What do these resistors do?

[JJman]

What do the two "33k-100k" resistors do in this PPIMV ?

 

[antik]

Volume,

Edit - sorry I misread the schematic and thought the question was about the pots.

 

[Lanesplitter]

Keven O'Connor recommends this resistor as a mod to a Marshall master volume. He claims there can be up to 10pF of parasitic capacitance in the wiring between the pot and the grid of the next stage. The 3dB point (1/RC) moves with the pot. Adding the resistor makes the 3dB point more constant, so that it doesn't vary with volume. It may not be necessary in the PPIMV. I don't really know.

 

[mooreamps]

I am tending to agree with Kevin's assessment. It's a high impedance output going to the pot's, and long wires coming and going to the tubes. The possiblity for parasitic oscillations are probably a valid concern. For an existing build, the mod is reasonable. However, for a scratch build, my approach was to have the output of the PI drive a low impedance buffer, then a post PI Master Volume. This reduces the value of the master volume pots from 1 Meg, to 100K, and no issues with parasitics.

-g

 

[donnyjaguar]

Its hard to say because post master volume is an il-conceived design anyway. Anyone whom endorses such a design should be held in immediate suspicion of incompetence.

 

[Wakarusa]

Its hard to say because post master volume is an il-conceived design anyway. Anyone whom endorses such a design should be held in immediate suspicion of incompetence.

So this is the second thread where you've thrown this out -- that the use of this style of master volume is essentially an announcement of the builder's/designer's backwardness and incompetence. Someone else in the other thread picked up your gauntlet there (and hasn't yet received a response), so I'll pick it up here. Please either explain your position, or knock off the flame bait.

 

[phsyconoodler]

I'd sure like to hear that explanation too.I use a PPIMV in many amps with great success.No explosions yet.Just great tones.

 

[JJman]

I removed the resistors. Also the .1 caps on the output side of the pot(s.) It took me a while to realize they are only their to keep negative DC bias voltage off the pot(s.) My 6v6s are cathode biased so no DC needs to be blocked on that side. May also help allow a tad more bass thru. No sign of anything bad happened. The wires are a little long but I designed the layout so they would not be near others and would cross at 90degrees over the screen supply wire. So far so good.:BEER

 

[TheAmpNerd]

What is the schematic's source?

 

[Swarty]

The first commercial example I've seen is Marshall Club & Countrys, circa '79. They were also in the early JCM800 2205 and combo equiv. I started using it a few years prior, around '77-'78, and still do (but have never added the resistors... just 4 caps and dual 1m pot).

 

[donnyjaguar]

I've already addressed the reasons why in other posts. I don't have to prove its bad practice by virtue of the fact its never been shown to be good practice. Its roughly akin to driving your car whilst resting your left foot on the brake pedal.

 

[phsyconoodler]

You mean like an attenuator or a distortion pedal or a power brake or any other master volume?
Tell me how a properly executed PPIMV can harm an amp or is a bad design?
Why is it no one else agrees with you?

 

[JJman]

What is the schematic's source?

Bruce Collins
Mission Amps
01/14/98

I did not capture that portion of the graphic when I uploaded it.

 

[Swarty]

Here is a link to a schematic for a '79 Marshall Club & Country:

http://schematicheaven.com/marshallamps/jcm800_clubncountry_100w_4140.pdf

 

[John Ward]

However, for a scratch build, my approach was to have the output of the PI drive a low impedance buffer, then a post PI Master Volume. This reduces the value of the master volume pots from 1 Meg, to 100K, and no issues with parasitics.

-g

Gary, did you use cathode followers for the low impedance buffer? I am curious because I built this amp http://vtg-amps.com/Melissa/Melissa-2-1a.pdf that has a similar PPIMV that Swarty is talking about.
John

 

[darkbluemurder]

I tried the PPIMV in several amps, also in several styles. The biggest drawback is when you need a Master Volume to drastically reduce volume, i.e. for living room purposes. The reason is that the PPIMV is within the negative feedback loop. The feedback loop works best when it has enough signal to work with. As soon as the PPIMV is turned way down the feedback loop collapses. This results in a harsh non-musical distortion to my ears.
Of course if your amp does not have negative feedback the PPIMV should work well.

 

[Swarty]

Gary, did you use cathode followers for the low impedance buffer? I am curious because I built this amp http://vtg-amps.com/Melissa/Melissa-2-1a.pdf that has a similar PPIMV that Swarty is talking about.
John

Actually Melissa is missing the second pair of coupling caps.

 

[John Ward]

Actually Melissa is missing the second pair of coupling caps.

Good point, it is cathode biased and, as JJman said, those caps are not necessary. For fixed bias they would be required.