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

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Re: Reading tube amp schematics 101: by request UPDATE: new info- "how tubes wo

Post by øøøøøøø » Wed Aug 06, 2008 7:39 pm

Some common tube types and what they do

Just a brief post on the OSG øøøøøøø amp-DIY blog today.  ;)

There are many different tubes used in guitar amps.   There are many, many, many more that are not commonly used in guitar amps, and are not useful for much of anything anymore.  Some of these could be used in new guitar amp designs, if you were willing to design around them.  But that's sort of an "advanced level" proposition.  For now let's focus on the types you WILL see if you get into amp DIY.

First, know that tube names are very confusing!  Many tube types will have identical or compatible tubes with very different names.  For example, the 12AX7, the 7025, and the ECC83 are the same tube.  The EL34 and the 6CA7 are compatible (6CA7 is the same, but is slightly more rugged... usually they are used interchangeably as the vital specs are the same).  The 5881 and the 6L6WGB are equivalent, as well.

On the other hand, many tube types with VERY SIMILAR names are different enough that they're NOT compatible!  Or to put it another way, they may be "forwards compatible" but not "backwards compatible."

For example, let's look at 6L6 types.  While many people just use the term "6L6" too refer to any 6L6 type,  they are NOT all interchangeable!

Strictly speaking, a 6L6 is a metal-bodied tube which can only tolerate low-ish plate and screen voltages.  a 6L6G is the glass-envelope version of that tube.  DO NOT try and use either of these tubes in an amp that calls for 6L6GC tubes!  They WILL blow up as they cannot handle the plate and screen voltages of, say, a Super Reverb.  Off the top of my head, in increasing order of ruggedness, there are the 6L6 and 6L6G, the 6L6GB, the 6L6WGB/5881, 6L6GC, 7581, and 7581A.  All of these are 6L6 types, and all of them are different!  You could use any one to the right of whichever one your amp calls for.  For example, a 6L6GC in a tweed bandmaster is totally okay!  In some cases (checking how your tube sockets are wired) you could even substitute other, still more-rugged 6L6-family tubes like the 7027 and 7027A.  However, never use one to the left of what your amp calls for!  If you put a 5881 in a Super Reverb, for instance, it might work... for awhile!  If it doesn't go down immediately, tube life will possibly be very short.  A 6L6G would probably only last minutes.   In other cases you could get away with it.  It depends on the plate and screen voltages of the amp and the ruggedness of the individual tubes. 

Further compounding that issue is the fact that tube-rebranding companies like one whose initials are "GT" have muddied the waters further by adding their OWN suffixes that have nothing to do with the tube's operating parameters, but rather with their tube's country of manufacture or something else.  For example, the GT company sells a tube called the "6L6GE."  There is no such tube.  Their "6L6GE" is a 6L6GC tube made "in the style of" the old General Electric American-made tubes.

Confusing indeed!

But wait... there's more!  Most all new tubes now are rebranded tubes imported from Russia, Eastern Europe, or China.  In recent years there have been some "American" style designs and "European" style designs, but before that (and still to this day, with some tubes), what you were buying under one name was actually another tube with different specs that was produced for the Russian or Chinese military, but was deemed "similar" enough to the American/Euro tube type to be re-labeled and re-pinned with that name and pinout, and sold under that type in the Western world.  For example, as I understand it, the Sovtek "5881" is not a 5881 at all.  It is a Russian military tube with some other name in Russia that has operating parameters somewhat similar (but not TOO similar, actually) to the American 5881.  They had a bunch of those in surplus.  So what they did was re-pin them with the 5881's pinout and sell them as "5881" tubes domestically. 

So you can see how you really have to do some digging to know what you're REALLY getting when you are dealing with tubes.  That's one reason (of many, the biggest being superior sound and performance) why I like to stick to old-production US and European made tubes, when I can afford or find them.   

Now let's look at a few compatible types, off the top of my head:

12AX7, ECC83, CV4004, and 7025, ECC803, 12AX7A, which are "premium" 12AX7s selected for low noise.  The 7025 has spiral-wound filaments for low noise.  There are a few other "premium" 12AX7 types that I can't think about off the top of my head.

12AT7, ECC81, CV4024

12AY7, 6072

5881, 6L6WGB

5AR4, GZ34

5Y3G, 6106

6V6GTY, 5992

EF86, 6267, EF806

EL84, 6BQ5, 7189A ('beefier' EL84 type)

6CG7, 6FQ7

12DW7, 7427

EL34, 6CA7


There are many many more, but that's just a few off the top of the head.  That ought to be helpful!

What's weird is that there are so many different naming styles.  There is the Euro "letter letter numbers" style (EL84, ECC83, GZ34 etc), the American "number, letters, number, optional letter suffix" style, (5AR4, 12AX7, 6L6GC etc) and the US military and old-school style "numbers only" (5881, 7427, 5751, 83, etc).   

I don't know what they all mean or what the scheme for numbering them is, except for one thing:  On the American "consumer-market" number-letters-number-optional suffix style, the first number is the filament voltage.

For example, a 5AR4, 5Y3, 5U4GB all have a 5 volt filament.  6L6GC, 6V6, 6BQ5, and 6EU7 all have 6.3V filaments.  The 12AX7, 12AT7, etc all have 12.6v filaments (but can be wired for 6.3v operation, and they are in most guitar amps).

Well, I guess I didn't get around to "what they do," but maybe that will be for another day.  I'm tired of typing.  :)
Last edited by øøøøøøø on Wed Aug 06, 2008 7:45 pm, edited 1 time in total.

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Re: Reading tube amp schematics 101: by request UPDATE: new info- "how tubes wo

Post by øøøøøøø » Wed Aug 13, 2008 10:27 am

New post to the AMP DIY blog!   ;D

Am I being an annoying pedant here?  If so just tell me.   I'm not trying to be all 'whatever,' I just like typing about working on amps.  It's what I'm interested in this month... next month will be something different, and then about a year from now I'll get into it again.  :)

Today I'm going to type about different types/brands of coupling caps and their advantages/disadvantages.  Some people swear up and down that "a cap is a cap."  Those people are entitled to their opinions, but real-world experience has given me a different opinion.

I don't really know how to organize this,  so here they are "in no particular order."  This will be disorganized as hell.  I reserve the right to organize by brand, type, age, price-range, at will.   Maybe you can make some sense out of it.  ;)

Paper-in-Wax  Example: the vaunted "yellow astron." Edit: possible correction!  See below!
Image

These capacitors are the ones used in almost all Tweed-era Fender amps, and some early brownface/blond.  You will sometimes see the occasional one all the way up into the early blackface era, but for the most part Fender stopped using them around late '61/early '62.  They are made by taking two pieces of foil, putting paper on either side like a "foil sandwich" with paper on the outsides and one piece in the middle as a dielectric.  This "paper-foil double-decker sandwich" is rolled up like a cigarette, and impregnated with wax to 'seal' everything in place.  Then, as in the case of the Astrons, sometimes a plastic sleeve covers the whole shebang.  The most common color for narrow-panel Fenders is yellow with green writing, but they also came in red.

With all paper-in-wax caps, they have advantages and one major disadvantage. 

Advantages: In the case of the Astrons, anyway, they have the definitive sound for Tweed fenders.  Very little else will get that sound exactly.  They are worth the trouble in that sense.
Disadvantages: If they get too hot, the wax can deform and cause internal shorts and leakage.  They have a very high failure rate, with many old ones going leaky.  It's not unheard of to find an old tweed Fender where every single one of these is leaking DC.

Update:  Someone told me they dissected several of the yellow Astrons and they are film-in-foil, NOT paper-in-wax.   Who knew?  I haven't seen firsthand either way.

Mallory "blue hot dog" poly
Image

This is the famous 'blackface' cap.  Also in later brownface ('62 or so and later).  They're a poly cap (polypropylene, I think?) which means that instead of a paper dielectric, they have a sheet of thin plastic.  The whole thing is then sealed up in a blue hot-dog looking tube (the ends are usually more rounded than the one in the pic). 

They sound pretty neutral to my ears.  Maybe my ears are just attuned to the "blackface" sound as the standard, who knows.  In my opinion they're fine but nothing special.  Some people rave about them.  Much more durable than the paper/wax type, I'll give them that.

The british "mustard" cap

Image

These are the caps used in the vintage Marshalls, vintage Neve recording consoles, the WEM copicat, and tons tons tons of other vintage British gear.  Also found their way into Traynor amps sometimes.  These are great sounding caps.  So vaunted that a company cloned their innards (not cosmetics)...

"Sozo" caps

Image

these are new boutique copies of the old "mustard" caps.  They have a "standard" line that's expensive, and a "premium" line that's ungodly-expensive.  People rave about them.  I've never tried one.  Expect to see more clone-caps like this in the future... like I just heard of a new one called...

Jupiter Condenser vintage tone caps

Image

The Sozos are a clone of the Mustard cap, and these are the clone of the old Astron.  they're not paper-in-wax, but they supposedly went through several prototypes to get the sound just right.  I haven't tried them, but I might pop them in to the bandmaster build, if I can ever afford to build it.  Jupiter also makes a bona-fide wax cap that's expensive and not recommended in amps for the same temperature stability reasons listed under Astron above.

The classic Sprague "Orange Drop."
Image

People often think that an orange drop is an orange drop, but this is not the case.  There are actually 3 at least six different models; the 715P (lowest cost polypropylene), the  716P (higher quality polypropylene) and the PS (polyester).  Additionally, they have been made for a VERY long time (dating back to the 1950s at least) in much the same cosmetic form,  but the older caps are different, too.  There are also other models like the elusive 418P (Torres sells them from his website, many people think these sound the best), the 225P (similar to the 418P, different who knows how) and 192P (axial lead).  Sprague is now owned by Vishay and many of the caps say "SBE" on them.  If you think "an orange drop is an orange drop," then you're wrong.   :)

Of the current ones, most people prefer either the PS or the 716P in guitar amps, but the most common one you will see is the 715P.  Many people prefer polyester to polypropylene in guitar amps generally speaking. 

Orange drop 715s are often seen in amps as replacements when the originals failed.  Many people (self included) tend to think of them as a little 'boring' sounding.  The 716P and PS series have nice fidelity, big bottom and big top.  Not my choice usually, but whatever.  I am biased against them from all the times I've opened a cool vintage amp only to see that someone had replaced every single coupling cap with orange drops.  These are by far the most common "quality" poly cap.

Other "mustard" clones
Image   

These include Mojo Dijons and TAD "mustard" caps.  The Mojo Dijons get good reports.  Never tried them. 

Oil-filled caps
Image

These include "Paper-in-oil" caps aka "foil-in-oil."  the paper dielectric is impregnated with oil (usually mineral oil).  I'm actually not totally sure about all the details of construction to tell the truth.  The most popular historical one is the Sprague Vitamin Q.  There are some obscenely expensive high-end modern foil-in-oil caps made.  Jensen makes some and Allesandro high-end makes some, too.  Both are very expensive, sometimes 20 bucks per cap or more.   On the other end, you can often get Russian military-surplus paper-in-oil caps for very inexpensive, and they're very good.

Mallory 150 series

Image

These are polyester caps.  Lots of people like them.  I like them okay.  They are made without foil layers... instead, conductive paint is used to 'paint' foil right onto the polyester dielectric.  Pretty cool.  For a long time these were in most peoples' opinion the best replacement for the old Astrons.

Mojo Vitamin T

Image

Sort of a hybrid design.  The name suggests that they are like the Vitamin Q but this is not exactly true.  I heard that they're similar to a Dijon, but impregnated with mineral oil.  I have used them and think they sound really good for the price.  As I understand it is not a true paper-in-oil, but not a polyester either.  Pretty interesting. 

Ceramic disc capacitor
Image

The cheapest kind of cap.  Most people don't think they do audio any favors.  They can actually distort (hysteresis) at high voltages.  They are grainy sounding.  For certain things, they might be just the ticket.  Not usually though.

Silver Mica caps

Image

These are usually only available in very small values, like 1000pf or less.  They are very nice in audio circuits and sound much smoother and more beautiful than small-value ceramics.

"Tropical Fish" capacitors
Image
These were the cheapest type of poly cap available in the 70s.  They found their way into lots of cheap vintage audio gear... Colorsound wah wahs, etc.  They are kind of crappy sounding which is great for certain things.  They sound great in the colorsound wah wah.  I don't usually prefer them in amps, however.   May have been made by a few different manufacturers, the color stripes are there to indicate value.

the infamous "Chocolate Drop."
Image

these are the most common caps in silverface Fenders, at least until the late 70s when they started using the "smurf turds," which are dark blue polypropylene caps.  The chocolate drops (aka "rabbit turds") are polypropylene as well, and are generally low quality.  They are dark red in color, sort-of brownish with age, sometimes, hence the name.  I've heard them described as "numb" sounding, and I can say that fits my experience as well.  It's hard to describe it.  I never cringe if I see that these have been replaced.  It's not unheard of to see one or two of them amongst the blue hot dog Mallories in later blackface amps.

-------------------------------
These are some of the most common caps you will see used in guitar amps over the years.  In some cases, they are just representative examples of type, and you can see other brands from time to time.  For example, Xicon makes a reddish-brown polypropylene cap that's decent, sort of like a small orange drop.  There are also millions of other old poly caps like Sprague "bumblebees" and many old paper-in-wax caps of different makes.   

The deal with caps is, if all the ones in the amp are changed, you will notice a difference.  Changing one cap somewhere in the amp is not likely to matter much.  The differences tend to be cumulative.  So one mallory 150 in an amp full of Astrons will still sound like an amp full of Astrons, but if you replaced all the Astrons with mallories it would sound different.

Hope this info is interesting to someone besides me!
Last edited by øøøøøøø on Mon Sep 29, 2008 6:47 am, edited 1 time in total.

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

Post by quarterpound » Wed Aug 13, 2008 8:33 pm

Thanks! ...annoying pedant.  :ph34r:

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

Post by fuzzking » Wed Aug 13, 2008 11:30 pm

thanks again, brad!

i'll need to print this out and read all through it unhurriedly. covers all the stuff i wants to know!
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Re: Tube amp schematics 101, etc: NEW UPDATE: coupling capacitor summary.

Post by øøøøøøø » Fri Aug 15, 2008 7:18 am

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.  ;)

 

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Re: Tube amp schematics 101, etc: NEWER UPDATE: 5F1 champ schematic walkthrough!

Post by quarterpound » Fri Aug 15, 2008 7:48 am

You're my absolute hero, man. I went to the shops to price some components and buy some enclosures today. Shopping for components here is a bit of a pain in the ass but the prices are unbelievable. They only want like starting from $12 USD for aluminum amplifier chassis, so I may well be building one of these next week. little aluminum stompbox enclosures from $3 USD. Not crappy stuff, either, perfectly durable.

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

Post by luau » Fri Aug 15, 2008 11:51 am

Great write up. Thanks!
øøøøøøø wrote:The only thing I haven't discussed is 22k resistor AA.
Replacing AA with a pot would give us a presence control, correct? Or is it not quite that simple?
Bigger in sum than parts.

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

Post by øøøøøøø » Fri Aug 15, 2008 11:55 am

luau wrote: Great write up. Thanks!
øøøøøøø wrote:The only thing I haven't discussed is 22k resistor AA.
Replacing AA with a pot would give us a presence control, correct? Or is it not quite that simple?
Not quite.  :)

Replacing AA with a pot would give you a global negative feedback control. 

For a presence control, you would leave resistor AA and add a pot and a cap to ground... the same type of configuration as your guitar's tone control.  That would shunt off some highs from the negative feedback loop, meaning less treble gets cancelled out in the feedback loop, meaning an increase in overall treble response on the output of the amp.  :)

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

Post by luau » Fri Aug 15, 2008 12:08 pm

øøøøøøø wrote:Not quite.  :)
Derp. Got it. Thanks. :)
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Re: Tube amp schematics 101, etc: NEWER UPDATE: 5F1 champ schematic walkthrough!

Post by øøøøøøø » Fri Aug 15, 2008 12:14 pm

No problem!  a global NFB control is a useful thing too, though.  It is like a volume control for the NFB loop, whereas a presence control is like a tone control for the NFB.

Many amp manufacturers have started putting a NFB control on their amp.  They call it all sorts of things, like a "raw" control, a "soul" control, etc etc.

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Re: Tube amp schematics 101, etc: NEWER UPDATE: 5F1 champ schematic walkthrough!

Post by Maggieo » Fri Aug 15, 2008 12:16 pm

So THAT's what a "soul" control is!

Who does that? Carr?
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Re: Tube amp schematics 101, etc: NEWER UPDATE: 5F1 champ schematic walkthrough!

Post by øøøøøøø » Fri Aug 15, 2008 1:12 pm

Maggieo wrote: So THAT's what a "soul" control is!

Who does that? Carr?
I honestly don't remember where I saw it called that before.  But that could be right. 

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Re: Tube amp schematics 101, etc: NEWER UPDATE: 5F1 champ schematic walkthrough!

Post by Jay » Fri Aug 15, 2008 2:13 pm

Allen does the "Raw" control but IIRC from reading elsewhere on the interwebz, it actually bypasses the entire tone-stack to increase overall gain (pretty dramatically) as opposed to affecting negative feedback in any way.  Pretty cool trick that works great in pedals too, especially the big muff.  :)

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Re: Tube amp schematics 101, etc: NEWER UPDATE: 5F1 champ schematic walkthrough!

Post by tribi9 » Fri Aug 15, 2008 9:19 pm

I haven't been here too long but I think  it's pretty cool what you do Brad.

Im sure there's lots of us that truly appreciate it! 

Cheers!
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Re: Tube amp schematics 101, etc: NEWER UPDATE: 5F1 champ schematic walkthrough!

Post by 1946dodge » Sat Aug 30, 2008 4:43 pm

Remember that this stuff is all according to my understanding, and I'm totally self-taught... no electrical engineer here, just a dorky amp guy... so I'm not responsible if there are gaps and possibly small accuracy quibbles.  I do try to only post stuff I know for sure, though.  Wink
Well I gotta tell ya, I am an electrical engineer and I am learning stuff from you (mostly long forgotten or never understood). You have just succeeded in waking up in me what I learned in the 60's when I went to school!
For that I thank you, my talented smart friend.

I am looking forward for anything you give us. It is the most lucid explanation I have seen in 40 years.

Thanks a million.
Last edited by 1946dodge on Sat Aug 30, 2008 5:55 pm, edited 1 time in total.
A  man studies and learns all of his  life, and  attains wisdom only when he finds that  he knows much and understands nothing.

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