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

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

Post by øøøøøøø » Sat Aug 30, 2008 7:03 pm

Wow that's quite a compliment!  I just like to read books.  ;D

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

Post by 1946dodge » Sat Aug 30, 2008 7:55 pm

Me too. I read all the time. I am thankful that you read books, simplify what you read and give it to the rest of us.
That is science and engineering at its best.
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Re: Tube amp schematics 101, etc: NEWER UPDATE: 5F1 champ schematic walkthrough!

Post by Superfuzz » Sun Sep 07, 2008 2:44 am

This thread must go into the library section..never learned so much in a sunday morining.. :-*
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Re: Tube amp schematics 101, etc: NEWER UPDATE: 5F1 champ schematic walkthrough!

Post by 1946dodge » Sun Sep 07, 2008 5:38 am

I think putting it in the Library is a great idea. Then it won't be hard to find.
However, I see that ####### has already put it as a pretopic so it will stay in one place too.
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Re: Tube amp schematics 101, etc: NEWER UPDATE: 5F1 champ schematic walkthrough!

Post by øøøøøøø » Tue Sep 09, 2008 2:28 pm

Biasing your amp!!

Here is a quick primer on biasing your amp, since it gets brought up a lot. 

First, determine if your amp is "cathode biased" or "fixed bias."

If your amp is "cathode biased," your bias is self-adjusting and will need no adjustment-- your output tubes are plug and play.  All preamp tubes on all known guitar amps  are cathode-biased, as well.

If your amp is "fixed bias," then it will need an adjustment for new tubes.  Most amps have a convenient internal trimmer that allows you to set bias that way.  Some amps (Pro Jr., Blues Jr., all Mesa Boogies, all fixed-bias Fenders made before about 1964, etc) do not have a bias pot and bias must be adjusted by changing the bias resistors.  In theory, you should check and maybe change resistors every time you swap tubes in these amps; in practice, many people just plug the new tubes in and hope they don't smoke 'em (and replace 'em if they do).  I think you know which approach I subscribe to.  ;)

Now the question is, "how do I determine where to set bias on my fixed-bias amp?"

Well, the real answer is "you set it so the idle current so that the tube's plate is dissipating no more than 70% of its rated power at its quiescent point."  Now, I assume that most of you are making one of the following faces after reading that statement:  ???  :-[  :(  :wtf:  :?  :ph34r:

Here's the good news:  You can go to this page: http://www.webervst.com/tubes/calcbias.htm

This has a calculator and several tables for many popular tube types.  You plug in your plate voltage and tube type (and operating class), and it spits out the maximum plate current your tubes should be drawing.  This will tell you everything you need to know about where your bias should be set.

Now I know what you're asking: "but I don't know what my plate voltage is?  I'm confused and I haven't even gotten to the point where I can measure it yet!"

Fear not!   ;D  That info is coming up next, but first the standard warning:

If you aren't careful, you can kill yourself poking around in an amp with live voltages.  You might have to measure plate voltage which can be over 400 volts DC sometimes.  If you are measuring, clip the black lead of your meter to the chassis and keep ONLY ONE HAND in the amp at any given time.  You do NOT want a path to ground through your body!  Don't let any jewelry, etc dangle over the chassis, and wear rubber-soled shoes if you can.  And/or, ideally, just be careful and don't touch anything "hot."  But if you do happen to grab the wrong thing, the above procedures can help you stay alive.  No need to fear the high voltage, but there's a big need to "respect" it.

OKAY!  So with that out of the way (and bear in mind, I do this all the time-- you'll only get hurt if you're not careful), on to the fun stuff!

The plate in the most common-for-guitar octal-base power tubes, including 6V6, 6L6, KT66, KT88, 6550, EL34, 5881, etc etc etc, is "pin 3."  On an EL84, it is "pin 7."

If you do not know your plate voltage, you can clip the black lead of your meter to the chassis, set it to read high voltage, and measure voltage on the appropriate pin.  Now be advised, as you adjust bias current upwards, your plate voltage might go down somewhat.  A "ballpark" figure is fine.

Alternatively, you could find a schematic on your amp, and it would usually have an approximate plate voltage marked.  Look over by the power tubes, look for the plate symbol, and look for a really high number specifying voltage.   :D  This ballpark figure would probably be adequate for calculating a rough idea of what your bias setting should be.

---------------

NOW that we've got our plate voltage, where should we set our bias?  Well on the Weber page, the tables have "hot" (70% dissipation at idle) "medium" (60%) and "cool" (50%).  I tend to shoot for the "medium" range, except in Deluxes or Deluxe Reverbs.  The reason is, a Deluxe or Deluxe Reverb tortures 6V6s.  They run the plates way high (maximum is 350 on the 6V6 spec sheet, but most Deluxes run them at 420 or higher!)  Since raising the bias current will drop the plate voltage somewhat, I try and hit that "middle ground" where the plate voltage is somewhat sane (around 420v or less) while the max dissipation isn't too far off.  Usually I end up around 23-26mA (this is 'hotter than hot').  JJ and NOS 6V6s can handle this fine.  Many others might not.  This challenge is unique to Deluxe-amps and Deluxe Reverbs, and amps modeled after them.  Long story short, you have a choice:  Either run the plate voltage 100v over design maximum but don't exceed max dissipation, or exceed maximum dissipation by a little bit and run your plates only 50-70v over design maximum.  I know, neither choice sounds great, but such is life in a Deluxe Reverb!   :D

-----------------

OK, now:  "how do I actually MEASURE bias current so that I can adjust it?"

Thought you'd never ask!

There are actually several ways.  Some ways I do not recommend.  I don't recommend doing it exclusively 'by ear' as some people do.  Your amp might sound great but tube life might be measured in minutes.  I don't recommend using an oscilloscope and biasing to "just where the crossover notch disappears" either.  It's not precise enough.  I also don't recommend what's known as the "transformer shunt method," since it's dangerous and completely not necessary.  I also fried a meter doing it that way once, so I'm biased against it (har har).

Now that you know what I DON'T like, here's what I DO like:

My favorite method is the "precision 1 ohm resistor in series with the cathode" method.  This can take the form of an external device like a "bias probe," or it can be an actual 1 ohm resistor that you put in the amp yourself.  It's important that the resistor be EXACTLY 1 ohm, so using precision resistors is important.  Basically what you do is put the resistor in between the cathode and ground, instead of just tying the cathode straight to ground.  Cathode is pin 8 on the 8 pin tubes listed above (and others), and pin 3 on the EL84.  Then you set your meter to read millivolts.  Put one lead on each side of the resistor.  That reading (even though it's in millivolts) is the same as plate current in milliamperes.  So if the meter reads 23.5mV, then your tube is biased for 23.5mA of current.  Simple, huh?  A bias probe has the resistor built in--it interrupts the cathode connection before the tube even gets to the socket.  So you plug the bias probe sockets into the tube sockets, and plug the tubes into the bias probe sockets.  Then it gives you your reading.  No muss, no fuss, and you don't even have to open the amp!  Bear in mind, if you use your own resistors, you should just install them permanently.  If using your own resistors, you have to disconnect the cathode from ground first, or no current will flow through the resistor and you won't get a reading.  The bias-probe, like I said, interrupts the cathode-ground connection at the socket, so you can leave the amp 'stock.'

If you have several amps that you need to check bias on, but have no technical aspirations, the bias probe is a worthwhile investment.   I don't use one, but I DO install the 1 ohm 1 watt resistors between cathode and ground in most of my fixed-bias amps.  It doesn't affect the sound, and is a nice time-saver.

My second favorite method is to measure it at the transformer and calculate it.  This can be done on amps without bias test point resistors if you don't have a bias probe.  It involves some math.

Step 1:  With the amp off and cold, locate the primary side of the output transformer--these will be the wires that connect to the tubes.  There will be 3 wires, 2 are connected to the tubes (probably pin 3, or 7 in an EL84) and the third will be connected somewhere else, maybe the power supply before the choke.  This lead is the center tap of the OT.  Put your black probe there and with the amp still off, measure resistance to both sides of the primary (the other two wires).  In other words, you are measuring the resistance of each 'half' of the OT with respect to the center tap.  The numbers will be dissimilar, as much as 10% difference is normal.  Only if they are drastically different is it any indication of a problem.  This will be in the 20-100 ohm range, typically.

Anyway, write these numbers down. 

Now, turn the amp on, and measure the voltages the same way... black lead on the center tap, red lead on each half one at a time.  Be very careful-- high voltage is in this area.  But the measurements you take will usually only be a volt or two (you are not taking the measurements with respect to ground, but rather with respect to the center tap, remember?  It can still shock the shit out of you, in other words).

Write those measurements down.

Now, divide the voltage by the resistance to get the current (ohm's law).  You might want to come up with a "target voltage" first by doing your calculating beforehand, and then work toward that point by adjusting the bias trimmer.  Remember, it's safest to clip the test probes in place with alligator clips while the amp is off, then you can take your measurements "hands free."

Hope this helps!
Last edited by øøøøøøø on Tue Sep 09, 2008 2:40 pm, edited 1 time in total.

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

Post by noirengineer » Tue Sep 09, 2008 3:03 pm

your the man!  8)

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

Post by Maggieo » Mon Sep 15, 2008 3:12 pm

Hey Brad, can you explain what's up with "matched pair" power tubes?  In, for example, a 6V6GT powered amp, do both power tubes have to be the same brand or what?  I was thinking about dropping some of the spare 6V6GTs I've got for my Vibro Champ into the AST, but I only have single examples of any given brand of tube.
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by øøøøøøø » Mon Sep 15, 2008 3:53 pm

eh, "yes and no."

The main thing that matters is that both tubes draw within a few mA of the same amount of bias current.  Is the swart cathode-biased? (self-biasing?)

If so, it would probably totally okay to try.  I'm honestly not totally sure of the ramifications of using drastically mismatched tubes in a cathode-biased design.

If the amp is fixed-bias, when you're biasing it up for the new tubes, make sure they're not drastically mismatched.  Let's say your amp wants to see 24mA of current on the plates at idle (a made-up number for demonstration).  If one is drawing 21mA and the other 27mA, that's probably borderline, but okay.  If one is drawing 18mA and the other 33mA, you probably don't want to use those tubes.  If they're drawing 22mA and 24mA, or something close like that, they're a good match.  Those numbers were all invented, I don't know what they should be biased to in your Swart.  It's just anecdotal to give you an idea of what constitutes 'matched.'

Basically, I try to get tubes that are within a few mA of each other at the operating point in the amp.

Now there are other factors.  In a Hi Fi amp, I would definitely say "only use matched tubes, same brand and all."  The reason is, in a push-pull amp, the matching of the tubes is critical to cancel out bias supply hum, filament hum, etc.  It also means both top and bottom of the waveform will clip in the same way at the same time.

But in a guitar amp, using different tubes (as long as their current draw is relatively close within reason) can yield some interesting character to the distortion, at the possible expense of slightly noisier operation.  With mismatched tubes you might get a slight increase in even-order distortion products and a slightly more characterful, asymmetrical clipping.

That may not be a very definitive answer, but it's about as good as I can do. 

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

Post by Maggieo » Mon Sep 15, 2008 4:36 pm

Yep, the Swart is cathode-biased.

So, different 6V6GTs will draw different amounts of current?  Why is that?
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by øøøøøøø » Mon Sep 15, 2008 7:47 pm

Maggieo wrote: Yep, the Swart is cathode-biased.

So, different 6V6GTs will draw different amounts of current?  Why is that?
Yes, but I mean different individual tubes, not just different brands.

As for why, I confess I don't know exactly!  But it's why "matched" tubes are necessary.  There is a lot of variation from tube to tube.  It has been my experience that the new production tubes are even more inconsistent in that regard than the old production ones.

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

Post by Maggieo » Mon Sep 15, 2008 8:28 pm

Wow!

I guess I'd better go looking for some NOS matched pairs to play with the Swart.

The Vibro Champ is cathode biased, too, right?
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by øøøøøøø » Mon Sep 15, 2008 8:46 pm

yep, it sure is!

beware that NOS matched pairs are VERY expensive.  Fortunately, 6V6s are a bit cheaper than 6L6s.

I might suggest looking for some "used, test strong" matched pairs of old-stock tubes.

there are lots of tubes out there floating around that have tons of life left in them.

The best is if you can find some tubes that were pulled from old military equipment.

Back when lots of military equipment had tubes in it, they would get new tubes and install them in all the stuff.  Much of it communications or testing equipment.

In peacetime, the stuff just sat there with brand new tubes in it.  When transistors came along and made tubes obsolete, all that equipment and never-used tubes were sold as surplus.  The tubes have been installed, but are essentially new.  But if you buy them, they will be sold as "used, test strong."

If you can find some stuff like that, you can pick up really good deals on hardly-used or never-used tubes.  Lots of times those tubes end up on ebay. 

Of course, you CAN just go the NOS route.

I really wish that Mike at KCA had some pairs of Marconi 6V6Gs left.  I had a pair of those in my Deluxe and they were delightful, and I believe they were only $60 per pair.  They were a real sleeper.  He had a bunch of them but I think they're all sold out now.  They were too good for the money; they didn't last long.  I think he went up to $80 and then $105 per pair when stock got low.  Now they're all gone. 

Anyway, good luck!

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

Post by Maggieo » Mon Sep 15, 2008 8:54 pm

Thanks for the "used, test strong" tip, Brad!!!

I'll keep my eyes peeled.
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Re: Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by Regan » Fri Sep 19, 2008 4:45 am

Brad, all I can say is, "WOW"!!!!! Oh, yeah, and,  "THANKS"! I would say more but my brain is about to explode from all this information!!!


You 'da man!!!!!


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

Post by luau » Mon Nov 03, 2008 9:17 am

øøøøøøø wrote:Here's the good news:  You can go to this page: http://www.webervst.com/tubes/calcbias.htm
Broken link. http://www.webervst.com/tubes1/calcbias.htm
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