Non-Marshall PPIMV

[Chuck H]

I don't understand how putting a big value (2 or 2.2 M) resistor across the pot can have no effect on the sound. Doesn't that resistor then figure into the equation all the time?

Yes, that resistor IS in the equation. But it's relative huge-ness means it doesn't effect the pots reistance all that much. BUT...Since the bias circuit draws very little current there is an appropriate amount of bias voltage at the end of that 2.2M resistor to keep the power tube from meltdown if the pot fails. Adding the 2.2M resistor changes the circuit value to 225k. Since the original resistor value was 220k this is actually BETTER than the dual gange 250k pot by itself. At least from the perspective of keeping the circuit close to the original values.

Not sure whether or not I'll use one, just thinking about a MV as opposed to an attenuator, which initially I liked but after using it for 6 months or so I can definitely hear a very decided effect on the un-attenuated tone of the amp and character of the overdriven sound. An effect which I don't care for all that much. Thin and fizzy.

Well IMHE a master volume won't sound any better than an attenuator. Thing is, alot of the way your amp sounds when it's loud is due to the fact that it's loud. When you make it less loud some things change. There is alot of talk about speaker response at lower power and room acoustics, yada yada... What is most responsible is the Fletcher Munson curve (not sure I spelled it correctly) which has to do with the human ears perception of EQ and dynamics at different db levels. For this reason primarily, quiet will never sound like loud.

A master volume would actually remove an element of overdrive from your tone because the power tubes will no longer be distorting. And in this case it's worth mentioning that most players feel that it's preamp distortion that is buzzy/fizzy and power amp distortion that sounds best. There has to be a reason most players feel this way but I personally don't agree. Power tubes buzz and fizz with the best of preamp tubes. As you've noticed by using an attenuator. Once you turn the whole amp down you can actually hear more of the distortion componants you don't like and less of those you do.

Unfortunately there is no ideal solution as yet. Mostly because everyones "ideal" is different. Power scaling, attenuators, master volumes, etc. are all employed and sometimes together. The players that find their tone with these devices are lucky if only because most of us never quite get there and we never relax about it.

Cheers

Chuck

[hasserl]

I don't understand how putting a big value (2 or 2.2 M) resistor across the pot can have no effect on the sound.

Because the value is so large it won't have an effect on the sound. The 2M2 resistor is paralel with 250k pot; when the AC signal voltage sees the two it takes the path of least resistance, the 250k pot. If the wiper of the pot should loose continuity, the 2M2 resistor provides a path for the bias voltage to reach the grid and maintain the bias for the tubes. It might not be perfect, but it will keep the voltage present on the grids and it will keep the tubes from running away while you dick around with trying to figure out what happened.

[Chuck H]

P.S.

You can find the value of two parallel resistors by multiplying their respective values (250,000 X 2,200,000 is 550,000,000,000) then dividing that figure by the sum of the two resistances (2,200,000 + 250,000 = 2,450,000 / 550,000,000,000 = 224,489) or 225k for all intents and purposes. I used our 250k pot and the 2.2M parallel resistor for the example.

Cheers

[EFK]

Hey guys - both of you, THANK YOU! Perfect explanation - path of least resistance I understand. I really enjoy soldering and electronic work in general, a nice diversion from my day job but at the same time I'm very respectful of something that can make my wife a widow so I try to get as much factual information as I can before I touch anything. Iwant touse the MV to just take a little off the volume. I'm fortunate to have no neighbors within 2 miles so I can play loud, but my Weber attenuator (I have it to just take some volume off to save my hearing) is too much 'all or nothing' and even at light settings really modifies the tone with no inherent way to modify it. For what a supposedly better attenuator costs, say a UA, I could buy a cheap valve junior or little giant and just crank that. But I have yet to hear an el84 amp that speaks to me in any way - whole different can of worms. I think a MV will be an interesting experiment. Thanks!

[EFK]

WOW! As a follow-up, this is spectacular. Took some time today to build this into the amp and I am overjoyed. You know I should explain my problem with the Weber - it's a nice attenuator, but the way it is designed, it is either bypassed or when turned on there is no 'neutral:' there is a "high" switch and a "low" switch, and either setting modifies the tone asymetrically (cuts more highs or cuts more lows). So you have a whole lot of eq work just to use it. What I've been looking for is a way to cut the volume yet maintaining the amp tone settings and this does it! All I have to worry about it maybe tweaking a knob or two on the amp itself, not the attenuator as well. Also, I have to tell you, the overdrive sound is better - at much reduced settings there is a slight difference but at say 4 or 5 and above on the MV knob I can not detect any added harshness or fizzy sound. This is a great ppimv design.

I'm curious - aside from added wear on the tubes (obviously) can this increase wear or diminish life in regards to any other components? Also - consider the wiring for the master volume - should these wires be kept away from the PT or transformer lines, or does it make no difference?

[Chuck H]

I'm curious - aside from added wear on the tubes (obviously) can this increase wear or diminish life in regards to any other components? Also - consider the wiring for the master volume - should these wires be kept away from the PT or transformer lines, or does it make no difference?

Excellent. Glad it's working well.

Hey Steve, are you seeing this? Maybe you could call it the KF master volume "Chuck version"... I've been trying to get a spot on the Blue Guitar for awhile

You should actually get LESS tube wear with the master volume than with the attenuator because you are not driving the power tubes as hard. No components will experience any added stress because of this circuit. Except for the possibility that you may play through the amp more than you use to As far as layout, it's not as critical as the preamp stages. If it looks neat and doesn't make any funny noises your golden.

Chuck

[Victor Denance]

Hey Chuck!
Do you mean the PPIMV arrangment shown right above on post number 11 works with a bias vary trem (even when turning down the MV)?

Thanks for sharing!

[EFK]

From my perspective, the amp tremolo is about as useful as tits on a bull but I tried it today specifically to answer this question. I installed the MV exactly as Chuck explained his version of it and my trem and reverb (which I also don't use much) perform exactly the same regardless of where I set the MV pot (I say 'pot' because I still don't have a knob to fit the solid shaft). Thanks again!

[Guitarist]

Speaking of master volumes, I tweaked someone's Soul Control (Variable Feedback off the OT) and found it worked much like a MV. I was pleasantly surprised! Simple and it sure reigned in the volume - not wholly off - but alot.

[Chuck H]

IIHO the rub with using NFB for a master volume is that it effects the tone in other ways that I don't like. When you use NFB for volume reduction you also reduce speaker damping, seriously alter the harmonic content of the signal in the PI and power tubes and reduce distortion in the PI. Which is why one chooses a PPMV to begin with. It doesn't matter as much in the power tubes because they aren't really working all that hard with a master volume in use. But the overall effect just seems to dry out the tone to my ears.

Chuck

[erikpaul]

PPIMV problem

I just wired in a 250k dual-ganged pot on my Silverfaced twin, as seen with ken fisher design and now have lost almost all of the volume. It's faintly audible. Is there any possible things I should check imparticular? Any help would be greatly appreciated. Thanks

[Chuck H]

The KF PPMV works perfectly with a SF Twin. If you are having a problem it is almost certainly an error in the wiring.

I'm not trying to be a dick or anything but that's the brass tacks of it.

Chuck

[crunchman]

Based on this layout, http://www.brownnote.net/forum/downl...=717&mode=view Could someone kindly walk me through the few connections please,

based on the Ken Fisher diagrams above it should be fairly easy, I'm not 100% sure which C/R network to hook too, I'm using a dual 1M audio pot ..Thanks a bunch folks.

[Chuck H]

Ideally you would want a dual ganged 500k for that amp. You can do it with the dual 1M by soldering a 1M resistor from each wiper to ground and otherwise wire up as usual. This would also offer some protection from "runaway" in the event of a pot failure. You could also use the dual 1M as the old Marshall style PPMV like this:

[crunchman]

Thanks Chuck, not sure which way to go yet.