PPIMV bypass with 4 way switch. (Power Broker)

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Guy77
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PPIMV bypass with 4 way switch. (Power Broker)

Post by Guy77 »

I changed the master volume in my Liverpool from a Type 3 Master ( cross line master) to a PPIMV master. I found that the PPIMV was better sounding than the cross line Master . I wanted to add a Bypass switch on this PPIMV since I had one on my previous Master vol. I have drawn up a layout ( sorry its pen and paper) with a 4 way (on-on) switch. I wanted to get peoples opinion before I started to solder it into my amp.
I have removed the 220k resistors from the board where they are with a stock Trainwreck and placed them on the switch.
Thanks.
I left the 2 2.m resisters off the pots to keep it tidy.

Cheers
Guy
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Last edited by Guy77 on Fri Sep 15, 2017 11:21 am, edited 1 time in total.
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martin manning
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Re: PPIMV bypass with 4 way switch.

Post by martin manning »

Not necessary IMO. When you turn the PPIMV all the way up you have 250k to the bias voltage from each PI coupling cap. If you add a 2M2 resistor from the wiper of each pot to the bias voltage (CCW lug), then you will have 220k to ground and a bit of insurance against a pot failure. In short, it's back to the original circuit when full-up. This is what's called a Lar-Mar PPIMV, after the two guys that developed it from the KF type 2.
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Guy77
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Re: PPIMV bypass with 4 way switch.

Post by Guy77 »

Yes very true, thanks Martin.
I am doing it since my faceplate has an empty spot now that says Master Vol Bypass and didn't want to leave it empty.

Cheers
Guy
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martin manning
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Re: PPIMV bypass with 4 way switch.

Post by martin manning »

Such a thing might be useful as a boost switch with the ability to return to the same reduced volume level. A simpler switching circuit would do though; see below. I don't know if it will pop when switched or not... one way to find out!
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Guy77
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Re: PPIMV bypass with 4 way switch.

Post by Guy77 »

I like your version better than mine , much simpler!
One question though, what is the item labelled Rg in your diagram? Is it a power tube grid resistor .(Ex in the case of my Liverpool it would be a 1K resistor). Also in your diagram I would leave the 220K resistors from my Phase Inverter on my board were they are and not remove them?

Thanks Martin.

Guy.
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martin manning
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Re: PPIMV bypass with 4 way switch.

Post by martin manning »

Yes Rg is a grid resistor. In a Liverpool there will be two 1k5's on each side. The dual pot and the two 2M2's will take the place of the 220k's on the board, as in a standard Lar-Mar PPIMV. The point labeled Vbias will go to ground since you have a cathode biased stage.
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Guy77
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Re: PPIMV bypass with 4 way switch.

Post by Guy77 »

Thanks. I will hook it up and report back!

Cheers
Guy
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Guy77
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Re: PPIMV bypass with 4 way switch.

Post by Guy77 »

Hi Martin, I used your diagram to wire up my ppimv on a dpdt and it worked great! It did pop but otherwise it was fine. In the end I think I will go with just the PPIMV and use the spare hole for my vvr instead.

Thanks
Guy
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RJ Guitars
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Re: PPIMV bypass with 4 way switch.

Post by RJ Guitars »

A common request I get from guitar players is that they want a MV that is linear as you dial it... of course that's not so easy to deliver in a tube amp.

Another thing about MV controls (and also with power scaling) is that you need to get rid of a huge piece of the signal (voltage in power scaling) to get roughly half of the volume. Then there is some minimum volume before things start to sound kinda ratty. This usually means you are only using some small percentage of the rotary motion on the control. Based on the various thresholds of usable volume levels (including full on) you would have the need for an odd non linearity in the resistance curve of the control.

So I came up with the idea to use a multi-position rotary switch and give myself six settings that cover the useful range in linear sounding increments... I like it a lot and it's not especially expensive or complicated although it does take some extra soldering and some R&D to figure out your resistance values.


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Re: PPIMV bypass with 4 way switch.

Post by JazzGuitarGimp »

RJ Guitars wrote:A common request I get from guitar players is that they want a MV that is linear as you dial it... of course that's not so easy to deliver in a tube amp.

Another thing about MV controls (and also with power scaling) is that you need to get rid of a huge piece of the signal (voltage in power scaling) to get roughly half of the volume. Then there is some minimum volume before things start to sound kinda ratty. This usually means you are only using some small percentage of the rotary motion on the control. Based on the various thresholds of usable volume levels (including full on) you would have the need for an odd non linearity in the resistance curve of the control.

So I came up with the idea to use a multi-position rotary switch and give myself six settings that cover the useful range in linear sounding increments... I like it a lot and it's not especially expensive or complicated although it does take some extra soldering and some R&D to figure out your resistance values.


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RJ, would you be kind enough to post a schematic with resistor values? From the pic, it's hard to discern some of the colors of the resistor bands. Thanks!
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TUBEDUDE
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Re: PPIMV bypass with 4 way switch.

Post by TUBEDUDE »

RJ, you have such meticulous work displayed, and nice components too.
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Re: PPIMV bypass with 4 way switch.

Post by RJ Guitars »

Thanks guys!

Here are the values I used in my MV (sorry I forgot to paste these in earlier)

Step 1 = 1.2K
Step 2 = 2.7K
Step 3 = 10K
Step 4 = 22K
Step 5 = 68K
Step 6 = 220K

I posted the build thread here: The build thread is here: http://diyguitaramps.prophpbb.com/topic18.html


Meanwhile I'll try to hand Guy77 his thread back... what did you do with your MV implementation

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Guy77
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Re: PPIMV bypass with 4 way switch.

Post by Guy77 »

That's a great idea RJ ! I will have to try that one day.
I measured my alpha dual gang 250k pot and each one was also off a bit (by 1 or 2kb).

I ended up removing the bypass switch ( it popped , but it did work successfully). I liked the idea of using it as a volume boost as Martin mentioned. I later realised I could use one of my effects pedals that has a level control and turn off all the effects and just use the level control to boost volume. I will be ordering a new faceplate. I also have a lot less wires running around the inside of the amp now, Simplicity is the ultimate Sophistication, to quote a famous renaissance man.

I am off to build my airbrake now. Parts just came in from Mouser!
I am interested in hearing the difference between the amp with the master vol engaged and running with master off and just the airbrake to attenuate the sound.


Cheers
Guy
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Re: PPIMV bypass with 4 way switch.

Post by RJ Guitars »

I think you'll like the Airbrake. I prefer that sound to any form of MV or power scaling but still at bedroom levels they all sound kinda ratty. I think you can actually get lower volume and still have some tone character left with an attenuator but nothing is ever as satisfying to my ears as "Loud" is.

Interesting thing on the Airbrake - I have done the opposite thing of my MV scheme in replacing the fixed resistors with a big Rheostat. You have to consider a lower wattage limit for the final unit based on the wattage of the available rheostats. http://diyguitaramps.prophpbb.com/topic33.html



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Re: PPIMV bypass with 4 way switch.

Post by Guy77 »

I got my Airbrake together this weekend, used a Hammond 9x5x2 enclousure.
I am really happy with the results with my Liverpool. I am running a G12H65 speaker.
You were right RJ, it sounds better than my ppimv master!
My search for tone has finally ended, I have built several amps and this amp now has the overdrive sound I was looking for , at none ear shattering volumes, thanks to the Airbrake and everyone on this forum who has contributed over the years!

Cheers
Guy77
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