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Hello all, not really sure if this is the right area to post this, but need advice from anyone with amp building experience. I'm trying to understand how tube amps work so I can potentially mod a fender of mine, but the handbook I'm reading keeps referring to "power rails", which are confusing me because:

a) I conceptually really don't understand what they do (but get the gist that they transfer DC voltage back to power the tube

b) I don't understand what object in the Fender Princeton schematic the handbook provides is supposed to represent the "power rail(s)". I've included the schematic below. Apparently, the plate (pin 1) of the first triode of the preamp tube is supposed to run the signal through the 100k resistor, said resistor being connected to the "power rail" at some point. The problem is that I have no idea what the schematic is using to represent this "power rail" that the 100k resistor is wired to.

Any help from anyone experienced with this kind of stuff will be greatly appreciated. I'm decent when it comes to understanding a system conceptually, but if I can't visualize a component of the system then there's no hope for fully understanding the whole.

You might have better luck if you also post this in the DIY part of the forum.
viewforum.php?f=151
Since our amp makers/modders tend to lurk there more than general?

1) Check out ampgarage.com and music-electronics.com for more tube amp focused help, those guys are great.
2) This is a layout, not a schematic, I'll post a schematic
3) I'm 90% that the power rail is the filter caps and the dropping resistors, together they regulate and feed voltage to different parts of the circuit.


So, the power rail should be the power from the tranny, after the choke. Basically, the 8mfd 450v caps and beyond.
Maybe someone with a better understanding can correct me.

D.o.S. wrote:You might have better luck if you also post this in the DIY part of the forum.
viewforum.php?f=151
Since our amp makers/modders tend to lurk there more than general?

Ok, will do and thanks!

WORMDIRT wrote:1) Check out ampgarage.com and music-electronics.com for more tube amp focused help, those guys are great.
2) This is a layout, not a schematic, I'll post a schematic
3) I'm 90% that the power rail is the filter caps and the dropping resistors, together they regulate and feed voltage to different parts of the circuit.


So, the power rail should be the power from the tranny, after the choke. Basically, the 8mfd 450v caps and beyond.
Maybe someone with a better understanding can correct me.

Ok thanks for those sites, I'll check them out

In regards to layout vs. schematic, I meant to post the layout (as that's what the handbook uses) but called it a schematic instead by mistake. My problem is not only that I don't understand what is representing them on the layout I posted, but also the fact that I really don't understand where the "rails" come from (as in where are they getting the power they're supplying, the transformer?).

Search for "how tube amps work" on youtube. A dude made a badass video series, sectioned off into different sections of the amp. He uses the same amp you posted in his videos too.


No worries on the schematic/layout thing, I hope I didn't come off as a dick.

WateryDomestic wrote:Hello all, not really sure if this is the right area to post this, but need advice from anyone with amp building experience. I'm trying to understand how tube amps work so I can potentially mod a fender of mine, but the handbook I'm reading keeps referring to "power rails", which are confusing me because:

a) I conceptually really don't understand what they do (but get the gist that they transfer DC voltage back to power the tube

Please forgive me, as I'm going to be blunt: If you don't know what a power rail is you have no business poking around inside something that connects to a mains supply.

WORMDIRT wrote:3) I'm 90% that the power rail is the filter caps and the dropping resistors, together they regulate and feed voltage to different parts of the circuit.

You are 90% sure and 100% correct, this is a good (albeit difficult) ratio to maintain.

The function of the power rail is to charge the active elements of the circuit. Without a healthy potential at anode in respect to the cathode of the tube there is little chance of an electron path to guide your signal from start to finish. Huge oversimplification there, just understand that without B+ (power rail) nothing is getting done.

WateryDomestic wrote:b) I don't understand what object in the Fender Princeton schematic the handbook provides is supposed to represent the "power rail(s)". I've included the schematic below. Apparently, the plate (pin 1) of the first triode of the preamp tube is supposed to run the signal through the 100k resistor, said resistor being connected to the "power rail" at some point. The problem is that I have no idea what the schematic is using to represent this "power rail" that the 100k resistor is wired to.

As WORMDIRT pointed out, this is a layout, sort of a secondary document to assist in the actual assembly/troubleshooting of the build. To answer your question, the B+ is sourced at pin 2 of the 5Y3 (which is the rectifier for the secondary high voltage winding), it is a yellow color - because it shares the 5 volt AC filament supply due to how that particular dual diode works. The power rail then goes through a series of dropping resistors and smoothing caps. It is best depicted on the schematic as the horizontal line between the lower power supply section and the upper preamp/amp section.

It will reveal itself to you when the circuit is charged.

WORMDIRT wrote:Search for "how tube amps work" on youtube. A dude made a badass video series, sectioned off into different sections of the amp. He uses the same amp you posted in his videos too.


No worries on the schematic/layout thing, I hope I didn't come off as a dick.

No, you're fine, thanks for pointing me to that video.

crochambeau wrote:Please forgive me, as I'm going to be blunt: If you don't know what a power rail is you have no business poking around inside something that connects to a mains supply.

Not to be rude, but refer to:

WateryDomestic wrote:I'm trying to understand how tube amps work so I can POTENTIALLY mod a fender of mine

"Potentially" as in "when I understand everything about the circuitry of an amp competently and completely then AND ONLY THEN will I mess with one". I know that isn't explicitly saying that but that's the gist I was trying to give. I realize that tube amps are seriously dangerous and, like the Wu Tang Clan, ain't nothin to fuck wit.

I also understand that I need to be well versed in electronic concepts to really get into amps but I'm starting to get confused as to how acquire the necessary knowhow, because its starting to seem elusive. For example, I've read, studied (as in have done recall and flash cards), and done the projects that are part of this electronic book I've been reading for awhile, and therefore feel I have a decent grasp on basic electronics and the associated components, schematics, values, etc. What books like that don't teach are the amp-world vocab like "power rails" and "mains supply" that I can't seem to find decent explanations for. I guess I'm really baby when it comes to learning new things; I need for everything to be introduced to me in the simplest of terms. Though, I have a feeling that a lot of the people I've talked to about amps learned hands-on from someone else.

crochambeau wrote:It will reveal itself to you when the circuit is charged.

Sounds cryptic. Hope I'll make it there someday.

Not cryptic, when the neighbor's cat curled up in the amp chassis fries, you know it's been revealed.

Lol, yea crochambeaus last statement kind of goes back to the first one.

Be careful with this stuff is all. Read about discharging capacitors and that sort of thing too. I'm sure there are modern tube amp specific books to get you in the right direction.

WateryDomestic wrote:I guess I'm really baby when it comes to learning new things; I need for everything to be introduced to me in the simplest of terms. Though, I have a feeling that a lot of the people I've talked to about amps learned hands-on from someone else.

Terminology is going to be somewhat fluid, so I call B+ and ground what someone else might call a power rail, the facts about a given device remain the same - however. I tend to reserve "power rails" as a reference to bipolar power supplies, it just makes sense to me that way and is not to be considered a correct (or even accurate) discrimination of the term, it's just the vocabulary I adhere to.

What I'm driving at, is just: be careful. Especially with immediate access to some of the shit passed off as knowledge on the internet. I have seen some cringeworthy tutorials regarding tube circuits on youtube, usually people will tear it apart in the comments. Here's a massive library regarding the technology: http://www.tubebooks.org/technical_books_online.htm

WateryDomestic wrote:

crochambeau wrote:
It will reveal itself to you when the circuit is charged.

Sounds cryptic. Hope I'll make it there someday.

It is cryptic, and I did give you the answer in the preceding paragraph... but, I find it a LOT easier to trace the power section when the amp is on - with a multimeter... a decent one, having a meter arc while poking around 600 volts is NOT a good time. *Always* probe a live circuit with one hand in your pocket or behind your back.

You have my genuine apologies if you took any of my preceding post as an affront, my reaction to this sort of thing has become more cautious since the lick your finger and poke around bending school of electronics became widespread. Electrons do not need my assistance in making people respect them, they can gain the respect on their own.

Seriously, it can be an enriching experience. I'm self taught too, I'm also pretty lucky, in retrospect, that I'm not dead. Wheeeee

crochambeau wrote:
Terminology is going to be somewhat fluid, so I call B+ and ground what someone else might call a power rail, the facts about a given device remain the same - however.

Yeah, I guess that's true for more disciplines than merely the tube amp domain. Should probably just accept that and keep searching; I'm sure those books will come in handy.

crochambeau wrote:You have my genuine apologies if you took any of my preceding post as an affront, my reaction to this sort of thing has become more cautious since the lick your finger and poke around bending school of electronics became widespread.

It's all good, I didn't want to make a big deal, and honestly should've been more explanatory with my intentions. One of the main reasons I've been trying to learn about amps is because I think I want a new one, and therefore want to know enough about them to expand my options (maybe even into boutique territory). But I get that "safety first" especially means "SAFETY FUCKING FIRST" in the amp world. I haven't had to worry about it with the caps on my hobby-project boards, but I do understand the basic function of a cap is to store up voltage, and the big boy caps in big boy circuits will let you know that.

Truly, you are a scholar/gentleman and I deeply appreciate any advice you give

Ok, so I sorta think I "get it" better in regard to the layout I originally posted. Functionally speaking, the positive DC voltage (which should be somewhere around 150V)is flowing from that little circle (which I think represents a transformer on the other side), through the 22k resistor through the 100k resistor and is hitting the 12A7 preamp's first triode plate (pin 1), with all that positive voltage exciting the electrons from the low AC guitar voltage signal, thereby boosting the signal in this first gain stage.

Still have no idea what is actually representing a "power rail" in this layout, but that's my understanding of whats happening in the pre-amp stage from a functional standpoint. Pls correct me if I'm wrong

That layout lists the voltages as +n at nodes between the B+ and ground. Here's the cheat sheet you have been asking for:



B+ secondary (secondary is a reference to position in relation to the power transformer) circuit is the fuzzy coloring, reddish on the high end, dropping down to yellowish at the "low" (still 150 volts) voltage end. I went ahead and touched all the wires of the primary (mains) circuit with a hard red, note the ground referenced cap I drew and X over, that's best removed from circuit (that's what is commonly referred to as the death cap), if you're building your own, just grab a current UL drawing and follow THAT. Pretty much anything built in the last 20-30 years should be fine, but watch out for anything preceding the 1980s.

You'll note immediately that the voltage gets cut in more than half through the passage of the initial choke (not depicted, but it's part of that early circle with four wires near the 6V6), 22K dropping and 100K load resistors.

This illustrates why I don't think of the B+ as a power rail. Power rails are a more solid point, say +24, and they are tapped along points that feed the circuitry, like spurs of a railroad, whereas B+ is like a river, wider at one point than the other but still flowing and able to carry goods and drown people.

Anyway, hope that helps.