I've decided to get into troubleshooting old radios a little deeper than recapping and tube replacement and have read a couple of books on the subject. I'm stumped at what B+ is and where it's measured. I assume it's DC voltage and think one side is measured at the rectifier tube plate pin but where does the other voltmeter (-) lead attach? Is B- just the chassis ground or electrical ground? Any good books with PICTURES available?

Thanks

Well, recapping is a good idea, as capacitors cause a huge amount of trouble. It is wiser, in my opinion, to first replace all the capacitors before replacing tubes, that is, unless one tube is obviously burned out.

At any rate, probably 90% of radios take the B+ off of the cathode of the rectifier. To a beginner, this seems strange, as one thinks of the plates as being positive. Here's the reason why, though. When you heat the cathode of a tube, it emits electrons. They are knocked loose. The reason the plate is positive is because when a current source is hooked to the tube for amplification and various other purposes, you want a positive voltage source at the plates to collect the negative electrons. The negative side of the source will be connected to the cathode to supply more electrons. When you use a tube as a rectifier, however, all you are doing is using its one way characteristics to act as a valve. Every time the AC current in your home flows in the direction stated above like the direct current source, it will flow through the tube, and every time it flows in the opposite direction, nothing will flow. The reason why the cathode of the rectifier supplies a positive supply is because the negative feed that would otherwise be connected to the cathode is connected to the other side of the amplifier (or other load), and must flow all the way through the amplifier to get to the cathode. The cathode would then be positive with respect to the current source connected to the other side of this load. I don't think I described this as well as I could have. I'm kind of tired right now. Hopefully someone can correct any places where I am to vague or confusing. In most cases, you will connect the positive lead of your meter to the cathode of the rectifier tube, and the negative to the center tap of the high voltage coil on the transformer in AC sets, and to the side of the line cord not connected to the rectifier tube in AC-DC sets. There are cases, though, where the radio either has filter chokes/resistors in the negative supply before B- is established, thereby making the center tap or line cord wire being more negative than B- as listed on the circuit diagram, or resistors are placed between established B- and the center tap/return for tube biasing purposes. When these cases arise, the negative lead of your multimeter is placed where it says zero volts or B- in the diagram. To place the negative lead in a set like this at a terminal marked at perhaps -100 volts, would make all of your B+ voltages 100 volts higer than listed in the diagram. At any rate, B+ is the positive side of the direct current supply, and B- is typically the negative side of the direct current supply, though, as stated above, this can be an established neutral within the set, which is often connected to the chassis of AC sets, with other voltages falling both higher and lower than this neutral.

Thomas

:I've decided to get into troubleshooting old radios a little deeper than recapping and tube replacement and have read a couple of books on the subject. I'm stumped at what B+ is and where it's measured. I assume it's DC voltage and think one side is measured at the rectifier tube plate pin but where does the other voltmeter (-) lead attach? Is B- just the chassis ground or electrical ground? Any good books with PICTURES available?
:
:Thanks

Thomas,

Thanks for a great explanation! I'll get started right away. Have 5 radios I can check this procedure out on.

Regards, Stephen

Stephen

In addition to what Thomas wrote, you will need to know this if you ever come across a pre-1938 battery-powered set. It is also useful when working on other sets.

There are three voltage sources in tube electronics, called 'A,' 'B' and 'C.'

'A' voltage is the high-amperage source of power to the heaters/filaments. It is either AC or DC.

'B' voltage is the moderate-amperage (usually 15 to 200 ma) *positive* DC voltage source that powers the plates and screen grids. ("B-minus" refers to the "floating ground" line in AC/DC sets.)

'C' voltage is the low-amperage *negative* DC voltage source that provides bias to the control grids. After World War 2, most sets eliminated 'C' voltage sources by adding cathode bias resistors. (AVC performs a similar function to 'C' voltage. However, true 'C' voltage comes directly from the power supply.)

Most pre-1938 battery sets are powered by separate 'A,' 'B' and 'C' batteries. When you read schematics for these radios, you will see references to these three sources.

:I've decided to get into troubleshooting old radios a little deeper than recapping and tube replacement and have read a couple of books on the subject. I'm stumped at what B+ is and where it's measured. I assume it's DC voltage and think one side is measured at the rectifier tube plate pin but where does the other voltmeter (-) lead attach? Is B- just the chassis ground or electrical ground? Any good books with PICTURES available?
:
:Thanks

And do be careful. We have become so accustomed to working with low-voltage solid-state stuff these days that it is easy to get casual when taking voltage measurements, but this old tube stuff can run several hundred volts B+ and believe me, you don't want to be the ground path for that!

Hmmmmmmmmmmmmmmmm.......I think my description of how and why a rectifier is hooked up was done rather poorly. One of the reasons why tubes can be rather confusing is because we have all been taught that the positive side of a battery emits the power. That makes you think that the power flows from positive to negative. Truth is, the negative side emits the electrons. You could say that it is the positive side, but since scientists have decided that electrons are negatively charged (for reasons I am unsure of), they label it the negative side. You could simply think of the battery in a slightly different way. The negative side of the battery is losing electrons, and the positive side is gaining electrons. Perhaps that will clarify things a little for you. An electron tube will only allow electrons to flow from the hot cathode to the cold anode (plate). The AC current in your home has negative current (electrons) flowing one way and then another 60 times a second. If the rectifier is placed across this line, current will flow through it each time the AC current is arranged to supply the cathode with negative electrons. (Do not wire a rectifier tube directly across the line, or you will blow it up quite explosively.) If you drop the idea that the rectifier tube, as wired in most radios, supplies the radio with positive energy, you will not confuse yourself as much. The part of the radio people label as positive is actually where the current is exiting. Instead, think of it as a valve that will only allow electrons to flow from cathode to plate. It could be wired so that its cathode was tied to one side of the AC line, and its plate was tied to the cathodes of all the other tubes, and the plates of all the other tubes would be tied to the other side of the line (theoretically...usually there are resistors and other stuff....I'm simplifying things). In this manner, the tube would only allow negative current to flow in through it. The cathode would supply the plate with electrons, and so the plate would have a negative supply for the radio. The radio, tied to the other side of the line, would be the plate's positive source, but please do not think of the positive side as the source, as it really is not. It is simply where the electrons exit. If the tube was wired in conventional manner, where the amplifier, etc., was connected to its cathode, and its plate was then tied to the line cord, then it would serve to only let negative energy out (through the cathode, to the plate). If you always think of current flow with tubes as cathode to plate, and consider what most people call B+....the positive source, as actually the exit for the electrons, tubes will be much less confusing to you, as will solid state diodes, etc. Now you may ask: "So, if all tubes only let electrons flow one way, then why have a rectifier tube?" Well, if you were to hook the AC line directly to the amplifier tubes, current would flow through them only one way, but would stop flowing 60 times a second, which would introduce horrible noise into the audio and radio signals. To solve this, a separate rectifier is used, and filtering equipment is put on this rectifier.

Hope all of this helps. I know it's long.

Thomas

: And do be careful. We have become so accustomed to working with low-voltage solid-state stuff these days that it is easy to get casual when taking voltage measurements, but this old tube stuff can run several hundred volts B+ and believe me, you don't want to be the ground path for that!

It is my understanding that until the edison effect was discovered, it was assumed that electrons (electric current flow) went from positive to negative. But the edison effect, (and later, vacuum tubes based on it) could not be explained until scientists decided they had it wrong- and that current must flow from negative to positive.

.........which makes you wonder.....how do they know that an electron is negative? Does it suck things towards it? It obviously doesn't suck other electrons towards it, as it repells electrons (like charges), and why is a proton positive? Does it emit something? I think they should have renamed the side of the battery that supplies the electrons the positive side. Oh well.

: It is my understanding that until the edison effect was discovered, it was assumed that electrons (electric current flow) went from positive to negative. But the edison effect, (and later, vacuum tubes based on it) could not be explained until scientists decided they had it wrong- and that current must flow from negative to positive.

Well this is where the confusion comes. Way back when they first began experimenting with electricity they knew there was a current flow but they didn't know which way it was flowing. Somebody (seems like I remember it was Franklin) arbitrarily named the two polarities positive and negative. He had a 50-50 chance of being right- but he guess wrong- which has lead to confusion ever since. Even today, you find people who insist current comes from the positive side of a battery.

:.........which makes you wonder.....how do they know that an electron is negative? Does it suck things towards it? It obviously doesn't suck other electrons towards it, as it repells electrons (like charges), and why is a proton positive? Does it emit something? I think they should have renamed the side of the battery that supplies the electrons the positive side. Oh well.
:

:I've decided to get into troubleshooting old radios a little deeper than recapping and tube replacement and have read a couple of books on the subject. I'm stumped at what B+ is and where it's measured. I assume it's DC voltage and think one side is measured at the rectifier tube plate pin but where does the other voltmeter (-) lead attach? Is B- just the chassis ground or electrical ground? Any good books with PICTURES available?
:
Stephen: I think you need to procure a good book on basic radio priniciples such as ELEMENTS OF RADIO by Marcus - (Prentice Hall). This is rather an old reference but is very well illustrated and is tilted towards vacuum tubes.
I noticed there was some discussion of plus and minus with some direction of current flow confusion.
Being an engineering professor I can attest to the fact that 99.99% of engineers use "conventional" current flow which is plus to minus. "Conventional" actually means that it was the consensus of the majority of the group by consensus. Even the schematic symbols for solid state devices (arrows) indicate the flow direction of conventional current and are accepted by everyone. However, "all the fuss about plus" does not really matter in that the circuit quantities and math yields the exact same answers either way!! I can remember that a few EE textbooks were available in the "conventional" or "electron current" versions and the only differences were that the direction arrows were pointing opposite ways in the circuits illustrations. We could diverse and ask what is the real differences between a scientist, engineer, and technician but that would encroach on the occupational handbook. Keep in mind that education is "the changing of cocksure ignorance to thoughtful uncertainty" - good luck in your studies.
:Thanks

Tom, read my second posting.

T.D.

::I've decided to get into troubleshooting old radios a little deeper than recapping and tube replacement and have read a couple of books on the subject. I'm stumped at what B+ is and where it's measured. I assume it's DC voltage and think one side is measured at the rectifier tube plate pin but where does the other voltmeter (-) lead attach? Is B- just the chassis ground or electrical ground? Any good books with PICTURES available?
::
:Stephen: I think you need to procure a good book on basic radio priniciples such as ELEMENTS OF RADIO by Marcus - (Prentice Hall). This is rather an old reference but is very well illustrated and is tilted towards vacuum tubes.
: I noticed there was some discussion of plus and minus with some direction of current flow confusion.
:Being an engineering professor I can attest to the fact that 99.99% of engineers use "conventional" current flow which is plus to minus. "Conventional" actually means that it was the consensus of the majority of the group by consensus. Even the schematic symbols for solid state devices (arrows) indicate the flow direction of conventional current and are accepted by everyone. However, "all the fuss about plus" does not really matter in that the circuit quantities and math yields the exact same answers either way!! I can remember that a few EE textbooks were available in the "conventional" or "electron current" versions and the only differences were that the direction arrows were pointing opposite ways in the circuits illustrations. We could diverse and ask what is the real differences between a scientist, engineer, and technician but that would encroach on the occupational handbook. Keep in mind that education is "the changing of cocksure ignorance to thoughtful uncertainty" - good luck in your studies.
::Thanks
:
:

Tom & Nat,

Wow, this has been quite a discussion! I have purchased a couple of books: Old Time Radios! Restoration and Repair by Joseph J. Carr, and Antique Radio Restoration Guide by David Johnson. Now the question becomes "where is B- found?" These books are replete with circuit diagrams and techniques but again no illustrations of WHERE TO HOOK UP TEST LEADS. Now I've learnded that there are 'chassis grounds', 'earth grounds', and 'counterpoise grounds'. Most of my radios are of the AC/DC type. I've determined that a B- bus runs through the circuit and that B+ can be found in several places, but that it's slightly different depending on where you measure it (e.g. at the rectifier cathode or at another stage in the circuit). I can generally find the locations for B+ and B- on the circuit diagram, but finding them on the radio is another thing...especially the B- bus. I've gotten to where I'm looking for the common (floating??)ground around the filter cap cans. I assume that (again in AC/DC radios) most filter cap cans are electrically insulated from the chassis but have lugs which are connected to the "counterpoise" ground. Does this sound right? I'm especially concerned about the fact that these radios may have high voltage connected to the chassis via a capacitor and can be dangerous (Nat). I've been replacing these capacitors with the new Y2 isolation types which fail open. I assume that is a good thing to do. Again I appreciate the tremendous amount of expertise out there and, even more, the willingnes to share. THANKS

Stephen

Stephen, a pretty sure fire way to identify "B-" is that it will be the negative side of the main filter capacitors connected to the rectifier. In transformer sets B- is almost always connected directly to the chassis so you can hook your meter neg lead to any chassis connection. On "AC/DC" sets it varies- many early sets just connected one side of the AC-line right to the chassis (making it B-) despite the potential hazard of this. So you should always check to make sure the "low"(grounded) side of the AC line goes to the B- point- rather it is the chassis or a floating ground. You can check this by measuring the AC voltage from the chassis to a external ground- like a water pipe. You should get almost no voltage. If you do- reverse the plug. I would then replace the plug with a polarized type that can't be plugged in wrong. Most newer electronics come with polarized plugs now but they weren't so safety conscious in the old days. If the AC/DC set uses a floating ground- the negative side of the filter cap is still the place to find B-.

- Nat

:Tom & Nat,
:
:Wow, this has been quite a discussion! I have purchased a couple of books: Old Time Radios! Restoration and Repair by Joseph J. Carr, and Antique Radio Restoration Guide by David Johnson. Now the question becomes "where is B- found?" These books are replete with circuit diagrams and techniques but again no illustrations of WHERE TO HOOK UP TEST LEADS. Now I've learnded that there are 'chassis grounds', 'earth grounds', and 'counterpoise grounds'. Most of my radios are of the AC/DC type. I've determined that a B- bus runs through the circuit and that B+ can be found in several places, but that it's slightly different depending on where you measure it (e.g. at the rectifier cathode or at another stage in the circuit). I can generally find the locations for B+ and B- on the circuit diagram, but finding them on the radio is another thing...especially the B- bus. I've gotten to where I'm looking for the common (floating??)ground around the filter cap cans. I assume that (again in AC/DC radios) most filter cap cans are electrically insulated from the chassis but have lugs which are connected to the "counterpoise" ground. Does this sound right? I'm especially concerned about the fact that these radios may have high voltage connected to the chassis via a capacitor and can be dangerous (Nat). I've been replacing these capacitors with the new Y2 isolation types which fail open. I assume that is a good thing to do. Again I appreciate the tremendous amount of expertise out there and, even more, the willingnes to share. THANKS
:
:Stephen