What the heck is B+, and that A on AA764 Champ Schematic?

[Shontsy]

I keep seeing reference to B+ over and over. What is that?

On one Champ (and V. Champ) layout I have, the AA764 layout (not schematic) from Fender Amp Field Guide, there is actually an A node on the upper left corner of the circuit board, but no B's shown anywhere. What is that "A" doing there?

Does this tell the whole story for guitar amp guys, or does B+ mean something different to us?

http://wb0nni.dakotamade.com/basic.html

Thanks in advance! You guys are a great help!

[52 Bill]

I keep seeing reference to B+ over and over. What is that?

Back in the olden days when guys like me were still in diapers, those big tube radios our folks had were run on batteries. The main power source for the tubes was referred to as the B+ source. The term just stuck, and now it basically refers to the main power supply.

The letter designation on most schematics refers to connecting points. The letter "A" on one node of a power supply needs to connect to the letter "A" somewhere in the circuit. This way there doesn't need to be line drawn from point "A" to point "A".

[Shontsy]

So the "B+ rail" is just the Hot side of the amp?

I'll look at my amp tonight and see where that "A" wire is going, to see if I can find the mating "A" on the schematic. I figured that is what it would mean, but I can't find the mating "A" so far.

Thanks!

[52 Bill]

I'll look at my amp tonight and see where that "A" wire is going, to see if I can find the mating "A" on the schematic. I figured that is what it would mean, but I can't find the mating "A" so far.

Look in the middle of the fiberboard. That line goes to the two 100K plate resistors for the 12AX7.

[Rob Mercure]

Shontsy,

I'm not sure how you would define "hot" but if you take it as anything that might kill you then the line voltage in is "hot" - some tubes have heaters over 50 V and would be "hot" and some high powered radio and industrial tubes have grid bias supplies with sufficient voltage and current to kill you.

Based on the battery supplies that Bill mentioned the heater voltage is the "A" supply, the plate and screen grid supply are the "B" supply and this is usually positive while the bias supply, usually negative, is the "C." Supply. If I remember correctly there is a AGC "D" supply also not found in audio.

Why don't you get a hold of some basic electronics texts -there are lots to recommend like some of the RCA Receiving Tube Manuals or the ARRL Handbook, etc., and do some basic research on tube electronics? You'll learn more in one evening than you will from 20 posts here.

But we're always glad to help.

Rob

[Enzo]

To add:
The B voltage in an amp is the high voltage supply for the tubes, typically several hundred volts.

In the old days, the heaters ran off the A voltage, but we never refer to "the A supply" anymore. And the negative supply for bias was the C supply. I have not heard someone say that in a long time either.

Back 50 or more years ago, before transistors, there were portable radios - with tubes even. ANd you bought these large batteries with multiple voltages in them. They had a little socket, and the radio had a mating plug. A typical battery might be called an "ABC battery." As I recall, the ones in my Mom's radio were about the size of a carton of cigarettes.

But "B" stuck around. Like any battery, the B battery had a + and - end. The - end would be grounded, leaving the B+ end for the tube plates. Now the B supply needs to be higher voltage for the power tubes than the preamp for example, so we typically have a string of resistors dropping the B voltage along the way for the various stages. AS mentioned above, rather than draw wires everywhere, at each node (junction) we simply give that voltage a designation. In this case A, B, C, D etc.

SO don't confuse the B supply with the B node.

YOu COULD make the + side of the B supply ground, and then the cathode resistors would all trail down to the -400v rail, which would be called then B-. You don't see that often, but now and then some compelling reason exists and circuits are made that way. COmes to mind certain Seeberg Juke Boxes where the tubes that trigger the selections are wired that way.

[Shontsy]

Ah yes, found it waiting in the doctor's office today. I was checking out the schematic, visualizing some mods I was reading about, and there it was, finally!

This low-fi print of a low-fi hand drawn layout makes finding/reading things hard sometimes!

Thanks for the response!

[Shontsy]

Crazy, man!

Back 50 or more years ago, before transistors, there were portable radios - with tubes even. ANd you bought these large batteries with multiple voltages in them. They had a little socket, and the radio had a mating plug. A typical battery might be called an "ABC battery." As I recall, the ones in my Mom's radio were about the size of a carton of cigarettes.

Man, are you serious?! That's pretty crazy! Did the B section of the battery acutally output over 300V?!

And I thought the 9 v trasistor radio I carried to school to listen to at recess was primitive, even by 1975 standards. I had no idea that portable battery powered tube radios even existed. I thought that was the whole big deal about transistor radios was the big boom in portables. I guess I just assumed there WERE no portables before then.

[Shontsy]

Shontsy,
Why don't you get a hold of some basic electronics texts -there are lots to recommend like some of the RCA Receiving Tube Manuals or the ARRL Handbook, etc., and do some basic research on tube electronics? You'll learn more in one evening than you will from 20 posts here.Rob

Just for the record, this post isn't really aimed just at Rob. I think many readers will get a lot of insight from it, and maybe reflect on how THEY got here and how they learned what they know today. It's a lot different for us who are new to this subject, trying to learn it in the the silicon age than it was for the guys that actually lived and worked on these things when they were in common everday use. It's a whole different world for us, like Back to the Future!

I have some Radio Shack books, I bought the entire London Power / Power Press tube amp book set, I've tried to read about all this stuff before. I had basic circuits in High School. I've put together pedal kits. I can "read" a schematic sort of, but I have no clue what most parts of the circuit are doing just by looking at the diagram. Book learning doesn't keep my attention like the posts/threads do right now.

It's hard for me to sit down with a book and read all this "esoteric crap" and RETAIN it for future use when you don't have any perceived need/application/reason to learn the stuff you are reading at that time. When I get on here and read the real world problem descriptions and solutions, I can IDENTIFY with that, becuase it's not just some example problem in a book, it's REAL problem.

I know, it should be very similar reading about ficticious problems and solutions just like real problems and solutions, but for me it's not. I have a mental block when I know it's "just book learning". I blame college for ruining me like that. The fact that I'm even motivated enough to try to learn about tube electronics when electronics has always come so hard for me, is a minor miracle.

Not that I will NEVER read the books, I just am trying to get my feet wet with something REAL to motivate me to read the Power Press books. Believe me, I actually am very ANXIOUS to be able to sit down and make heads or tails of what the hell that guy is talking about. I've tried!

I figure if I make myself a Champ expert by working through every cap and resistor of THAT simple amp, then learning about JCM800's and Fender twins will come MUCH easier, since I'm starting from "ground zero" with a very difficult and outdated subject matter you couldn't learn at ANY college today even if you wanted to pay the tuition to do so, except for maybe the Egnater amp class, but that's not a university. Vacuum WHAT???

I've always been GREAT with mechanical stuff, but very weak with electrical anything. My experience with EE 345 at MSU buried me even further. For ME's they took about 4 years of EE learning and jammed it all into 1 semester for ME's?! Um, excuse me, but we are NOT the experts in this field. We need a REMEDIAL class, not an AP-style class! DOUBLE imaginary number schemes to figure out circuit values?!! Are you kidding me?!! Get real, literally! The fact that it's the ONLY EE class we took and it's a 300 level class ought to tell you that I'M not the one who is crazy. The profs/dean/whoever should know better than that!

I had another class about electro/mechanical stuff like motors, later at U. of Cincinnati night school. A lot of radians and phase graphs and blah, blah, blah. What do I need this crap for? I'm not DESIGNING motors, I'm just picking one from a catalog. Waste of time!!

To look at it another way, you can lead a horse to water, but you can't make it drink. When I had the opportunity to learn this stuff in college, I wasn't thirsty and I suspected the water was polluted. Even if I HAD learned it then, I haven't used it enough in the last 15 years to retain it anyway.

I know what all the components DO, I just don't know how they react to make a circuit, especially one as complicated as a tone generating machine that is a guitar amp. You guys all KNOW an amp is much more than the sum of it's parts. Most EE's are just happy if the damn thing basically WORKS. WHAT IS TONE?!! That's CRAZY talk to most EE's that I know. Tubes superior to transistors?!! Are you MAD?! Anyway, the circuit designing part, which actully WOULD have been interesting to me, we never even touched on. I can calculate the time constant of an RC deal-ee-o and the voltage drop across a resistor, great, now what heck do I do with THAT?

They don't tell you how to discharge caps on tube amps in college so you don't get electrocuted, at least not in the late 80's/early 90's. And guess what. They never EVER will again, either. Really THINK about that for a few minutes. The ONLY new blood coming into this field are hobbyists, or kids coming out of college that wind up getting hired by some RF transmission tube manufacturer and learning on the job. There is NO training left for tube technology. I take that back, the Military is still teaching this technology on an on-going basis, but as far as I know, that is it.

"Stupid punk kids" can design computer circuits like a mofo. I can buy a computer for $350 that I could never hope to build myself and does way more than I'll ever need. I don't give a hoot about that. In fact, most computers you could dig out of a dumpster these days are good enough for me to get on-line and do CAD drawings. I want to know how to buy $350 worth of tube amp parts and put together an amp worth $2500. Now THAT is useful knowledge!

That's why I'm here.

Soapbox going on fire pit now. Sorry! I hope you were entertained!

[defaced]

DOUBLE imaginary number schemes to figure out circuit values?!!

We had the same thing at Ohio State. The 300 level EE class butchered everyone in my class, including me when we got to AC analysis. I still wouldn't mind knowing how to compute values using phasors.

-----
Your question about B+ is legitimate. Had I not worked on cars as a kid, I wouldn't have known that B+ is the live side of a circuit (replacing starters is where this term is used often). Coming here I found out that B meant "battery".

There are two ways to do this whole build/design an amp thing. 1) analytically understand the components and their interactions prior to building/designing, or 2) take a circuit that works, build it and get it working, then start trying to screw it up by swapping out components and seeing what happens then researching the result. Being an engineer, I understand the desire to go about this using method 1, but if EE stuff doesn't come easily for you (as much as I wish it would, it really doesn't for me either), then ditch the engineering methodology and take the trial and error path. I can't read an RCA tube manual and actually get anything out of it; my brain just hasn't got to the point where I can visualize the interactions that happen in a tube circuit.

For what it's worth, I just started doing tube amps a few months ago when I was staring at a schematic and it just clicked and I "got" how a tube works to amplify. I didn't know the details, but I formed a basic "a small vibration here makes a big vibration there" sort of understanding of things. From there, following the schematic was easy. Once you get that part down, then you can work on the pieces, anode parts, cathode parts, grid parts, and all of that stuff. There are circuits I still don't understand (tone controls for instance), but hey, my first project so far is a success because of the two big things I learned in college: how to learn and where to look for answers.

[Shontsy]

Rock on!

We had the same thing at Ohio State. The 300 level EE class butchered everyone in my class

There are two ways to do this whole build/design an amp thing. 1) analytically understand the components and their interactions prior to building/designing, or 2) take a circuit that works, build it and get it working, then start trying to screw it up by swapping out components and seeing what happens then researching the result.

For what it's worth, I just started doing tube amps a few months ago when I was staring at a schematic and it just clicked and I "got" how a tube works to amplify. I didn't know the details, but I formed a basic "a small vibration here makes a big vibration there" sort of understanding of things. From there, following the schematic was easy. Once you get that part down, then you can work on the pieces, anode parts, cathode parts, grid parts, and all of that stuff. There are circuits I still don't understand (tone controls for instance), but hey, my first project so far is a success because of the two big things I learned in college: how to learn and where to look for answers.

Glad to hear MSU is not alone in that madness. Were you ME too? Or did you get butchered even as an EE? I could easily see that happeing too. That was some deep $hIt for a bunch of 20 year olds stuck between the tube age and the computer age. Computers had arrived, but I still couldn't envision how MUCH they would change our future even though everyone was talking about it constantly. They never had any examples, just "Things will change, a LOT, wait and see!" I hated high level math too, that killed me in a LOT of classes! Even when I TOTALLY GOT the concepts, I had trouble working out the equations that went with it. I take the blame for that. I flat didn't study hard enough in calc I and II and paid for it for the rest of my college career.

Yeah, I'm definitely a Route 2 guy. I can't just read it with the hopes of doing it someday anymore. I gotta get my hands dirty, and fast, or I start to loose interest. It's not real to me until I make and do. Doing it in real life REALLY drives it all home. I almost never forget the experience. Book reading I WILL forget given enough time.

Heck, I even TOTALLY GET how the tubes work. I sometimes forget which is Cathode and Anode, but I totally understand the heaters, electron stripping, the grid, oxide coatings, flash pot, getter, ALL that. I did nothing BUT study tubes for about 2 months a couple years ago. I was trying to help a friend of mine study the feasibility of starting a tube company based in MI. Heck, I even know what materials are IN them and HOW they are made, right down to the machinery and processing. I've got 3 words for ya, Vineland New Jersey (I think that was the name of the city, if not, it's very close to that, definitely in NJ). That's where it's ALL at, the machines, the processing knowledge, the raw materials, the know how, it's all in NJ. It used to be GE tube and lightbulb central, before the US went solid state and all the tube tech went 3rd world. You can find some of the key knowledge elsewhere like glass blowing/sealing technology in PA for instance, and stamping just about anywhere, but Vineland is still the tinder box that with a hot enough spark could reignite the American Tube Mfg industry. WHAT are we waiting for?! Good tubes NOW! Fire up those damn Sealex machines and let's show the world how to make GOOD tubes again!

Like you said, it's all the supporting circuitry and the tone stuff I'm still struggling with. I feel I can't be too far off at this point though. I'm getting there. Heck, I've come a LONG way in 1 year, and I haven't even been studying full time. It sinks in over time as you read and re-read the material and run across the same info. Just today I started to get a handle on the PT and what all it is feeding by reading Joe Popp's JCM800 building blogs. Seeing the Mercury Mags transformers with all the labeling made it really obvious! But I start getting lost with the multiple wires going to the tone controls, the PI, the negative feedback loop, and some of that stuff. I'll get there!

Rock on!

[mooreamps]

We had the same thing at Ohio State. The 300 level EE class butchered everyone in my class, including me when we got to AC analysis. I still wouldn't mind knowing how to compute values using phasors.

Wait a minute. You had difficulties with phasors in school ? It really not that hard. Maybe you should take a look at that again....

-g

[Enzo]

The B battery was more like 90 volts on that portable - it wasn't driving 6L6s or anything like them. No portable tube radios? Ever watch a World War 2 movie? Buys comunicating with the base with hand held walkie-talkies? Tubes. Batteries.

Car radios have been around a whole lot longer than transistors. Tubes, 12v battery.

Learning is learning. I know it sounds cliche, but after well over 50 years of electronics, I can still honestly say I learn something every day. Things I never quite understodd fall into place, or things i just assumed and never really thought about pop up and provide epiphany. You read something, then read it again a few years later, and then again later still. You absorb more and more of it.

One of my favorite books is "Troubleshooting analog circuits" by Bob Pease. I have it here on the shelf. I have read the whole book at least a half dozen times. And each time I gain new insights from it. I never expected to absorb it all in one try. You keep going over your material, and read the same thing by different authors, as they each communicate things differently.

You guys were in EE programs. At MSU when I attended (I was in physics) we all called the EE major "Pre-Packaging." It seemed that a high percentage dropped out of the program because it was too tough, but the prereqs for packaging were about the same as for EE. SO you could switch over without losing much credits. So a large number did just that.

[Steve Conner]

I'm a big Bob Pease fan too

Anyway, if you're the "learning by doing" type, then you will love tube amps. They really reward the trial and error approach, for two reasons: Tube circuits are simple enough to just play around with, and nobody knows how to calculate and simulate tone. If an amp has good tone, it probably got that way by trial and error, or even by accident. Marshall probably stumbled on their house sound, what they call the "open roar", by accident in 1960-something.

I started out as a kid who loved to take apart old TVs and washing machines. In my teens, I decided I wanted to do that for a living, so I went to university to study EE. The heavy math and theory that you learn complements the "learning by doing" skills, which EE programs don't teach. When you're designing a circuit and it doesn't work the way the Spice simulation says it should, you need the same skills that you used to fix the old TV you found in a dumpster, or turn an old PA amp into a deafening Mesa Boogie clone, which are probably the skills Bob Pease writes about in that book, too.

I asked one of my professors why these practical skills weren't taught on the EE course, and his reply, which I'll never forget, was: "We're training you to be managers, they get paid more than engineers"

To cut a long story short, I passed and have been making a living as an EE for a while now. Most of my work is with DSP, microprocessors and precision analog circuits, since that's where the money is, and some days it feels like I ended up as a second-rate programmer. I wouldn't ever want to design tube equipment for a living: playing with tubes is too much fun to be spoiled by specs and deadlines I have 2 weeks vacation coming up, and I plan to spend much of it restoring an old Crown tube amp for my stereo.

[martin manning]

Thanks for the memories... As an MSU ME grad I remember that circuits class... the EE's were usually paid back in the ME Thermo series though. I really liked Thermo, but the EE class didn't bother me too much. I was probably the only 5th grader at my elementary school who owned an oscilloscope.

MPM