The PI Tube Matters!

[s2]

OK. Here we go. I haven't picked a fight in a while, so...

I've heard the PI tube referred to as a utility position, etc. I've also heard it said the tube you stick in there really doesn't matter. I disagree.

The PI produces some good OD in a cranked 18 Watter. It is also the preamp's traffic cop so to speak for what is sent to the power amp. I think it really does matter. I see it as the pinch point in the hour glass. If you don't have a good one, you are really cheating your tone.

Sure, V1 matters a lot. But, I contend the PI matters just as much. Let the arrows fly... LOOSE!

[Plexi]

"""I've heard the PI tube referred to as a utility position, etc. I've also heard it said the tube you stick in there really doesn't matter. I disagree. """


Can't pick a fight,because i agree with you. All of the tubes matter.
And most of the 18 watter people know this section is alot of the magic in the 18. I think what they mean by function and tube used.alot of times a preamp tube may be microphonic, but in other sections of the amp,it may work without the noise,or in some amps like fenders,for a trem section.

Now i'm sure alot of people will get into debates over matching the tube sides for the PI,and all that. But also it matters about the circuit itself.
One thing for sure,when something goes bad in this section,you have no amp..lol Its a common problem in some fenders to have to fix the PI/driver circuit.

But yes,i agree the PI is very important,as you can see in the CBS fenders,they changed values to make the amp clean..or if the cathode resistor is off or changed in value or modded,it can make a difference.

I think in all sections,finding the right tube if everything else is as it should be..can make a big difference. These tubes also work pretty hard,and should be changed more often than the preamp tubes.

Richie

[zaphod_phil]

Like Richie says, every tube matters. Although maybe you could argue the toss about the rectifier.

The PI produces some good OD in a cranked 18 Watter.

I would be interested to know what you base this deduction on. Most folks experience with MV controls suggests otherwise, and I'm feeling too lazy at the moment to get out the signal generator and 'scope...

[rjgtr]

Now i'm sure alot of people will get into debates over matching the tube sides for the PI,and all that. But also it matters about the circuit itself.

...These tubes also work pretty hard,and should be changed more often than the preamp tubes.
Richie

I agree completely. I do think for clean tones a balanced tube in the PI is better than an unbalanced one, unless the amp is like the 18 watt, where the sides are not balanced. However, an unbalanced PI can make distorted and clean tone more rich and interesting.

I personally think the PI tubes works almost as hard as the power tubes. I think it should be changes more often than the preamp tubes, but not every time the power tubes are changed. Only when it starts to lose tone.

[zaphod_phil]

I personally think the PI tubes works almost as hard as the power tubes.

Really, why? An LTPPI is just a differential amplifier, working in the same signal range as pretty much any guitar amp gain stage, apart from the very first one. And in fact with an LTPPI the output siwng is somewhat limited, due to the presence of the tail resistor reducing the effective rail voltage. Please correct me if I'm missing something here...

[rjgtr]

zp, that's what my experience has shown me. The PI wears out quicker than the other 12ax7s. I may be wrong or crazy, but I don't want to argue about it.

[zaphod_phil]

I don't see any need to argue either. I just wanted to understand more about what you were saying...

[rjgtr]

The phase inverter has a lot of current passing through it to drive the output tubes so that has to contribute to what I've seen in how it wears. Voltage swing is just one part of the equation, current is the other. That's all I was getting at. Plus the tubes that pass the most signal are gonna wear and the signal is pretty strong at the output of the phase invert as well as at the input.

I'm not saying the PI should be changed every time you change output tubes (I hope that doesn't start something!), as it doesn't wear anywhere as much!

[Darryl]

Is PI the same as V2? Thanks, Darryl

[T-bones]

No doubt the phase inverter should be the strongest of the 9 pin bottles. I can't really say that the triodes being matched makes a hudge difference in a guitar amp. Although you want it in your HiFi rig for the best seperation possible. For the P.I. tube I use Tung-Sol, Mullard and anything German- Tele, Siemens, RFT. Testers are good for getting you started on the strengh and life test. Not dissing any of the other guys at all. just about all NOS are well worth there price. The ones I have listed last the longest in that position. Any comments welcome. T-bones.

[rjgtr]

Daryl,

In a standard 18 watt Trem V2 is the PI.

But in a TMB it is usually V3, but could still be V2 in some cases. For a TMB either look at the schematic if you built it yourself or open up the amp and trace the circuit if you didn't.

If you are building a TMB, use a layout with the PI in V3. For a Trem, V2 works just fine.

[NitroLiq]

So, is it just me or do all the TMB layouts currently show V2 as the PI? From looking at the trem and tmb layouts both here and at nik's site, in the tmb, V3 always has the 220uf cap connected to it and V2 always has the 3 resistors (2 470ks and 1 820)...at least that's how I'm differentiating the PI section. Am I looking at this wrong?

[rjgtr]

No you're seeing it right. However, I think there are a few layouts in downloads or in the gallery that have the PI moved to V3 for the TMB, which is a lot better.

[NitroLiq]

Did a bit of searching but didn't see anything. I've actually been thinking of re-creating an Illustrator doc based on Nik's layout but with V2 and V3 switched. All the connections from the board are the same except that now everything going to V2 goes to V3 and vice versa, yes? ( the 220uf would now go to the same place on v2 and the the 3 resistors on V2 would got the same place on V3?). Doesn't that make for a messier lead dress having to cross wires over like that?

[tkroeger]

As I was telling the sarge the other day, I don't use a preamp at all with astounding results.....and I ain't goin back! I don't now how to describe the tone, except that it is a very 'balanced' sound. Of course you're not going to get big distortion out of this arrangment, but the base-mojo is there and my pedals pick it up from there.

Just my pedals straight into PI. (balanced GE)

Volume wise this works out fine....the same volume as when running into an attenuator.

My power amp section is basically a half-powered AC-30 (2xel84).
Big overkill Hammond OT 1650F.

I pulled my preamp tubes out and I'm stunned at my newly found discovery.

Terry

[rjgtr]

Nitro,

You would want to re layout the board. If you don't you'll have worse problems. The idea is to eliminate all wire crossing and have the tube positions follow the signal flow. So if you put the PI in V3 you need to move the PI circuit past the TMB channel circuit. Chris's Gallery and Minky's Gallery both contain layouts with the PI moved to V3. While not true 18 watts, they shoul give you enough to go on.

[NitroLiq]

Thanks, I'll take a look.

[MU-tant]

Correct me if I'm wrong but from my research EVERY tube matters in distinguishing the final sound of the amp. Even the rectifier, although no signal passes through it, has an effect. The ability, or inability, of the rectifier to supply current to the rest of the tubes during a high demand (on 11) can cause a sag in the flow. This drop produces a kind of compression on the signal and gives the amplifier its "feel" and dynamics.

[/quote]If it ain't broke kick it harder[quote]

[s2]

Nitro,

Here is the layout for my TMB:

http://18watt.com/modules.php?full=1&se ... _photo.php

It has the PI in V3 and it sounds great with no monkey business.

[MooreCowbelle]

FYI: I'm in the process of doing a layout derived from the JA 2.0 TMB, but with the PI moved to V3. I'll post it when I get it working.

S2, you have convinced me to switch to a star ground...can you describe your strategy?