Tube Theory...True RMS meter to detect signal? Renamed

[Fischerman]

I am confused about one thing though. In the amp I am checking, the signal seems to go to the grid of the first PA tube. Then it exits at the plate. Then following the "Normal" line, it enters the next PA stage at the grid and exits at the cathode. Why does it switch like that?

When you say "PA tube" I assume you mean 'PI triode' i.e. phase-inverter triode. In the first triode you are right...the signal is input into the grid and signals input at the grid are amplified out-of-phase at the plate. But the 820ohm cathode resistor is not bypassed by a cap...so the signal exists on the cathode (and it's in-phase with the input signal). Also notice that the cathodes of the two triodes are tied together. So the input for the second triode is at the cathode. And signals input at the cathode are amplified in-phase at the plate. So there's your two opposite-phase signals for the power tubes.

[ampgeek]

Well.....although I usually reserve...."Oh my god".... type exclaimations for more orgasmic experiences....the use of the device (or more accurately, an understanding of what it could be used for) certainly neared an epiphany for me!

Not sure how, or if I even dare, to weigh in on the other thread vein going on here. Suffice it to say that I don't believe one needs genius to do the tube amp gig successfully. However, one can not be stupid!


Brian,
I believe that you are looking at the cathode follower (CF) stage in your schemo. As Fischerman states, the output signal there is in phase with the signal at that section's grid. You will see CF's typically used ahead of very lossy circuits like tone stacks. CF's are capable of supplying a lot more current than plate couplings.

Dave O.

[tele_player]

nevermind...

[mchauck]

OK....I need to be clearer in my question. By PA, I am referring to the Pre Amp tube (V1). Sorry about that. What I mean is that the signal enters the amp at the grid of V1a and exists. (Signal path grid to plate) Then it bypasses V1b via the Normal pot and enters V2a @grid and exits at the plate immediately entering V2b at the grid, exiting then at the V2b cathode. I just wondered how or why this happens and I think I can learn a very fundamental lesson here if I can get it.

Thanks and sorry for the mix-up....

Brian

[Fischerman]

Suffice it to say that I don't believe one needs genius to do the tube amp gig successfully. However, one can not be stupid!

The same could be said for being a degreed engineer...and I might be a living 'test' of the 'can not be stupid' claim. Note: I'm not EE obv.

[mchauck]

I guess what I really am beginning to do is to look at these things a bit more closely. I build guitars and am not an electrical engineer by any stretch of the imagination. I started building amps as an avenue of sharing music with my kid. I like to tinker. It is really no problem for me to builde an amp according to a layout and I am begining to be able to build one just using a schematic. I am learning how to follow the signal thru the amp as well. But I am limited in my understanding of the magic that happens in the vacuum there. So I am hoping that by asking some uestions that may seem a bit rudimentary to y'all, I may gleen a bit of light.

Thanks (and sorry for the stupid jokes)

Brian

[drhulsey]

So I am hoping that by asking some questions that may seem a bit rudimentary to y'all, I may glean a bit of light.

That's what this forum is all about Gaining an understanding of why amps work is a worthwhile endeavor. It'll keep you off the streets and out of trouble It will also keep you out of trouble INSIDE the amplifier
I don't know about the rest of you, but I don't plan to put a major dent into any body's amp repair or production business. I do, however, thoroughly enjoy the fact that I built a great sounding amp with my own hands. The knowledge to do it came mainly from this forum (for which I am very greatful) Quality Well, lets see how it hold up to the gigging schedule

Suffice it to say that I don't believe one needs genius to do the tube amp gig successfully. However, one can not be stupid!

1++++++ I sure don't qualify for genius, but I am trainable

... I usually reserve...."Oh my god".... type exclamations for more orgasmic experiences...

Me, too, but I usually say it repeatedly at those times

[tele_player]

OK....I need to be clearer in my question. By PA, I am referring to the Pre Amp tube (V1). Sorry about that. What I mean is that the signal enters the amp at the grid of V1a and exists. (Signal path grid to plate) Then it bypasses V1b via the Normal pot and enters V2a @grid and exits at the plate immediately entering V2b at the grid, exiting then at the V2b cathode. I just wondered how or why this happens and I think I can learn a very fundamental lesson here if I can get it.

Thanks and sorry for the mix-up....

Brian

V2b is a cathode follower. A cathode follower has unity gain, but has a very low output impedance. Google 'cathode follower' for more details.

[mchauck]

Thanks tele

[gearhead]

I highly recommend reading NEETS module 6. It will give you a solid understanding of how tubes work, and a common-language explanation of the basic cathode-biased circuit. AFAIK, don't think it has the common cathode circuit, but google results are decent.

http://www.phy.davidson.edu/instrumentation/NEETS.htm

Be aware that they refer to electron flow, and not current flow. For that little tirade, read the aforementioned thread in this forum

[mchauck]

Thanks gearhead...I WAS in the Navy and I will complete this course. I think it will really teach me alot. Thanks for the gear "heads-up"

Brian

[mchauck]

This course rocks!!!! Thanks gearhead!!!!!!

Brian