like one in the post above this one but with added pot connections.

I don't see any added pot connections.   

The ground for the LTPI is missing, the 2nd R for the NFB voltage divider, the R that goes to ground.   

[Papa Jim]

Sorry about that. I corrected the wording and the schematic. I was in a hurry when I added that.
Do try it though. I think you will be pleased. In the other post by tubenit the similar circuit is the same it would just be opposite rotation. (clockwise would be more headroom not less) I believe.