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 » Sun Nov 09, 2008 9:36 pm

The EZ81 rectifier tube (and all rectifier tubes) operate a little differently than an amplifying tube.

A rectifying tube's job is to rectify AC into DC.  It does not amplify.  It has no control grid.

So some different rules apply. 

Honestly, I've never gotten that crazy with understanding what goes on inside a rectifier tube.  I guess I just never cared enough.  I just know that you apply 5V (or in the case of your ELK, 6v) to the filament, and high voltage to the anodes, and DC comes out.  :P

Better to be honest than to pretend I know what I'm talking about, right?   ;D

Since the 5Y3 and EZ81 etc. are dual diodes configured as full-wave rectifiers, it might help to consider what a full-wave rectifier is and how it works.  Maybe we can figure it out together, right now.

The graphic I posted earlier in the thread shows a full-wave rectifier with 4 diodes.  This is called a bridge rectifier, and I will talk about that and post the graphic again later in this post. 

The following graphic shows it with 2 diodes and a center tapped transformer, which is how it's done in your amp (the common amp rectifier tubes are dual diodes).

Image

Let's break down the graphic.

The resistor "R" represents the "load."  In our case, resistor R would be "the rest of the amp circuit."   :)

Looking at the transformer, the secondary (B+ voltage winding) is center tapped.  Remember that the center tap, in addition to being connected to one side of the load R, is also grounded (in fact, the common ground connection IS what connects the two, but that's not important now).  When a center tap is grounded, voltage on one half of the winding is positive, and on the other half of the winding is negative.  Make sense?  In other words, say you have 500v on the secondary.  Instead of having one end of the winding be 0 and the other end be 500, we have 0 in the middle, +250 on one end, and -250 on the other end.  This is known as a 250-0-250 secondary.  If it's still cloudy, picture a straight line.  You have a point in the middle labeled "0," and one end says "-250" and the other end says "+250."  Now picture wrapping that line around an iron core, and connecting the zero point to ground.   :P  That's what a center-tapped secondary is like.  The distance from end-to-end is still 500, we've just put a "zero reference" in the middle.  Ground IS the zero reference for voltage.

Does that make sense?  I hope it does.

Let's say that the top half (above the center tap) in the diagram is positive.  It has alternating current going through it, and it's constantly cycling back and forth.  Whenever it swings in one direction (positive), it is allowed through diode D1, and when it swings the other direction (negative), D1 blocks it.  This rectifies half of the AC, but in and of itself would only be a half-wave rectifier, like so:

Image

BUT, since the transformer is center tapped, we have the bottom, negative half of the winding, too.  Since it's inverted from the top half, the alternating current flow through D2 will be of opposite polarity of that through D1.  So whenever D1 is passing DC, D2 will be blocking it.  Whenever D1 is blocking DC, D2 will pass it.  We then combine the two together to give the full-wave rectified AC, or pulsing DC.  This feeds into the other side of our load R, completing the circuit.

Does that make sense?

Note where the cathodes are.  (the cathode, on a Diode schematic symbol, is the thick black line).  The filament IS the cathode in a rectifier tube.  Actually, the EZ81 has an indirectly heated cathode, but the 5Y3 and 5U4 do not.  Most schematics show the filament as the cathode on the schematic whether it's directly heated or not.  But this is why the filament connects, in addition to the 5v winding, to the rest of the amp's power supply.  In addition to being the filament, it's also the cathode, which is where the rectified AC (pulsing DC) is spit out.

Makes sense to me now, don't know about you!   ;D  Clear as mud, right?

The disadvantage of the dual-diode tube full-wave rectifier is that the transformer secondary must have twice the voltage you would want to see.  So our 500VAC center-tapped secondary above only gets us 250V rectified DC. 

There IS a way for us to get the full 500VDC rectified out of it, but we would need what's called a bridge rectifier, which requires four diodes, to do so.  You can look that up if you're curious.  Almost all solid-state rectifiers are bridge rectifiers, and almost all tube rectifiers are full-wave dual diode rectifiers.

Bridge rectifier:
Image


I hope that is at least partly understandable.  I'm bad at explanations.   :-[

But hey, I'm glad you asked, because it made me take the time to figure it out!   ;D
Last edited by øøøøøøø on Sun Nov 09, 2008 9:50 pm, edited 1 time in total.

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

Post by tribi9 » Sun Nov 09, 2008 10:31 pm

On this pic..
If the yellow lines represents the now rectified and an unconstant DC power that will be smoothed out by our filter caps then what does the red line represent? Why do those meet at that point?   8)  :o  :)

Image

PS that was a pretty good post there dude!

I want to build an amp sooooo bad!!

On the 12AX7 symbol what is represented? Plate, Grid and Cathode? From top to bottom.
Last edited by tribi9 on Sun Nov 09, 2008 11:00 pm, edited 1 time in total.
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by øøøøøøø » Sun Nov 09, 2008 11:01 pm

The red line represents the same thing.  Note that they're connected together!

This part is hard to understand, I know.  A full-wave tube rectifier is weird.  Like, I guess the 5VAC rides "on top of" the rectified high voltage AC?  And then maybe it gets filtered out by the filter caps?  I'm honestly not sure.  How does the rectified high voltage not burn up the filament?  Again, no clue.

Sorry, I think you've reached the point where my expertise ends.  ;D  I'm sure I'll learn some day.  It's probably really basic.

What you're seeing is the breaking-down of knowledge as a result of being self-taught though practical learning, as opposed to someone who has actually had formal electronics training.

BUT, it has never stopped me from repairing, overhauling, and building amplifiers and getting them to sound awesome!  ;D

Which brings my to my next point... you don't need to know every single thing about how an amp works to be able to perform most repair jobs, or even to build an amp from a pre-existing design.  You really only need to understand this stuff if you're designing an amp from scratch, or perhaps if you're doing a repair and something gets confusing.

Don't get me wrong, the more knowledge the better, but the only knowledge required to get started is the safety knowledge.  The rest you will learn as you go, much like I have/am.

I learned SO much building that bandmaster clone, for instance.  :)

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

Post by øøøøøøø » Sun Nov 09, 2008 11:03 pm

tribi9 wrote:
On the 12AX7 symbol what is represented? Plate, Grid and Cathode? From top to bottom.
That is correct!

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

Post by tribi9 » Sun Nov 09, 2008 11:12 pm

øøøøøøø wrote:

Sorry, I think you've reached the point where my expertise ends.   ;D  I'm sure I'll learn some day.  It's probably really basic.


Hahahaha.... Awesome. I think I've crammed enough for the weekend. Thanks for all your help. This is interesting yet mind numbing stuff.

What was really cool was reading and understanding your last post. It felt good.  8)
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by øøøøøøø » Sun Nov 09, 2008 11:15 pm

You know, nothing will make you understand it like actually putting an amp together for yourself.  There will be moments when you go "now WHY would this be connected to THAT?" or maybe "Is this RIGHT?  Does this REALLY go here?"  Then you pore over the schematic and convince yourself it's right, and then you hook it up and go "hm... well, it works, so I guess it's right."

Then sometime down the road you'll be thinking about it (or read something in some book somewhere) and go "AHA!  now I get it!"

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

Post by tribi9 » Sun Nov 09, 2008 11:31 pm

I agree, nothing beats hands on training. I'd love to maybe in the future become an amp tech!   :ph34r:
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Re: Tube amp schematics 101, etc: NEW UPDATE: coupling capacitor summary.

Post by mezcalhead » Mon Nov 10, 2008 4:54 am

tribi9 wrote: Would anybody have some clear nice shots of the guts of a champ?
The first page of this thread might be helpful .. the circuit on that one is 5F1 (mostly, thanks Brad).
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Re: Tube amp schematics 101, etc: NEW UPDATE: coupling capacitor summary.

Post by tribi9 » Mon Nov 10, 2008 8:41 am

mezcalhead wrote:
The first page of this thread might be helpful .. the circuit on that one is 5F1 (mostly, thanks Brad).
Perfect, that's exactly what I was looking for. I knew I'd seen those gut pics somewhere.   :)
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by tribi9 » Tue Nov 11, 2008 1:28 am

Found this really neat schematic and layout for the champ. Makes it a easier to understand what's going on in there.

http://www.dockeryamps.com/web/Docs/5f1 ... ematic.pdf


http://www.dockeryamps.com/web/Docs/5f1 ... layout.pdf


:)
Last edited by tribi9 on Tue Nov 11, 2008 1:41 am, edited 1 time in total.
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by tribi9 » Tue Nov 11, 2008 3:35 pm

And one more with a labeled PT for us rookies, but of a Blackface champ...   :P




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

Post by atomicmassunit » Mon Mar 16, 2009 4:53 pm

Hey guys, new to the board.  I work as an amp tech, so if you need any help with any of this stuff, let me know.

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

Post by rugby style » Tue Mar 31, 2009 12:12 pm

Another question for the veterans - The schematic for the amp I'm working on specifically lists 1W and 2W resistors near the filter caps, but no wattage is specified in other areas.  Can I assume 1/2 watt where not specified?

On an electronics component site I found the following, relating to replacing older carbon resistors with newer metal oxide versions:
Wattage: Available at 1 Watt and 2 Watt. Replace your 1/2W carbon comp with 1W metal oxide..
So I guess it's not really an exact science, and going larger when in doubt is safe?
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by øøøøøøø » Tue Mar 31, 2009 1:48 pm

For most resistors in most amps NOT in the power supply area or B+ string, 1/2w is fine.

Look again at your schematic, sometimes in the corner or somewhere there will be a little note saying "all resistors 1/2w unless otherwise specified" or some such indicator.

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

Post by rugby style » Tue Mar 31, 2009 4:25 pm

My schematic doesn't, but I found another revision online, which does indeed spec 1/2w unless otherwise noted.  Thanks for your help!

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