Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by øøøøøøø » Mon Nov 03, 2008 3:03 pm

thanks for the fix.

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Re: Tube amp schematics 101, etc: NEW UPDATE: coupling capacitor summary.

Post by tribi9 » Sat Nov 08, 2008 1:40 am

øøøøøøø wrote: It has occurred to me that the original request was to talk about "reading amp schematics."  Well, maybe the best way to do that is to take an actual schematic and dissect it piece by piece.

A very common circuit small enough to make me not have to write forever but that covers the basics is the 5F1 tweed champ.  So here goes:

Image

First, after you plug into the wall, the power goes through switch A on one side and fuse B on the other.  The functions of these should be obvious.  Then 'death cap' C was on the original but is not necessary if a 3-prong cord is installed.  I included it for completeness only.

Then we get to power transformer D.  You can see that the wall power hooks to the coil on the left side.  This is called the "primary" of the transformer.  The coils on the other side put out all different voltages.  The one on the bottom is for the 6.3v filaments in all the tubes and connects to them all.  The other two are the 5v rectifier filament and the high voltage winding, and connect to the full-wave rectifier tube E .  The tube is a 5Y3.  Back up for a second to the power transformer and notice that the middle winding is center-tapped, with the center tap grounded.  This means that the winding is "zero" in the middle, making the voltage on one end positive and the voltage on the other end negative.

Okay, from the rectifier tube we have the filter caps F, G, and HF is the main high voltage, or B+ supply, AKA plate supply.  This supplies the really high voltage to the plate of the power tube.   Notice that it also goes through the primary of the output transformer BB.  This works fine, because the OT primary is connected to the power tube plate, and since the high voltage is DC, it cannot make it through to the other side of the OT, so it stays off the speaker.  A transformer allows AC to pass but not DC. 

Moving along, between the first two filter caps there is a 10k dropping resistor I.  This resistor's job is to separate the two stages of filtering and also to drop the voltage a bit for the screen supply.    Filter cap G is the screen supply's filter cap.  There is lower voltage at this point as a result of dropping resistor I

After a 22k dropping resistor J brings voltage down considerably, we hit another filter cap H... which feeds the preamp tube's plate supply.  This goes through two resistors K and L, both 100k, which are the plate load resistors for the two halves of the 12AX7.  Read earlier posts if you need to know what the plate load resistor is for.  I'm too lazy to type it all out again.  :)

Now, we've covered most of the power supply and then some, really.  Keeping that all in your mind, let's shift over to the input of the amp for a minute.  Input jacks M are obvious.  The two 68k resistors N help to separate the two inputs from one another to keep the two instruments plugged in somewhat isolated from each other.  1 meg resistor O is a reference to ground.

From the junction of the two 68k resistors, we go to the grid of 1/2 12AX7 P.  Remember that the 12AX7 is two triodes in one bottle, so we will treat each triode as if it were its own tube.  We'll call the other half Q.  The 1500 ohm resistors R and S are the cathode resistors, which bias the tube.  They go between cathode and ground.   What they do is put a slight positive voltage on the cathode.  When biasing a tube, what we are trying to do is get the grid "negative" with respect to all other elements of the tube.   There is a large positive voltage on the plate, but the plate voltage in practice is only the plate voltage minus the cathode voltage.  For example, if the plate voltage is 400v and there are 20v positive on the cathode, the 'effective' plate voltage is onle 400-20, or 380.  If the grid is at "zero," then what it really looks like is that it is at "-20"  So now in reality our plate is at 400, our cathode is at 20, and our grid is at 0.   But in practice, our plate is at 380, our cathode is at 0, and our grid is at -20.  Make sense?  A little confusing and I didn't explain it very well, but if you don't understand it, just realize the 1500 ohm resistors are cathode resistors that bias the tube.

Now notice that off the plate of triode P there is a .02 capacitor T.  This is a coupling cap.  What its job is is to keep DC from the plate off of the grid of the next stage, while allowing AC audio signal to pass through.  This is what a coupling cap does.  It joins one stage to the next.

In this case, instead of connecting directly to the grid of the next stage, it connects to volume potentiometer U.  Now the wiper of the volume pot goes to the next stage, which operates the same as the previous one, it's just amplifying the signal further before getting to its own .02 coupling cap, V.  This is coupling the preamp tube to the grid of the power tube.  220k resistor W is the bias feed resistor for the 6V6.  The cathode of the 6V6 (I have labeled the tube Z) is biased in much the same manner as the preamp tubes, with a small difference.  The cathode resistor in this case is resistor X, but the resistor is bypassed with a 25µf cathode bypass capacitor Y.  This allows audio AC signal to 'pass around' the resistor, while the DC goes through the resistor as it cannot pass through a capacitor.  This increases gain somewhat and keeps the bias stable, because the voltage across the resistor stays constant (the only thing changing is the audio, and it is going around the resistor and not through it).  This is a very common thing.  The preamp tube's cathode resistors could by bypassed in this way as well, if you chose to do so.  The designers just chose not to on the 5F1.  They were probably trying to control distortion... a tube biased with an unbypassed cathode resistor is harder to clip because the bias of the tube varies in opposition to the signal.

The output transformer BB changes the high-voltage, low current output of the 6V6 into low-voltage, high current signal that is needed to actually move the speaker DD's voice coil.  This is called impedance matching.  First it goes through jack CC, which is just the speaker output jack.  Self-explanatory. 

The only thing I haven't discussed is 22k resistor AA.  This resistor is the negative feedback loop.  I think I've explained this before, but here goes briefly.  It takes a small bit of the signal off of the output transformer and hooks it up to the cathode of triode Q, which is a point of opposite polarity (commonly called "out of phase").  This flattens frequency response but also reduces gain somewhat.  You can disconnect it at one end and get a more midrangey sound with a touch more gain.

That's it.  I didn't really go into depth much, but since this is free info you'll forgive me.  ;)

 




I just spent like 3 hours re-reading this post. :ph34r:

Would anybody have some clear nice shots of the guts of a champ? Also a pic of a power transformer without the cap would be nice if anyone's got one?

:)
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by tribi9 » Sat Nov 08, 2008 10:21 am

See when I first read this I thought the champ showed two 12AX7's on this schematic. I now think, that the schematic shows only one 12AX7. The two separate triode icons threw me off.  Am I safe to assume that all 9 pin tubes are dual triodes then? So does the tube work by allowing current to flow through the anode to heat the filament,  then releasing electrons that the grid traps,  grid opens up slightly when an audio signal passes through exiting through the cathode? Will that signal go out directly to the speaker?  What is the center tap on a dual triode then? the ground?  :-\  :)
Last edited by tribi9 on Sat Nov 08, 2008 10:25 am, edited 1 time in total.
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by øøøøøøø » Sat Nov 08, 2008 12:10 pm

tribi9 wrote: See when I first read this I thought the champ showed two 12AX7's on this schematic. I now think, that the schematic shows only one 12AX7. The two separate triode icons threw me off.  Am I safe to assume that all 9 pin tubes are dual triodes then? So does the tube work by allowing current to flow through the anode to heat the filament,  then releasing electrons that the grid traps,  grid opens up slightly when an audio signal passes through exiting through the cathode? Will that signal go out directly to the speaker?  What is the center tap on a dual triode then? the ground?   :-\   :)
Well, you're a little off, but I'll try to explain it more clearly below. 

:)

No, not all 9-pin tubes are dual triodes.  Many of them are, but many are not.  For instance, the EL84 power tube is a 9-pin power tube.  The EF86 is a 9-pin tube that's a single preamp pentode.  So the answer to that question is "no."

Here's how it works.

There is a low-voltage current that heats the filament just like in a light bulb.  This part of the circuit is separate from the audio portion.  The only job of the filament is to get the tube hot.  The filament (or heater) is not usually drawn on the schematic, since it functions similarly for all tubes.  There is a separate low-voltage winding on the power transformer... it's really not connected to anything else in the amp.  Its sole function is to heat the tube.  The cathode is NEAR the filament, and when the filament gets hot, it boils electrons off the cathode.

The anode has high voltage on it, called the B+ voltage or "plate supply," etc.  The anode is also called the "plate."  The high positive voltage on the plate/anode has a very, very strong attractive charge to the electrons that were boiled off the cathode.  Remember that electrons have a negative charge and opposites attract. 

So the filament gets the cathode hot, and the cathode emits electrons when it heats up.  These electrons are attracted by a positive charge on the plate, and this forms a 'stream' of electrons going from cathode to plate, traveling through the vacuum inside the tube.

The control grid is located physically between the cathode and the plate, so the stream of electrons has to pass through it.  As long as there is no electrical charge on the grid, it has no effect and the stream of electrons is unimpeded and goes at maximum force toward the plate.

Remember as I said that the stream of electrons travels through the vacuum because opposites attract.  Well you may also know that like charges repel.  By applying a negative charge on the grid, you can slow down the stream of electrons somewhat, as the negative charge applied to the grid repels the electrons a little bit back toward the cathode.  The charge on the plate will still be strong enough to overcome this negative charge, but the grid's charge will influence the stream of electrons somewhat nonetheless.

Now, if you have a steady-state negative charge applied to the grid AT ALL TIMES, it will slow the electrons down to a steady, less-than-maximum force.  The positive charge on the plate attracts them, and the steady-state negative charge on the tube slows them down somewhat.  They reach a balance where the electrons are still attracted to the plate, but their force is managed somewhat by the steady negative charge on the grid.  Now there is a happy medium.  By applying this steady negative force to the grid, we have biased the tube.

Now stay with me, because we're about to do some amplifying, and I think I might be able to give you a mental breakthrough where it all makes sense, if you understand all of the above.

What we have now is electrons moving from cathode-to-plate at their steady, managed pace.  Their force is less than maximum, but more than minimum.  Somewhere in the middle.

Now, keep that in mind, and let's look at a sine wave:

Image

Note that there is a "zero" in the middle (x-axis... ignore the "phase" measurements for now).  Everything above that zero is "positive" (to 1 in the graphic) and everything below is "negative" (to -1 in the graphic). When the signal comes out of your guitar pickup, it likewise has "positive" and "negative" regions, with zero in the middle.  When your guitar string swings one direction, it induces a positive charge on the pickup.  When it swings back the other way, a negative charge is induced.  So coming out of your guitar, we have positive and negative voltages alternating in succession to make electrical impulses that represent sound waves (in a sound wave, the "positive" is the compression of air, and the "negative" is the rarefaction, or de-compression, of air).

So let's go back to our tube that has a steady but managed stream of electrons going from cathode to plate, slowed down by a steady-state negative charge on the control grid between them.

In ADDITION TO that steady state negative charge (the bias voltage), let's ADD the signal from our guitar.  So in addition to the bias voltage, we now have a constantly-changing, cyclical voltage that alternates between positive and negative.

Whenever it goes positive, it counteracts the bias voltage somewhat, making it "less negative."  This allows the electrons to travel with somewhat greater force toward the plate momentarily.  Whenever the guitar's input signal swings negative, it makes the grid "even more negative" than the bias point, reducing the force of the electrons even more than the bias point. 

So now we have this steady stream of electrons that's constantly changing, and we're dealing with larger voltages and more electrons, but it's changing positive and negative in a manner that almost exactly resembles the positive-and-negative swings of the input signal. 

Make sense?

I hope so.  Sorry if I'm not a good explainer. 
Last edited by øøøøøøø on Sat Nov 08, 2008 12:16 pm, edited 1 time in total.

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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by tribi9 » Sat Nov 08, 2008 1:18 pm

øøøøøøø wrote:

There is a low-voltage current that heats the filament just like in a light bulb.  This part of the circuit is separate from the audio portion.  The only job of the filament is to get the tube hot.  The filament (or heater) is not usually drawn on the schematic, since it functions similarly for all tubes.  There is a separate low-voltage winding on the power transformer... it's really not connected to anything else in the amp.  Its sole function is to heat the tube.  The cathode is NEAR the filament, and when the filament gets hot, it boils electrons off the cathode.

Awesome! So in this case is the 6.3 V winding at the bottom of the schematic, I guess.
øøøøøøø wrote:
The anode has high voltage on it, called the B+ voltage or "plate supply," etc.  The anode is also called the "plate."  The high positive voltage on the plate/anode has a very, very strong attractive charge to the electrons that were boiled off the cathode.  Remember that electrons have a negative charge and opposites attract.

Is the voltage supplied by the middle winding on the transformer. Then is the job of the caps is to maintain the proper voltage in case of a drop or a spike?
øøøøøøø wrote: Remember as I said that the stream of electrons travels through the vacuum because opposites attract.  Well you may also know that like charges repel.  By applying a negative charge on the grid, you can slow down the stream of electrons somewhat, as the negative charge applied to the grid repels the electrons a little bit back toward the cathode.  The charge on the plate will still be strong enough to overcome this negative charge, but the grid's charge will influence the stream of electrons somewhat nonetheless.
So this would be when a resistor comes in handy? To control the negative charge that the grid gets?


On the transformer on this 5F1 schematic is there polarity only on the middle winding of the transformer?

Dude your explanations rock!!  :)
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by øøøøøøø » Sat Nov 08, 2008 1:35 pm

you're getting closer.

6.3v is indeed the heater winding in this case.  There is also a 5v winding for the rectifier tube.

In American civilian (non-military) tube nomenclature, the first number represents the filament voltage.  6L6, 6V6, 6BQ5, 6EU7, 6X4, 6SN7, etc. all need 6.3v to the filament.  5AR4, 5U4, 5Y3 etc. all need 5v.  12AX7, 12AT7, 12BZ7, etc. all need 12.6v.  This does not apply to military tubes like the 5881, 5751 or 7025, obviously.  That's a different numbering system.

BUT the 12xxx tubes can often have their filaments wired humbucking, in which case the center tap of the filament is utilized and they run off of 6.3v.  They can be wired either way, but usually they are wired humbucking.  This reduces noise and allows a single filament winding to supply the heater voltage for the power AND preamp tubes.  Gibson amps tended to use 6EU7s instead, which is similar to a 12AX7 but can only be wired conventionally as far as the heaters go.

The B+ (plate) voltage is indeed supplied by the high-voltage winding on the power transformer.  As for what the caps do, you're close, but no cigar.  A capacitor is not a good tool for voltage regulation.  Instead, the capacitors' job is to help smooth out the rectified/pulsating DC emitted by the rectifier.

Remember that wall power is AC, alternating current.  Just like your audio signal, it swings both positive and negative.  It comes out of the wall in the form of a sine wave.  The plates of your tubes need direct current.  This is NOT a sine wave and never swings negative-- it is a steady-state, non-cycling voltage. 

The full-wave rectifier tube is a device that takes the mathematical "absolute value" of all  the voltage.  In other words, it turns the sine wave on the left into the pulsating, all-positive DC on the right:

Image

If allowed to be pulsating like that, it would manifest itself as hum in your audio signal.  Each time there was a "peak," the plate would attract electrons,  and each time there was a "dip," it wouldn't.  This would contaminate the audio signal.

The "filter" capacitors' job is to 'filter out' this hum.  The brits often call them "smoothing capacitors" for obvious reasons... it takes the bumpy-ride pulsing DC and smooths it out to a steady-state DC that's similar to "clean" battery-powered DC.  They charge in the "peaks," and discharge in the "valleys," causing everything to smooth out.

On the 5F1 schematic, only the B+ winding (high voltage) is center-tapped, if that's what you mean by "polarity."  On the originals,  the filament winding was not center-tapped, and it's not on this schematic.  However, center-tapping the filament winding gives better hum rejection and so most new amps have center-tapped filaments, including most new tweed champ clones and builds people do.

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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by tribi9 » Sat Nov 08, 2008 2:35 pm

Cool, it is starting to make sense. How do you id the taps on a tranny? Multimeter?  :ph34r:

:)

On a somewhat related note..
I got a .05 cap @ 400VW that I want to replace on the ELK. All I can find is an orange drop cap .05 @ 600 V. Will that do? (actually I unscrewed the turret board and there were 5 Atlas Oil caps underneath that are bubbling up, so Im going to replace them all.


Thanks dude!!

:)
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by øøøøøøø » Sat Nov 08, 2008 3:21 pm

tribi9 wrote: Cool, it is starting to make sense. How do you id the taps on a tranny? Multimeter?  :ph34r:

:)

On a somewhat related note..
I got a .05 cap @ 400VW that I want to replace on the ELK. All I can find is an orange drop cap .05 @ 600 V. Will that do? (actually I unscrewed the turret board and there were 5 Atlas Oil caps underneath that are bubbling up, so Im going to replace them all.


Thanks dude!!

:)
I'll start with the caps:

You can ALWAYS replace a lower-voltage cap with one rated at a higher voltage.  The only concern would be if the higher-rated cap was physically too large to fit in the space occupied by the old cap.  What you want to be careful of is going the OTHER direction-- if you're replacing a cap with a high rating with one of a lower rating, make SURE that the voltage at that part in the circuit is not going to be beyond the limits of the lower-rated cap.

But in your case, you're absolutely fine.  You want same µf value, and same or higher voltage rating.

IDing the taps on a tranny is a little bit tricky.   Ideally, there is an external clue to tell you what's what-- i.e. either the tranny is already in an amp so you can just make a note of which wires are connected where, or alternatively the wires are color-coded in a manner familiar to you so you can identify them that way (or it's a new tranny that comes with a data sheet listing the color codes.

There's a semi-universal color coding scheme for power transformers.  Most transformers will adhere to this convention, but some may not.  You want to be sure, obviously.

Most transformers will go something like this:

black, white = primary coil (i.e., the side that connects to wall power)

secondaries:

red = B+ plate supply, high voltage winding.
yellow = 5v filament supply for rectifier
green = 6.3v filament supply
red/yellow = B+ plate supply center tap (will connect to ground, usually)
green/yellow = 6.3v filament supply center tap (also connects to ground in most amps)
red/white or sometimes red/blue = C- supply (also known as bias supply)

If you have a power transformer with these sorts of colors, that's probably what it is.

Not all transformers will have all of the above taps, of course.  A solid-state rectified amp may not have yellow 5v taps.  An older small Tweed fender may not have a filament center tap.  A cathode-biased amp will not need a C- supply and therefore will not have a red/blue or red/white C- tap.  You get the picture.

The only way to measure an unknown transformer is semi-dangerous: Hook up the primary to the wall and measure voltages on all the secondaries. 

For output transformers, a scheme you will often see is:

blue, brown = primary
red = primary center tap
green, black or white, black = secondary (speaker)

Again, the only way to know for sure is to measure an unknown transformer, but if you see those types of colors, it's a fairly safe bet that that's what you've got.
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by tribi9 » Sat Nov 08, 2008 3:36 pm

Awesome, I knew that about the caps I just wasn't sure if the drop shape made a difference on but I guess a cap is a cap after all.  :)

Cool, that clears it about the transformers. I think that as soon as I come up w/ some extra cash (I don't know if there is such a thing as extra tho') I might do a tweed champ....

You wouldn't happen to have a pics of a 5F1 guts anywhere on your HDD there, do u Brad?
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by øøøøøøø » Sat Nov 08, 2008 9:44 pm

none here, but I'm sure you can find some pics on the web without too much trouble.

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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by tribi9 » Sun Nov 09, 2008 3:42 pm

øøøøøøø wrote:

BUT the 12xxx tubes can often have their filaments wired humbucking, in which case the center tap of the filament is utilized and they run off of 6.3v.  They can be wired either way, but usually they are wired humbucking.  This reduces noise and allows a single filament winding to supply the heater voltage for the power AND preamp tubes.  Gibson amps tended to use 6EU7s instead, which is similar to a 12AX7 but can only be wired conventionally as far as the heaters go.

Sorry quick question about a 12AX7's that's wired humbucking...

Wiring said 12AX7 to the tranny....

Will a single 6.3  supply on either pin 4 or 5 plus wiring pin 9 to the trannys center tap do? Or do you wire to either 4 or 5 then short them, then wire your pin 9 to the tranny's center tap?   :ph34r:
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by tribi9 » Sun Nov 09, 2008 7:24 pm

:-[

I just looked inside the ELK and answered my own question. Pins 4 and 5 are shorted. On the ELK is the blue cable supplies the 6.3 v to pins 4 and 5 the white wire goes to the center tap.


:D

Image
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by øøøøøøø » Sun Nov 09, 2008 7:40 pm

Right.  Pin 9 is the center tap. 

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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by tribi9 » Sun Nov 09, 2008 8:29 pm

See, I think I get the picture as a whole but there is little gaps of info here and there that confuse me a bit.

See an example here....


øøøøøøø wrote:

Image


Then we get to power transformer D.  You can see that the wall power hooks to the coil on the left side.  This is called the "primary" of the transformer.  The coils on the other side put out all different voltages.  The one on the bottom is for the 6.3v filaments in all the tubes and connects to them all.  The other two are the 5v rectifier filament and the high voltage winding, and connect to the full-wave rectifier tube E .  The tube is a 5Y3.  Back up for a second to the power transformer and notice that the middle winding is center-tapped, with the center tap grounded.  This means that the winding is "zero" in the middle, making the voltage on one end positive and the voltage on the other end negative.

In this example if the bottom winding of the tranny is negative/ground, the middle winding is Positive/Center Tap/Negative then what is the 5 V filament heater winding on top? Shouldn't it also have a ground? Or is the ground to the rectifier supplied by the center tap of the high voltage winding? If so then is the top winding both Negative?

???

See on my Elk I got heater wires going to pins 4 and 5, and high voltage going to pins 1 and 7.  I looked up an 6CA4 (EZ81) and got some vague info pins 1 and 7. I believe are anode and cathode and 4 and 5 im going to assume are the filaments. (Freaking info is in German I think.. >)   >:(   So does that mean then that a 6CA4 is also a dual triode?

Sorry dude, Im trying here, So... I'd rather look dumb asking than to never learn. (Or I can just Google it and not look dumb...but then is much cooler having it all here in one place)   ???


;)  :) :P  8)  :)

EDIT:
Found out an EZ81 is a Dual Anode Rectifier... So I guess pins 1 and 7 are Anode and Anode. Ok but the first question about the top wiring I still don't know... :ph34r:

http://www.tubebuilder.com/images/tubed ... 1specs.pdf
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by tribi9 » Sun Nov 09, 2008 8:59 pm

Am also confused cuz see...
øøøøøøø wrote:
The anode has high voltage on it, called the B+ voltage or "plate supply," etc.  The anode is also called the "plate."  The high positive voltage on the plate/anode has a very, very strong attractive charge to the electrons that were boiled off the cathode.  Remember that electrons have a negative charge and opposites attract.
I f I Google anode an cathode

http://www.google.ca/search?client=fire ... gle+Search

The second entry there says that the flow of electrons is from Anode to Cathode. Yet you are telling me that the plate is attracting electrons boiled off the cathode meaning the flow of electrons is going from Cathode to Anode. What you explained makes sense but it just confuses me to see that on Google.

I knew I should have not dropped out of Electronics on my second day!! ARGHHHHH..........    >:(
Last edited by tribi9 on Sun Nov 09, 2008 9:29 pm, edited 1 time in total.
If i ever list my gear in this space, it means we're back in 1998.

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