Pre PI master vs post

[Smokebreak]

Ive got two pretty much identical builds here. Fendery 3 stage 2x6l6 single input. I basically copied what I ended up with on an earlier build, to give to a pal. Mine has a master before the PI cap entrance. This other one has a PPIMV because it was already there as a different amp at some point, so I figured I'd keep it to maybe hear some differences between same circuits, different master spots
Barring some massive snafu on my part, the difference is night and day at lower volumes. When cranking the preamp gain ,with the PPIMV at reasonable levels, there is way more distortion than there is with the Pre PI master amp. Both amps have the same character, just this difference in distortion at low volumes .

I haven't compared the two dimed, yet, but is this difference really the PI distorting/or not? The Pre-master is way more pleasing to my ears, again at lower volumes. It sounds "just right". Stones on steroids thing.

[wyatt]

There could be several factors.

Which PI design are you using? Fender's 12AT7 split-pair PI?

But the whole reason for a PPIMV is to include the tone of the overdriven PI. I many amps people finds it adds a thicker, more poweramp-esque quality to the overdrive.

[Smokebreak]

Yes, typical 12AT7 w/ 820r/100r. The PPIMV here seems to be having the same affect as it does with the Express, to me..just kinda harsh with no grunt. The pre master sounds thicker here.

[matt h]

(deleted)

[morcey2]

As wyatt said, PPIMV is usually to get the overdrive from the PI. (Exceptions are things like power-scaling the output tubes only.) If the PI overdrives well, it will add some thickness to the tone. If it isn't designed that way, it can make things sound much worse. Personally, I don't like the sound of an overdriven 12AT7 in any way, shape, or form. They're very, very harsh and just sound lousy.

Many of the non-high-gain amps get the start of their overdriven sound from the PI when it is the first stage in the amp to start clipping.

Matt

[V2]

I just built an 18-watt amp with 2204 preamp that has a bootstrapped MV (pre- PI) as well as a PPIMV. So I can use one, the other, or both to bring down the volume and get different levels of distortion.

Your ears are not deceiving you. The PPIMV gives more distortion because of the PI....[/img]

[Smokebreak]

Yes a pre and post is very fun. I've got that on a Marshall thing I love, but I'm also a huge fan of the almost cranked Fender at7 thing. They can get harsh, but always have a sweet spot. I did swap around 12ax7 in the PI it just made it worse..at least what I'm hearing. Of course it may sound better cranked I'm just suddenly really interested in perfect bedroom levels, for some reason. Hell, I guess that's important too. I'll switch around the master in the problem child and see what happens.
Something else that seems to go along with it..I've got the same boost switch on each...series 47K,1200p across treble cap. The effect is way more pronounced with the pre-mv, and again, sounds better.

[sepulchre]

The extra distortion and difference in tone that a PPIMV gives you comes not only from overdriving the PI, which is considerable, but also from the fact that it negates the negative feedback. The more it's turned down the less NFB there is and the less effect your Presence control has if you have one. With the PPIMV dimed it's absolutely transparent IME.

I have an amp with both and find that to be Very versatile.

[Smokebreak]

Ah yes, I have witnessed the presence control being ineffective in ppimv amps, but how exactly is this occuring? Or better yet, how is NFB reduced?

[Darkbluemurder]

NFB works by tapping the signal at the speaker and injecting it earlier into the signal chain (i.e. into the cathode of the long tail PI) out of phase in order to reduce gain and improve linearity. Now if you reduce the signal appearing at the speaker by turning down the PPIMV, less signal is fed back into the circuit. At one point there will no longer be enough signal fed back to maintain linearity - that's when the feedback loop collapses, i.e. it is no longer effective.

Since the presence control decouples the NFB for high frequencies, it also becomes inefficient once the NFB loop is broken.

The amps where I use a PPIMV do not have NFB.

Cheers Stephan

[Colossal]

Ah yes, I have witnessed the presence control being ineffective in ppimv amps, but how exactly is this occuring? Or better yet, how is NFB reduced?

Since the PPIMV reduces the magnitude of the signal reaching the power tubes, less power is then produced, which in turn causes a smaller signal to appear on the output side and be fed back into the non-inverting input of the PI. At low settings of the PPIMV, the sound is more raw since less feedback is smoothing the sound and the speakers are moving less air, giving the sonic appearance of less overall dimension and thickness. A larger value pot in series with a lower value fixed NFB resistor could be added to help compensate. I am doing this on a Plexi build on the bench right now.

[Smokebreak]

. Now if you reduce the signal appearing at the speaker by turning down the PPIMV, less signal is fed back into the circuit.

Thanks Stephan and Colossal. I understand the concept of NFB, but where I have quoted is where I am confused...in the sense that even if you have a non-mv amp, and you are playing quietly, using say a single preamp gain pot, isn't the signal at the speaker reduced, as well? What makes the ppimv (turned down) have less signal, than if you were playing at the same volume, with no ppimv?

[d95err]

The amount of negative feedback is the relation between the input signal to the phase inverter and the signal being fed back from the speaker.

So, when the signal is reduced after the phase inverter (PPIMV), the feedback is smaller in relation to the input.

With a pre PI master, the relation between input and feedback remains the same.

(At least that's how I understand it)

[d95err]

(double post)

[Tone Lover]

This has been chased many times. Bedroom levels need a bedroom amo 2 watts to 5 watts then even sometimes to much. PPIMV never sounds good at bedroom levels to me no matter what you do..
just my 2 cents.
Bill