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Post-PI MV, Try It!!!

FractalAudio

Administrator
Fractal Audio Systems
Moderator
Turns a lot of mid-gain amps into ripping monsters. I just tried it on the JCM800 and, dayum...

The only caveat is that, like a real amp, the more you turn the MV down the less effective Presence and Depth become (since the loop gain is reduced).
 

Slammin Mofo

Inspired
Love the post PI MV option. More people should check it out!

I was just reading the FAS amps thread... Cliff, could you design your own ultimate Post PI MV british voiced amp? I really like your takes on the plexi (Crunch) and the brown sound (Brit Brown) amps and I'm wondering how much you could improve this kind of MV design.

Anyway, thanks for giving us all these little cool options to tinker with!
 

Rotti

Fractal Fanatic
I'm just going by memory here, but I want to say the last time I tried this I got a very steep volume dropoff compared to the other MV placements (at same MV value)... pretty much had to bump up the MV a couple dB.
And iirc, the result was very compressed, but in a good way, like a VH4 kind of compressed with dump truck of saturation on top. But then again I was using the Uber as a starting point, so... :D
I'll have to give this another go, as it might be exactly what I'm needing for a tone I'm going after!
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
This is because PI distortion is really the sweet distortion players associate with non-master volume amps like marshalls or fenders. Power Amps don't clip all that much, but that PI saturates like crazy.
That's a common misconception. The swing of a properly designed PI is greater than the range of the power tube grids. For example a 6L6 will typically be biased around -50V. This gives a range of around 100V of swing at the power tube grids. A well-designed PI for an amp like this will typically swing at least 150V and usually closer to 200V or more. This means the grids clip before the PI clips.

Then there are the power tube plates. Most designers choose the output transformer so that the plates just start to clip when the grids clip. This gives maximum power since current is maximum when the grids are driven to clipping so you want the plate voltage to be at its maximum excursion at this point. This is a matter of taste. Some designers slightly undermatch since the impedance of the speakers increases at low and high frequencies and this gives a more open tone. Others overmatch as this give a more touch-sensitive overdrive (i.e. Trainwrecks).

When you put the MV after the PI you attenuate the signal from the PI going to the grids. This allows the PI to clip before the grids clip. The PI has a fair amount of negative feedback so it's a somewhat hard clipper which gives a fairly aggressive distortion.
 

Artean

Inspired
That's a common misconception. The swing of a properly designed PI is greater than the range of the power tube grids. For example a 6L6 will typically be biased around -50V. This gives a range of around 100V of swing at the power tube grids. A well-designed PI for an amp like this will typically swing at least 150V and usually closer to 200V or more. This means the grids clip before the PI clips.

Then there are the power tube plates. Most designers choose the output transformer so that the plates just start to clip when the grids clip. This gives maximum power since current is maximum when the grids are driven to clipping so you want the plate voltage to be at its maximum excursion at this point. This is a matter of taste. Some designers slightly undermatch since the impedance of the speakers increases at low and high frequencies and this gives a more open tone. Others overmatch as this give a more touch-sensitive overdrive (i.e. Trainwrecks).

When you put the MV after the PI you attenuate the signal from the PI going to the grids. This allows the PI to clip before the grids clip. The PI has a fair amount of negative feedback so it's a somewhat hard clipper which gives a fairly aggressive distortion.
You beat me to it... Ehum.
 

VegaBaby

Fractal Fanatic
What classic Rock songs "tones" would this be?
I think the question would be, which known modded amps or even regular amps (like Cameron or Bray amps which use some mods as standard) are using the Post-PI MV mod or to what amps is that mod usually done ?
 

Nagi Mysore

Inspired
That's a common misconception. The swing of a properly designed PI is greater than the range of the power tube grids. For example a 6L6 will typically be biased around -50V. This gives a range of around 100V of swing at the power tube grids. A well-designed PI for an amp like this will typically swing at least 150V and usually closer to 200V or more. This means the grids clip before the PI clips.

Then there are the power tube plates. Most designers choose the output transformer so that the plates just start to clip when the grids clip. This gives maximum power since current is maximum when the grids are driven to clipping so you want the plate voltage to be at its maximum excursion at this point. This is a matter of taste. Some designers slightly undermatch since the impedance of the speakers increases at low and high frequencies and this gives a more open tone. Others overmatch as this give a more touch-sensitive overdrive (i.e. Trainwrecks).

When you put the MV after the PI you attenuate the signal from the PI going to the grids. This allows the PI to clip before the grids clip. The PI has a fair amount of negative feedback so it's a somewhat hard clipper which gives a fairly aggressive distortion.
Aha.. Quite simple really!!!!!
 

Deaj

Experienced
Very useful - thanks Cliff! :)

The JCM800 sounds pissed off with the master volume is moved post phase inverter. Me likes!
 
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groovenut

Power User
Yup, the tone gets pretty hairy when you use a post PIMV. Some like it, some dont. I have both pre and post PIMV on my JCM800 and really like the tone with the pre MV turned up to about 5-6 and the post turned down from wide open about 30%. Gives it just a great amount of pissed off but doesn't affect the presence too much.
 
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