Real TrainWreck mojo!

[Alan0354]

Careful, your phase margin is approaching zero there.

Technically, I thought people should know when I said phase shift of 360 deg., that already stated very clear that phase margin is 0!!!! This statement is absolutely implied. If you said phase margin of 30 deg, that usually means phase shift of 330 deg.....that you have stability margin of 30 deg.

Sarcastically, I have been trying to tone down and try very hard to stick to technical. On certain things, I don't see eye to eye with a lot of the people here, so I just simply avoid getting into the difference and concentrate on the common things. But I guess the pass arguments are catching up, so be it. If he want to push this, bring it on.

[jazbo8]

Technically,
Sarcastically, I have been trying to tone down and try very hard to stick to technical. On certain things, I don't see eye to eye with a lot of the people here, so I just simply avoid getting into the difference and concentrate on the common things. But I guess the pass arguments are catching up, so be it. If he want to push this, bring it on.

Good lord, stop embarassing yourself, it's getting painful to watch... "stick to the technical"?! That's what galls people when you say things like that all the while spewing BS. Since you are a 40-year EE, surely you have access to mountains of textbooks, if not, Google it. No one refers to 360° when taking about phase margin, that's why I suggested that you either want to confuse people, or your are confused yourself. If you want to be technically correct, as you constantly claim to be, at least use the commonly-known definitions, instead of something that you make up in your mind - and stop implying others do not know their stuff, i.e., stop being a schmuck!

Here is quote from a textbook I assume you have in your collection, Horowitz & Hill - The Art of Electronics (2nd Ed.):
"Since a 180° phase shift at a frequency where the voltage gain equals 1 will result in positive feedback (oscillations), the term "phase margin" is used to specify the difference between the phase shift at fT and 180°."

BTW, the TWE is not just a "modification of the vibrato stage", to suggest such, only shows that you have not bothered to read the voluminous threads on the subject, or spent time digesting the contents if you had read them. Instead, your attitude come across as this - I am not going build this amp (what amp does not come with a MV!), since there is really nothing special about it, bah-humbug... Really! Way to go, old man.

[Alan0354]

Good lord, stop embarassing yourself, it's getting painful to watch... "stick to the technical"?! That's what galls people when you say things like that all the while spewing BS. Since you are a 40-year EE, surely you have access to mountains of textbooks, if not, Google it. No one refers to 360° when taking about phase margin, that's why I suggested that you either want to confuse people, or your are confused yourself. If you want to be technically correct, as you constantly claim to be, at least use the commonly-known definitions, instead of something that you make up in your mind - and stop implying others do not know their stuff, i.e., stop being a schmuck!

Here is quote from a textbook I assume you have in your collection, Horowitz & Hill - The Art of Electronics (2nd Ed.):
"Since a 180° phase shift at a frequency where the voltage gain equals 1 will result in positive feedback (oscillations), the term "phase margin" is used to specify the difference between the phase shift at fT and 180°."

BTW, the TWE is not just a "modification of the vibrato stage", to suggest such, only shows that you have not bothered to read the voluminous threads on the subject, or spent time digesting the contents if you had read them. Instead, your attitude come across as this - I am not going build this amp (what amp does not come with a MV!), since there is really nothing special about it, bah-humbug... Really! Way to go, old man.

I am glad you make the quote and shows your ignorance. You are WRONG, so WRONG!!! What you are quoting is transistor fT bass current to collector current unity gain BW in common emitter configuration. The stage is like simple common cathode tube stage, THE OUTPUT IS INVERTED, WHICH IS 180 PHASE SHIFT. Then you add another 180 degree, you get 360 deg and will be in phase and you can get oscillation. I hope you understand what is common cathode stage means!!!! BTW, do you know the meaning of phase shift?


Let me give you a lesson. for gain of 1 and 180 deg phase shift, the gain is -1.

For gain of 1 and 360 deg phase shift, the gain is 1!!!

What is talking here is phase shift from input to output. For an inverted stage, the 180 deg phase shift is BUILT IN already like the common emitter or common cathode stage......

Let me give you another lesson, when you say 360 degree phase shift from input to output, the phase margin is 0. When you say 330 phase shift, you know the phase margin is 360-330=30 deg. Learn this by heart so you don't make of fool of yourself.

You really really need to study up before you talk. BTW, Art of Electronics is a beginners book for associate degree only. Read Nyquist stability, Bode Plot and study La Place transform, these go much deeper into stability and definition of positive feedback.


Start reading your Art of Electronics before you talk. That's as easy as it gets. Make sure you understand before you quote.

[Chuck H]

I'm certain that jazbo8 was talking about not needing to indicate a 360* phase shift because it's somewhat pedantic. A signal can also be of 0* phase shift or 720* phase shift and still be PFB. Further, there are phase differentials within any loop that are specific to frequency. It's easy to ignore this on paper but a reality that bites many a designer in the ass. The bottom line is that if you have a PFB induced instability, there is a like phase coupling causing it. It may or may not be, in fact, 360* degrees shifted. And the circuits/s responsible may not specifically exhibit the outward property for analysis except for certain, unintelligible circumstances (like clipping) and operating conditions. So limiting any assessment to this criteria is absolutely detrimental to evaluation of the phenomenon.

In light of this, arguing your position can only serve to illustrate a lack of understanding. Sometimes the BASIC rules are only guidelines for technical exercises and don't specifically apply to every real world circumstance. I think this is especially true of guitar amps, where operating parameters are seldom idealized and operation is often outside of the normally accounted for limitations.

JM2C on that.

Peace

[Dave H]

Chuck, ‘Tis much ado about nothing. Alan never said 360deg phase margin. It’s almost as funny as the Python sketch.

(Walks down the hall. Opens door.)

Q: **WHAT DO YOU WANT?
M: **Well, I was told outside that...
Q: **Don't give me that, you snotty-faced heap of parrot droppings!
M: **What?
Q: **Shut your festering gob, you tit! Your type really makes me puke, you vacuous, coffee-nosed, maloderous, pervert!!!
M: **Look, I CAME HERE FOR AN ARGUMENT, I'm not going to just stand...!!
Q: **OH, oh I'm sorry, but this is abuse.
M: **Oh, I see, well, that explains it.
Q: **Ah yes, you want room 12A, Just along the corridor.
M: **Oh, Thank you very much. Sorry.
Q: **Not at all.
M: **Thank You.
(Under his breath) Stupid git!!

[nsubulysses]

yeah someone tell those queens to relax

[Leo_Gnardo]

<nsubulysses: yeah someone tell those queens to relax> including Professor Alan. Don't want to see you pop a gasket, dear heart.

[Alan0354]

Hey I did not start the argument, just replying to Chuck's post. I have tone down a lot and mostly avoid confrontations now. I did not even go all the way out until after being provoked. BUT don't take that I am going to remain passive. And yes, at this time, the phase margin is 0.

I think everyone know 360 phase shift means. Only he make a big deal about 180.

[g1]

90 degrees is a right angle. And it is also quite warm.

[J M Fahey]

[Justin Thomas]

Two wrongs don't make a right. But three lefts do... What was this thread about again?

I don't have a clue what I'm talking about. But phase shift to me seems to indicate junk going out of phase. I don&t really care how much as long as it sounds good. And we make pedals to make stuff out of phase anyway. People pay a lot of money for them. I did! Now, I do know that 360° is a circle, so to me a 360° phase shift is just an amp following a crappy drummer who's always a beat behind.

Lemme in the pissing contest! Lemme in the pissing contest! Will those who have NOT played, seen, and poked at the real thing sit down? Thank you! And take note: I PLEAD COMPLETE AND UTTER IGNORANCE UP FRONT! No arguing with me, for one cannot argue with dumb. But you know what? I don't care if my amp sounds like the real deal or not, as long as it sounds like ME! And all my favorite players sound great no matter WTF they play through. Everyone I know using or used a TW also spent 22 hours a day practicing & gigging & playing their ass off before ever laying eyes on a real TW. The only Phase Shift we need to worry about is phase-shifting our ass off the stool and into the bar. Every day & night. For years. Then maybe we will be allowed to play some famous guy's real one, and can find out what it's REALLY about. Analyze it all you want, you won&t be a single step closer to understanding it.

Justin

PS laugh a little, people!!!

[Alan0354]

Why don't we just get on with the TW, this is quite interesting.

[Alan0354]

Guys,
I reviewed this over the weekend and decided that we may be over thinking this.
The Express front end is largely a Fender preamp with an extra gain stage tacked on the end.
So what stands out from an examination of the Express Preamp:

1)The 150K "Gain Dumping" resistor as the grid leak on the input to the third stage
The RL of this 2nd stage would be 90K (100K parallel 1M) ordinarily but with a 150K grid leak following instead of a 1M then RL becomes (100K parallel 150K) = 60K.
Gain = mu.(RL/ (RL + ra) so the actual gain of the 2nd stage is dropped marginally from 58 to 48.
However the output signal is developed across RL so with 60K instead of 90K we are swinging 1/3 again as much current in the tube to get the same output voltage.
That means that the ra variation with signal current is larger.
AND
since RL is much lower then Delta ra vs RL is a much bigger ratio - that is important because delta ra vs RL is the distortion level.
So we have modified the 2nd stage to drop the gain by a bit less than 20% but are likely to have increased distortion by much more, at least 50% and possibly as much as 100%

2nd - the extra gain stage
Well it has a couple of things working against each other.
Gain is low - you could predict that just by looking at the schematic and noting that even if the triode had infinite gain then feedback theory would tell you gain would be RL/Rk or (100K parallel 1M)/10K = 9.
In fact if you do the math the gain is actually more like 7 to 8.
We are working at the low current end (estimated 300uA) of the triode characteristics which means high ra and low gm and also that when considering the ra vs Ia curve we are on a section of that curve with very high slope indicating large changes of ra for small changes of Ia.
Normally we would associate that with VERY high distortion but in this case the unbypassed cathode resistor will act to oppose the distortion - even so distortion is likely to be high.
The other thing this does is set the anode voltage of this stage close to its B+ (300uA across 100K means 30 volt drop) so the positive going signal swing will be limited to 30V peak. That 30 V is likely to be increasingly comperessed as we approach the B+ but clipping is unlikely or in any case will occur only at extreme volume settings. Conversely the swing in the other direction is not only larger but is more linear, the asymmetry will be significant, introducing primarily even order distortion.
If you draw a transfer function graph - that is volts out (anode) for volts in (grid) then the departure from a straight line indicates distortion and the sharpness of the curvature indicates the order of the distortion, sharp curves indicate high order harmonics.
Now my view is that we will have fairly gently curving transfer function for those positive going signal swings and that means EVEN and LOW order distortion products, mostly 2nd harmonic.

This can be "sharpened up" and some harder clipping introduced and for that you need to look no further than the final preamp stage of a high gain Soldano, same circuit but with a 39K instead of the 10K.

This suggests to me that the poster above who mentioned varying that 10K until he found what suited him (6K8 I think it was) was onto something.

So thats my erratic "pot shots at the barn door" thinking.
Sound/Tone is primary formed in that final cold bias stage with the 10K cathode bias resistor which is large enough to cause significant compression on one side of the signal but not quite enough to cause clipping.
There is some additional distortion being introduced in the second stage by that 150K "Gain Dumping" resistor which is also the grid leak for the 3rd stage.

Cheers,
Ian

This is very thoughtful. I printed out two of your posts to read in the waiting room doing the blood test today. I really like it. Make me want to go back and lower the current on the stages of my amp and see what happen. So far, I am using 1.5K cathode resistor like Fender for no other reason other than just a starting point.

[Chuck H]

6.8k

Why?

Just 6.8k that's why.

Seriously though... Part of the point of that circuit is to create asymmetrical effects that probably accentuate pleasant sounding harmonics. In my experience this effect is audible around 3.3k (barely) and reached a point of diminishing returns at 10k. At 10k or more the tone of my personal amp seemed to lose too much dynamic responsiveness. The bass was really "smooth" and NOT farty, but lacked definition. Just not my thing I guess.

My amp (designed before anyone knew what the TW circuit was like) is VERY similar to the Express and Liverpool preamp circuit. I cringe that there are a few of my custom amps floating around out there with this circuit because I fear someone will open one and accuse me of being another cloner. Sorry, I always feel the need to qualify myself on this since the TW is such a hot point design. I digress.

So, in my amp, which is similar enough to a TW type amp, I've found that the magic number for the third stage cathode resistor to get useful dynamics and accentuation of desirable harmonics is 6.8k. There's also the problem of just plain too much signal drive to the PI with a center biased third stage. You'd need to include a voltage divider to make the amp more intuitive from the average users perspective. I know other cascade amps have used much higher value resistances in cathode circuits to good effect, so this applies strictly to my own experiences with this particular type of circuit. YMMV

[Alan0354]

I really have to experiment the higher value resistor. Tomorrow is the chassis drilling day. After that, I'm going to experiment.

Hey Chuck, so you say you get better harmonics with 6.8K cathode resistor and no bypass? I am using lower value resistor to get more current. What kind of effect would I expect to hear differently if I lower the current of a stage? I know Ian explain very good that you get more distortion with lower current as rp ( or ra) increase with lower current. But how do you describe the change in sound. This is like I followed your suggestion to run the tubes at higher plate voltage ( 240V) and I do like it better, less muddy for lack of words.