# Tube Amp Walkthrough Help Please



## keeperofthegood (Apr 30, 2008)

Hey oh

OK OK I have googled, and read, and googled, and read. Lots of information, all stated in lots of different ways and still I seem to miss "the point" of some of the information. So, I have done a single tube mock up amplifier. Stripped down and simple (and delightfully non functional). I have labeled all the parts and shown the working voltages and currents at the peak negative output point in time (this is with all the pots set to their center values and the pot values are selected based on the many schematics I have looked over).

*What started me off on this was "ok, let us look to the GREAT and WONDERFUL youtube to educate me on BIASING AN AMP" and low there were videos, lots of videos, of guys with screw drivers twiddling potentiometers/trimmers *

 Ok, now that I got over "biasing for dummies" just what would I be twiddling in the amp? I have three main resistors (in a triode) amp that I could twiddle. The voltages and currents on the plate and cathode follow the wave form of the signal and the grid bounces all over the place, the current only flows into the grid during tube conduction too.Yea, my example is a fail to as I was not able to get a nice clean sin wave out of it (as I show in the little insert) but thems the breaks of a java applet XD










Hey and anyone else with questions, please, ASK!! I am sure there are others that have looked at this same circuit, read the same texts, and are every bit as confused as I am on the odd points here and there ... or everywhere XD


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## Wild Bill (May 3, 2006)

This is REALLY scary! Easy way to fry up a tube in short order!

First off, you have to remember that there are two separate things going on. You have the AC signal path and what loads it sees but also you have DC biasing and load conditions to set the tube into a linear range.

By linear we mean distortion - free. The signal should correspond on a graph one to one from the input to the output. The amplitude can change but the actual waveform shape must always be the same.

So first off, the plate must ALWAYS have some kind of a load! If the pot shown on the diagram for the plate resistor gets too close to 0 ohms then the tube is working into a short. Not a good idea. The optimum load is given in the tube data sheets and in other reference data for triodes used as voltage amplifiers.

Duncan Amps site has all this stuff. I believe you can also find it at the Triode Electronics site. There are others.

Second, the tube must NOT be conducting heavily at idle! This means that the bias voltage must be set so that the idle current puts the tube into a linear range of its plate current flow.

The grid controls the current flow, whether for the DC idling current or for the signal amplification. The grid must be negative with respect to the cathode. You can do this one of two ways. You either put a negative voltage on the grid or you use a cathode resistor that will develop a voltage drop to raise the cathode positive with respect to ground, which means the grid will automatically be negative compared to the cathode. Don't make the confusing assumption of always thinking about the grid bias voltage compared to ground. That only is true when the cathode is also DIRECTLY connected to ground! If it has a cathode resistor then the grid can read 0 volts but will still be negative compared to the cathode.

Once you start screwing around with the load values you have to keep in mind that extremes will really screw things up! First off, you might want to start off with the typical triode amplifier values for a 12AX7 found in most Fenders and most other guitar amps. Set the controls to have 100k as the plate resistance, 1500 ohms for the cathode resistance and wait a bit...OH OH! Look at that grid input circuit!

You've got a pot for the input grid resistor that can short all the way to ground, but you have no DC blocking coupling cap! This means that if that pot is set too low you can force the grid to ground, screwing up the bias current and of course, shorting out completely the input signal. All that input resistor is supposed to do is establish a constant input impedance for the grid. If a grid is completely "floating" it picks up all kinds of noise and crap! It also has a tendency to start squealing. Other than that, the input resistor is not supposed to have anything to do with any sort of bias or volume control.

So get rid of it! Put in a 1 meg resistor, leave the cap that isolates the signal generator on the input and just concern yourself with the plate and cathode resistances.

Now, looking at the plate load, there's little point in reducing the plate resistor. The output signal is developed across that resistor. A smaller value will mean less gain. You can increase the value and you will see more gain but if you go too far things will max out and then become very unstable. At that point the stage is not very useful, squealing away all by itself. 220k seems to be the common limit but you may come across the odd circuit that gets away with a higher value.

When you start dinking around with the plate load the optimim DC idle current point will also change. You will have to move the tube into a different region along its plate current curve (by changing the bias) or most likely you will find it is now outside the linear region and either the top or the bottom of the signal will get clipped and distorted.

So this is where we might alter the value of the cathode resistor. Raising it higher will start to really reduce the gain. Lowering it will increase the gain but as always, things interact and there are limits. The lowest we commonly see is 820 ohms, as in some classic Marshalls. Much less and you can't get enough cathode bias voltage drop to set the bias to the right point, regardless of the signal gain.

So that's it for the typical triode voltage amplifier! A designer will normally use standard values from standard circuits for his resistors. There's no point in re-inventing the wheel. You never expect to change the biasing arrangements in your preamp stages, unless you are deliberately hacking away trying to get more gain. It's mostly not worth the trouble. You can get a bit more but not enough to impress anyone and its too easy to just get yourself into trouble.

Case in point might be the JCM800 circuit. You will find a triode stage in there with a 10 k cathode resistor. The stage is not being asked for very much gain. It's really just there to provide isolation. Many newbies learn just enough to realize that they can reduce that 10 k resistor and bump up the gain. Unfortunately, they don't have a good sense of proportion and make DRASTIC changes in that resistor value, like lowering it from 10 k to 820 ohms! The amp starts squealing like a stuck pig! A lower limit of 4K7 is already pushing the limit. I like to use 6K8, which increases the gain by BAGS and yet is still quite stable.

Power amp circuits of course are a whole different animal! The principles are the same but you expect to have to adjust the bias, due to differences between new tubes after replacement.

Once again, hope this is useful!

:food-smiley-004:


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## keeperofthegood (Apr 30, 2008)

XD

Thanks Wild Bill, I will redraw this up to be more ... educationally experimental with safety measures (no ZERO ohms etc) later today.

Still, of the three, are you saying that the value altered in biasing an amp is the value of the cathode resistance?


And yea, I want to bread board this up eventually too so I can take the readings and draw out the curves for my self too.  learning is never ending!!


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## Wild Bill (May 3, 2006)

keeperofthegood said:


> XD
> 
> Still, of the three, are you saying that the value altered in biasing an amp is the value of the cathode resistance?


Not exactly. You don't change the bias of your preamp tubes. You have a design where the preamp tube stages are all biased by their cathode resistors. The triode stages are Class A amplifiers, which means plate/cathode current is at a continuous maximum. This causes a voltage drop across the cathode resistor, making a positive voltage at the cathode. The grid will of course now be negative with respect to the cathode by precisely that voltage.

So you design the bias point and pick your cathode resistor values for the preamp tubes and never change anything again! Talking about biasing an amp has nothing to do with the preamp tubes.

Output tubes are a different story. New tubes will vary in specs and the bias control voltage will often have to be adjusted. You can have a fixed negative voltage that you apply to the grid, perhaps with a trimpot so you can adjust it. Or, you can also cathode bias an output tube. You calculate the resistance value necessary for a cathode resistor to develop the right bias voltage from the drop across the resistor caused by the idle current. You also calculate a safe wattage rating for the resistor.

Cathode biasing is a bit self-regulating. If you plug in a hotter tube you get a higher voltage drop across the cathode resistor. This means a higher bias voltage which throttles back the tube idle current. A colder tube will do the opposite. Cathode biasing output tubes also has a dramatically different tone when the amp is cranked! I happen to like it a lot, myself!

:food-smiley-004:


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## dtsaudio (Apr 15, 2009)

Nice explanation Bill

Not to get you confused any further, but you might want to aquaint yourself with plate curves. This tool is a graph of the current characteristics against voltage for given bias.

"Cathode biasing output tubes also has a dramatically different tone when the amp is cranked! I happen to like it a lot, myself!"

It's rare these days to find a designer that likes cathode bias. Too many guys want to squeeze out the last drop of power and don't like cathode bias because of the cathode voltage drop.


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## keeperofthegood (Apr 30, 2008)

dtsaudio said:


> Nice explanation Bill
> 
> Not to get you confused any further, but you might want to aquaint yourself with plate curves. This tool is a graph of the current characteristics against voltage for given bias.
> 
> ...



 that is actually part of this for me. I am sorting out a lot of the info I have read BUT I need to actually physically have an experimentation breadboard on which I can set up a tube, and then take and plot measurements. I have read over the use of "curves" a few times, and my mind is just not making the leap between them and the physical elements. Once I have something that I can "adjust" and monitor the results of that adjustment, and compare that back to the curves I know it will make a lot more sense to me. (that reminds me, I need to buy filter caps >.< )

Also why, when I read on 'biasing' and then I look at the schematics (what I can find) I am not seeing the connection. The schematics that I have don't have anything variable near any of the tubes, so I am not sure just what is being changed where or how and why I did the SUPER simple image at the top.

My brain took a momentary holiday this afternoon, I hope to fire up the java app again and redraw out something that will be able to be used for this


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## GTmaker (Apr 24, 2006)

With the outmost respect to all that replied especialy Wild Bill whom has taught me more about amps on this forum then I realy deserve to know.

I have a fender Bassman LTD.
It has a bias adjustable pot and a clearly marked resistor where I take a milliivolt readings. From research I know that a good range reading I should aim for is 65-70 mv.

Question....what the hell am I missing here. Why do I need to know anything more about biasing my amp. I dont build amps, I just play them.
The other thing that still confuses the hec out of me is this.
I read a lot about biasing individual power tubes.
My amp has 2 power tubes, 1 bias pot and one place to take a reading.
How could I ever bias one tube and not effect the other?

Obviously I am confused but I have learned to live with that fact.
Whats wrong with me?


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## keeperofthegood (Apr 30, 2008)

GTmaker said:


> With the outmost respect to all that replied especialy Wild Bill whom has taught me more about amps on this forum then I realy deserve to know.
> 
> I have a fender Bassman LTD.
> It has a bias adjustable pot and a clearly marked resistor where I take a milliivolt readings. From research I know that a good range reading I should aim for is 65-70 mv.
> ...


HAHA I don't think there is anything wrong with you! You know what you need to know. You have a finished amp and you know how to keep it in good working condition.

Me? I want to build. I have a plastic box full of loose bits and pieces and every time I think I have enough bits and pieces to start on something I find I am missing something. I also am the kind of person that doesn't like to follow other peoples works, because I want to feel what I did was all me, silly I know, I should just follow others at least until I have a better sense of what I am doing but hey  It has been YEARS since I have wanted to build anything, and the last half dozen have been really poorly for me. However, my mind has been returning to me, so I am pursuing the question "what is behind that chassis and why is it there and if I were to do it, what would I do?".


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## WCGill (Mar 27, 2009)

Well now, Bill explains this stuff so astutely, I don't know if I should even try. But I will. Ha.
Biasing just sets the operating point at a certain value that can be used in the designer's circuit. Since we're doing audio here, we want to amplify sounds (voltages) with our tube. If we over-bias, and by this I mean making the grid too negative compared to the cathode, either by means of fixed-bias negative voltage applied to the grid or using a large cathode resistor, it will take a large input signal to overcome this negative voltage and make the tube conduct and amplify. If the tube is under-biased, resulting from smaller negative voltage externally applied using fixed-bias, or by using a smaller cathode resistor in cathode-bias, the grid will be less negative compared to the cathode and it will take less of an input signal, or positive voltage to drive the grid positive and make the tube conduct. Over- and under-biasing are exactly what they mean-not optimum. There's a point in every circuit that gives the level of amplification needed by the designer with acceptable levels of noise and distortion without over-stressing the tube and this can vary according to circuit goals and designer intentions. 
This is a very simplified explanation, not even touching on plate resistances,
plate resistor values and grid resistances and output resistances, which also
play important roles in a tube's operation. Clear as mud?


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## Wild Bill (May 3, 2006)

WCGill said:


> Well now, Bill explains this stuff so astutely, I don't know if I should even try. But I will. Ha.
> Biasing just sets the operating point at a certain value that can be used in the designer's circuit. Since we're doing audio here, we want to amplify sounds (voltages) with our tube. If we over-bias, and by this I mean making the grid too negative compared to the cathode, either by means of fixed-bias negative voltage applied to the grid or using a large cathode resistor, it will take a large input signal to overcome this negative voltage and make the tube conduct and amplify. If the tube is under-biased, resulting from smaller negative voltage externally applied using fixed-bias, or by using a smaller cathode resistor in cathode-bias, the grid will be less negative compared to the cathode and it will take less of an input signal, or positive voltage to drive the grid positive and make the tube conduct. Over- and under-biasing are exactly what they mean-not optimum. There's a point in every circuit that gives the level of amplification needed by the designer with acceptable levels of noise and distortion without over-stressing the tube and this can vary according to circuit goals and designer intentions.
> This is a very simplified explanation, not even touching on plate resistances,
> plate resistor values and grid resistances and output resistances, which also
> play important roles in a tube's operation. Clear as mud?


Oh, I dunno! Seemed like good lookin' mud to me!:smile:

To answer GTMaker's question about how biasing affects both tubes in a push-pull pair, you're absolutely right! Of course when you have only one adjustment pot it is varying a bias voltage that is applied to both tubes.

The reason its not a problem is that tubes are supposed to be made reasonably close in specs. In the golden years of tubes manufacturers were expected to make tubes to very close tolerances so that their specs were as close as possible to the published data sheets.

When you put a pair into an amp they were expected to track reasonably closely. So the bias control voltage would adjust the current in both tubes but the tubes would track within a few ma. of current with each other.

Nowadays tubes aren't made to such close tolerances and vary a lot in their specs, even within the same brand! So tube sellers will test tubes for us and match them as pairs and quads when they sell them to us.

Many audiophile amps will have individual bias controls for each output tube. The idea is to match the tubes very, very closely to result in the least possible amount of distortion.

This is not at all necessary with a guitar amp. A few ma.'s difference in idle current actually sounds better! It gives a bit of a "swirl" with some extra harmonics. As I like to say about audiophile ideas being transferred into the guitar amp world, who the hell wants a hifi-sounding guitar amp? You're SUPPOSED to get distortion! The trick is to have good sounding distortion!

That being said, you DO have to worry about a BIG mismatch! A pair of tubes that is too badly mismatched will give you problems. One tube will "loaf" putting out less than its share of power and the other will try to flog itself to death! Instead of a 50/50 split you might get 75/25.

You will also have unbalanced gobs of current flow in your output trannie. You can actually burn one out!

I had a DR. Z amp in for service that had a bad hum. It turned out the owner had changed the output tubes with a pair from a music store. Many music stores don't even know about matching, let alone care. The tubes had a 12 ma. difference! I put in a new matched pair and the amp was quiet as a mouse.

My rule of thumb is to have no more than 3-4 ma. difference in idle current on a pair of 6L6's or EL34's, proportionately less with lower power tubes.

As Lily Tomlin used to say "And that's the truth!":smile:

:food-smiley-004:


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## GTmaker (Apr 24, 2006)

Wild Bill said:


> Oh, I dunno! Seemed like good lookin' mud to me!:smile:
> 
> To answer GTMaker's question about how biasing affects both tubes in a push-pull pair, you're absolutely right! Of course when you have only one adjustment pot it is varying a bias voltage that is applied to both tubes.
> 
> ...


Once again , Wild Bill comes thru in the crunch.
I have never seen or heard that there can be a bias adjustment for each power tube. To my simple mind, thats the only way you can ballance multiple tubes. I guess the moral of this story is to try and get as closely ballanced power tubes as possible when you have only one bias pot and a multiple power tube configuration.
I know that I should not even ask this as the response will probably involve some kind of "death trap " contraption that when used improperly will probably make me more bald then I allready am BUT I'll ask anyways.
Theoreticaly, if I buy a new set of power tubes, is there a simple home test I can perform to confirm if these tubes are matched?


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## Wild Bill (May 3, 2006)

GTmaker said:


> Theoreticaly, if I buy a new set of power tubes, is there a simple home test I can perform to confirm if these tubes are matched?


Nope! Although if you put 1ohm cathode resistors for each output tube as bias test points you can measure each tube's idle current, which is what you have to do when you set the bias anyway! You will see right away how closely they are matched.

However, any reputable vendor tests and matches his tubes for you. I use 

http://www.thetubestore.com

They're very nice people and even better, they're Canadian! When they grade their tubes they have a number of bias current testing machines that do a number of tubes at one time. They burn them in for 24 hours and mark the idle current as a number on the base of each tube.

So if you buy a pair of 6L6's from them you will see a number like "37" on the base. This number is only good for tubes from thetubestore. Somebody else's machines might have a completely different number for the same tube but it doesn't really matter. As long as all the output tubes have the same number they are matched!

Also, when the time comes to change tubes if you ask for tubes with the same number you theoretically won't have to change the bias adjustment. I don't do this myself. I like to check things each time. Too many things can drift in an amp. Still, lots of guys get away with it and if you're not comfortable doing your own biasing and you can't find a tech handy you should be ok most of the time.

:food-smiley-004:


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## dtsaudio (Apr 15, 2009)

> I had a DR. Z amp in for service that had a bad hum. It turned out the owner had changed the output tubes with a pair from a music store. Many music stores don't even know about matching, let alone care. The tubes had a 12 ma. difference! I put in a new matched pair and the amp was quiet as a mouse.


I just finished up a Fender Twin that has a control to balance the output tubes. You set it for the least amount of hum. Reduces bias in one tube and increases in the other as it's rotated. Rather nice approach actually. However the amp didn't have an overall bias adjustment.
I still prefer matched tubes though. Another plug for the Tubestore. They do a very good job of matching tubes.


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## Wild Bill (May 3, 2006)

dtsaudio said:


> I just finished up a Fender Twin that has a control to balance the output tubes. You set it for the least amount of hum. Reduces bias in one tube and increases in the other as it's rotated. Rather nice approach actually. However the amp didn't have an overall bias adjustment.
> I still prefer matched tubes though. Another plug for the Tubestore. They do a very good job of matching tubes.


Yeah, the balance control was a SilverFace thing, after CBS had bought the company. The new engineers were obviously more hifi guys than players. You could tell by looking at the changes they made to Leo's circuits. The balance control was a strong example. The overall bias was fixed to a very low idle current. Minimum hum and distortion but perhaps a much poorer overall tone. Most guys rewire the adjustment pot for the old Blackface circuit of overall bias and use matched tubes. As I posted before, there might be a little bit more mismatch than with a hifi amp but it lets you set the idle for a much warmer "sweet spot" tone!

:food-smiley-004:


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## dtsaudio (Apr 15, 2009)

I agree. The bias is quite a bit low. I personally prefer the sound from a hotter biased amp. However the owner likes it so I'll leave it.
The hum reduction is a nice touch though. Now if only they used both methods in combination.


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