Master Volumes Types

[Structo]

I thought I would start a new thread about MV's since the thread that had some discussion on it was sort of derailed.

On my home brew 6V6 amp that started life as a Hammond M3 organ amp, then a Deluxe that morphed more into a plexi type amp.

I have tried several master volumes on this amp.

The first was the cross-line MV, didn't care for that one and it didn't sound good and had a terrible sweep range, basically it was off then on.

Next I tried the pre PI type found on the Dumble amps, better but didn't like that one either.

Today I tried the post PI MV with a dual gang 1M pot.
Much better but sounded a bit fizzy when dialed down.

Then I tried the Bootstrap MV. Much better!
This one uses the dual gang 1M pot too but it takes the place of the 1M resistors on the long tailed pair PI.

This one sounds the best of all on this amp.
It retains the cranked tone better than the others I had tried.

The only problem I am having is the range seems a bit sudden.
It is difficult to have a low level as the range of sweep leaves something to be desired.
It's hard to dial in a low level volume before it jumps to a higher level.

Seems I remember some way of making the pot work better by tacking on a resistor to the pot(s) to make it a more gradual in the beginning of it's travel.

Can someone tell me how I would do this on the dual gang 1M pot?

If I increased the value of the 10K resistors feeding the grids what effect would that have?

Even the first stage 1M volume pot could stand to be more gradual in it's control of volume.

Here is how I have it now.

Thanks

[Structo]

So if I tacked a 1M resistor across each pot, that would lower the value to 500K, right?

Would that do anything for the taper?

[Sonny ReVerb]

RG covers all the variations over at Geofex:

http://www.geofex.com/Article_Folders/p ... tscret.htm

He shows you how to convert a linear taper to a psuedo log or reverse log, or change the curve of the taper. You'd have to apply the technique to each half of your dual-gang.

Are you already using a log taper? If so, make sure you have the taper on the 'right' side so it ramps up properly. Put the wiper approximately in the middle of it's travel and measure the resistance to each outer lug. The lower resistance side should connect to the tail resistor, if I'm picturing this correctly. That way, you'll have the gradual increase on the low volume side. Does that sound right?

Edit: After further reading, if the pot was hooked up backwards, the knob would have to be turned in the opposite direction to get a volume increase. So, that's probably not the issue.

Here's some more info:

http://sound.westhost.com/pots.htm

[Structo]

OK, thanks, got that sorted out.

One problem I notice with this Bootstrap MV is now the Presence control doesn't work well because the master volume when dialed down pretty much blocks the presence from the PI input.

Is there another way I can wire the presence/ feedback so it is not dependant on where the master is set?

To review what I have, the new dual gang Master pot takes the place of the 1M resistors on the long tailed pair. So the presence feedback is fed into that, so when the master pot is dialed down the presence doesn't work.

I suppose I could wire up a cut control?

[Alexo]

Forgive my ignorance, but why do we need R2 in your schematic to be a potentiometer? What would happen if you just left that as a 1M R and replaced the other 1M with a 1M pot?

....I wonder about this each time I put that extra cap from the wiper of a typical volume control that blocks the pi grid voltage, why not just replace that grid resistor with a pot and use the volume control there?

[Structo]

Well I'm no genius myself but I would guess that on the long tailed pair, you need to keep the balance on the grids.

If you just used one 1M pot, then when you turned that down there would a large signal making it through to the first grid, then the signal to the other grid would remain small.

So with the dual gang master pot, we are regulating both halves of the PI tube.

If you make the master volume the typical pre-phase inverter type, then you can use a single pot to control the voltage of the last gain stage before the PI.
That is how Dumble did it.
I tried that in this amp as well and didn't like it.
This amp only has the one 12ax7 before the PI so perhaps the gain isn't high enough to use that type.

This Bootstrap type seems to keep the tone the best of what I have tried, uses fewer parts as well which is always good.

I believe the grid stoppers ( the 10K) are there for stability reasons.

And of course we need to block DC high voltage from the pots or they will be scratchy, so we block the DC and allow the AC signal to pass.
That's what coupling caps do, or probably more accurately, decoupling caps.

[Alexo]

Well I'm no genius myself but I would guess that on the long tailed pair, you need to keep the balance on the grids.

If you just used one 1M pot, then when you turned that down there would a large signal making it through to the first grid, then the signal to the other grid would remain small.

The thing is.. there is no signal on the other grid, it just goes straight through the cap to ground. So by attenuating it, you're basically taking 0 VAC and cutting it down to say half of zero, which is of course still zero.

I should just shut up and try this some time, since I've been wondering about it for quite a while.

Regardless, what I'm suggesting probably won't provide a solution to the bugs you're experiencing.

EDIT: I guess that second grid is used as an input for the nfb - which is why the presence control stops working - but just reducing the first grid's signal should proportionately reduce the nfb that comes back anyway.

[ampdoc1]

I don't know how this would compare to the others mentioned, but I thought I had "invented" this when I first tried it in 2003. Turns out it was in one of the O'Conner books.
I thought it was one of the simplest and best I'd tried.

a'doc

[Structo]

Ampdoc, yes it seems to work the best out of the four I tried on this amp.
EDIT:
I was typing when you posted that schematic Dave.
So you just used one 1M pot huh?

Logically, at least to my addled brain, it would seem you would have to control both grids for it to work good?

And it works well?

Does it have a nice taper at the begining? In other words, does it turn up the volume gradually and not jump to a loud volume?

I'm still new to all these MV circuits so I'm learning a lot.



Alexo, I'm not sure I understand what you meant when you say, "there is no signal on the other grid, it just goes straight through the cap to ground. "

If you look at this PI which is from a AA763 Concert amp, the signal comes in through the .001uf cap.
From there it gets divided by the 1M/ 470R/ 22K network and supplies the grids of the PI with a signal.
There is no cap to ground there.

The only path to ground there is the 100R resistor that is part of the negative feedback loop.

Remember in my circuit, I am just replacing the two 1M resistors with a dual gang potentiometer with 10K grid stoppers on the wipers.

[Alexo]

Hey Structo,

I was talking about the "second" grid on the pi, the one that connects through a .1uf cap and then to the nfb and then to ground. Technically, this is used as an insertion point for the nfb, but looking at the long-tailed pair by itself, it functions by maintaining zero signal at that second grid, so you don't need to attenuate the non-signal, if you catch my drift.

In fact, what I was suggesting is exactly what Ampdoc posted in his schematic! Guess it works after all...

[Structo]

Oh OK, yeah I misunderstood.

I may have to give that circuit a try, the one Ampdoc posted.

I'll leave the dual gang pot and just try one section of it.

I wish I understood what the difference between the two would be?

Ampdoc?

[Andy Le Blanc]

Glad to see master volume discussed. I completely abhor MV, and go well
out of the way to avoid implementing it. Its easy to see why the VVR has
come into the light but I dont trust that either. But on the other hand I
recently built a super lead based amp with a MV for giggles.
That bootstrapped MV looks interesting and I have a platform to try it.
I've been obsessive about simple amps for so long that I didnt know what to do with a
high gain pre and master once I got it.

[Structo]

Well with me, I love the sound of this little 2 x 6V6 amp when its cranking good.

It's just too loud, the preamp is fairly weak so when I turn it down it cleans up.

I want to be able to keep that cranked tone but with low decibels.

I know it's a perfect candidate for the VVR, I just don't have the extra $$ for that right now, plus it's fun to experiment on an amp that I don't have much invested in.

The bootstrap MV so far has sounded the best of all the ones I've tried.

What also makes this amp cool is it will clean up when I roll the volume control on the guitar down.
And it will do that with this MV as well.

[Andy Le Blanc]

I always try to "keep it stupid, simple", seems to work well for most guitar
applications. All, and I mean all, long time professionals that I know, who get the best tone,
use an amp the is the appropriate size for the venue they
play, and never give it a second thought. MV's are a sticky wicket for many
players. At home you should practice unplugged, an amp is a distraction
from what you should be paying attention to. Playing out you lose dynamic
control of the of your instrument, end up sounding like a duck fart in a bath tub.
Its a pickle. I will be trying out the bootstrap MV. See if I can't overcome
my prejudice's.

[Structo]

Andy, from my limited experience I agree with you.

What I am finding out through experimenting on this amp that not all MV's are created equal.

No huge revelation I'm sure but I guess what I am trying to say is, it really depends on the type of circuit that you are trying to control whether or not a certain type of MV will work on it.

I tried the Crossline type which Matchless seemed to use on a lot of there amps. Sounded terrible on this one.

Tried the Pre-PI type found in the Dumble type amps, no good there either.

So I think a lot of it depends on what is happening before the PI as to whether or not a particular MV will work well or not.

Like I said, this amp sounds great at about 3/4 the way up, and I can clean it up with the guitar volume, but that is too loud to play for just practising and my old ears need some protection from high db's anymore.

I really like the MV on my D'Lite amp, seems to work really well.

But for that maxed out, fire breathing type tone, nothing compares to a duet or quartet of power tubes saturated and distorted.