The Watering Hole

All kinds of interesting things occur in the power section of Fender and/or other amplifiers.   There are two basic topologies used in the power sections of Fender amps - SE (Single Ended) or P-P (Push Pull).   The lower powered amplifiers (Champs, Bronco's, etc) are all SE style amplifiers and the higher powered amps (Twins, Deluxes, Bassmans, etc) are all P-P style amplifiers.

There are a LOT of myths, legends and creative ideas about what goes on in a power amplifier section, the "sound of tubes" and whatnot that are, in many cases, VERY untrue.   I'm going to try to talk through some of those, here, but may address some in a more generic topic on power amp sections in tube amps - another topic.   Many of the myths have much less to do with the actual tubes used and MUCH MORE to do with the components (i.e. transformer quality, biasing, screen resistors, speakers, etc.).  

Let's take a look, first, at the "easier" of the two topologies, the single-ended.   All single ended amps run in Class A.   Ooooohhhh... that venerable "Class A" sound, where the tube runs all the time and, supposedly, sounds better.   Let's start out by talking through some basic elements of most SE circuits.  In one of the first installments of this series of posts, I discussed, briefly, the purpose of the cathode resistor in a tube gain stage - remember that resistor on the bottom of the tube????    :o

This resistor does a bunch of things for the tube.   1) It sets the DC bias, or operating point for the tube.   The larger this value the more "linear" the tube is, especially for large input signal swings.   The smaller this value the more "non-linear" the tube is and the less dynamic range the input signal can have.   2) It creates what is called "negative feedback..."  This is similar to what your boss gives you, in the job.  Constant feedback that's equal, or nearly so, and opposite of what you'd like suppressing your creativity and output!    In truth, that's EXACTLY what negative feedback does - it controls the output AND reduces overall gain of the stage(s) it's fed into.  Negative feedback is always 180 degrees out of phase with the input.

All of that said, then, it can be seen that a well-designed single-ended amp does not need a feedback loop to control the gain.   You simply put in a well thought out and properly sized cathode resistor in the output tube circuit and the tube will self-regulate to the proper gain value and output level - automatically.   As the tube ages this resistor will keep the tube operating in the correct region, again automatically, until it finally fizzles out and dies.  

This lack of a feedback loop, in these SE amps, produced a unique character and sound that some of us really like.  First, they're "dynamic" and "spongy" and "responsive".  Great for blues players, especially.  You can crank them up and they transition from undistorted power amp stage to mildly distorted to distorted - just like the preamp tubes do.   They start out with soft clipping/limiting and move smoothly into distorted territory.  

Another thing about amps without a feedback loop is the fact that they have a very low "damping factor".   The "damping factor" is, to put it simply, the power amps ability to "control" the movement of the speaker.  A technical definition goes something like this:   The damping factor is the amplifiers ability to deal with back EMF generated by the speaker...   the higher the damping factor the lower the output impedance, the lower the damping factor the higher the output impedance...   when there is no feedback loop the power amp has a hard time controlling speaker movement especially at the low resonant frequency of the speaker/cabinet combination and the high resonant frequency of the speaker cone/voice coil combination.   This creates, in essence, a "mid-scoop" and a very non-linear (not flat) frequency response.   To most people this sounds more "natural" because, interestingly enough, it's the reverse of the Fletcher-Munson curve, de-emphasizing those frequencies to which our ears are the most sensitive and emphasizing those to which we're least sensitive.    

Because the tube has to be biased so "hot" to run in Class A, the amount of total output power delivered to the speaker is VERY low (5-15 watts).    Low output power = nice bedroom/studio levels.   They made great practice amps, and what not.    Low component counts, low wattage transformers = lower cost, as well.  In the Fender line the "Champ" was the most popular SE amplifier.   The Champ ended up coming in a LOT of flavors and styles over the years, some with tone controls (single and double), some without - all of them sported a single 6V6 output tube driving a transformer.

Remember I stated, in a previous post, that Fender was all about "clean" from input to output?  Well, Fender added, on many of their SE amps, a feedback loop. This was "tapped" off the output transformer and typically fed back into the preamp tube that fed the power tube.   Why???

Negative feedback, especially in a loop like this, increases the damping factor, decreases harmonic distortion and noise and improves power amp linearity (flattens frequency response).   By decreasing distortion, Fender was able to run those amps clean almost to the limits of the volume control.  The "bad" thing about a lot of negative feedback is that the power amp no longer smoothly transitions from "clean" to "slightly overdriven" to "distorted" it goes from clean to horridly distorted immediately with no middle ground and that latter distortion is not the nice "tubey" sound we like to hear, it's rather more like "digital distortion" or like "solid-state", highly "buzzy" distortion.  

Fender, in most of the power amp sections, used the Cathode resistor to bias the tube for DC operation (current) and bypassed it with a sufficiently large capacitor.  This capacitor increases signal gain to as high as it can go (limited by the circuit).  The output was fed to the output transformer and feedback, in many instances, was tapped off the output transformer to feed into the preamp to linearize the amp.

The more negative feedback fed back into the preamp/splitter section the more linear the output stage will get, the harsher the transistion from clean to distortion will become.   This also lowers the gain of the amplifer considerably by doing this.   Overall, though, the cleaner the output stage gets.  This high amount of negative feedback creates a much "tighter" output sound and it's more "focused".   Many Fender amps, especially the early ones, didn't have a lot of negative feedback in the SE amps, but they had some - just enough to "clean up" some issues.  Those small cabinets tended to resonate at very "nasally" frequencies.  The negative feedback, I believe, helped to control those resonances and make the amps sound a bit better, overall.

Light amounts of negative feedback will transistion the amp back to a more "spongy" feel, reduce the damping factor and have more of the resonances take precedence.   You can vary the amount of negative feedback, in the amp, by changing the feedback resistors.  We can talk about that further if anyone would like.   I have included a sample schematic, here, of an SE output Fender.   Note the line running from the speaker side of the OT back into the preamp.  That is afeedback loop.

Dar

Another great post!

Thanks again.

Randy  :)

In the final installment of this section, let's take a look at the P-P section of a Fender amp.   Like all stages of the Fenders, the power amps were designed to run clean and, as discussed in the last post, Fender used negative feedback to ensure that the amps ran clean, all the time.

The push-pull section runs in a mode called "Class AB" or "Class AB1", depending on implementation.  Most tube power amps use Class AB1 and I'll discuss why.  In a push-pull output section the primary side of the output transformer (OT) has 3 "taps" on it.   One in the middle, called a "center tap" and one on each end.   The center-tap is used, in reality, like a "ground".  IMPORTANT NOTE: It IS NOT ground, it is simply used LIKE a ground.  Ground, in any amp topology, can simply be a reference point from which measurements are made, or from which signals are sourced to.  

The two outside taps, then are fed by individual tubes - to be technical the top tube feeds the top tap, the bottom tube feeds the bottom tap.    :-X   Simpler that way.   The name of the stage is "push-pull".  In a pure Class B, push-pull, topology the top tube turns on IMMEDIATELY after the signal moves from 0V toward some positive value and stays on until the signal drops to 0V.   The bottom tube turns on IMMEDIATELY after the signal moves from 0V toward some negative value and stays on until the signal returns, or drops, to 0V.  When the signal is at 0V BOTH tubes are biased off.

Well, 50 years ago, tube construction techniques were relatively "poor" by today's standards.  In any given batch of tubes, two "matched" 6V6 tubes, or 6L6, or KT88's, or EL84's, etc could be "off" from one another by 20-30% in terms of accuracy.  The resistors, used to bias them and set their operating point, were also +/- 20% in accuracy, as were the caps, etc.   There's no way you could set up those power tubes to turn on/off at EXACTLY the right time.   Plus, after about 5-10 hours of operating the tubes would "drift" and they would continue to "drift" through the span of their lives.  If they don't turn on/off at EXACTLY the right time you end up with "crossover distortion" which is nasty beyond belief - even nastier than that transition from clean to distorted with negative feedback.

So, amp designers came up with a thought - why not leave the top tube "on" and have it amplify a little bit of the negative going portion of the signal just in case the bottom tube doesn't pick up right away... oh, and while we're at it, have the bottom tube amplify a bit of the positive going portion of the signal just in case the top tube gets lazy and Class AB was born.  The tube is biased on for MORE than 1/2 of the signal swing hence the "simul-class" operation of Class AB.  Class B, the tube is biased on for ONLY 1/2 of the entire signal swing.   BTW: This IS NOT simul-class as MESA defines it.  

Tube amp manufacturers soon found out that the Class AB side of the house didn't really cut it.  Earlier I noted that tubes that run with higher currents sound "warmer".  The Class AB amps sounded a bit "cold" and "harsh" and thus was born a new version of operation, Class AB1 which biases the output tubes "hot" and allows them to both operate for most of the signal swing, in both directions.  

As we move from Class B, to Class AB to Class AB1 to Class A, the efficiency of the power amp begins to drop dramatically but the overall "performance" and "palatability" of the power amp increases, so we end up with tradeoffs.  

Now, I know, that many of the "tube sites" show this little diagram with the top tube ONLY working on the positive portion of the signal and the bottom tube ONLY working with the negative portion of the signal.  That's simply NOT true.   Don't believe me?  Grab your favorite push-pull tube amp near you, pull all but one of the ouptut tubes and play your guitar!   You'll hear EVERYTHING and it's undistorted, in fact, you'll be surprised at just how freakin' loud that amp still gets without distortion!  And no, contrary to the many warnings on the internet you're not going to ruin your output transformer.   If anyone is interested in why, post the question, we'll talk through the actual science of that!   I've been running both my HRD and my JSX with a single output tube for well over a year, now.  

Ultimately, under normal operating conditions the real benefit of the Class AB, AB1 (push-pull) design is more power... RRRR   RRRR  RRR!!!!   But more power into the speaker LESS power drawn from the power supply.   Efficiency ratings are up folks!  This also produces less heat from the amp, too.

Fender power amps utilize 6V6 or 6L6 (most common) power tubes.  Why?  Because that was what was cheapest and easiest to get in quanties when Fender was making amps in America.   Those were the popular tubes used by most folks and they were always in stock.   It's really a simple equation, overall.   Fender "tapped" the 8 ohm output on the output transformer for the negative feedback loop.   There are, as you're aware, three typical taps out the output (secondary) side of a power transformer - 16 ohm, 8 ohm and 4 ohm.   The 16 ohm output has the highest voltage and lowest current, the 4 ohm has the highest current and lowest voltage and, of course, the 8 ohm sits there in the middle of the two.  

This "tap" will become important as we look at Marshall topologies.   :-)   There's something there, but not now kids.  

So, Fender used a "moderate" output voltage for the feedback loop.  MOST P-P Fender amps fed the negative feedback back into the "splitter" stage.  This helped to "balance" the splitter which was inherently "out of balance" because of component value tolerances.   It helped to make the power amp more "linear" in terms of frequency response, reduced power amp distortion, etc.  The Fender Bassman, as an example, had negative feedback, but there wasn't a lot of it.  That made this amp a bit "looser", which is why it's such a great amp for the Blues.   The Fender Deluxe Reverb ('65) is another example of an amp that has a low amount of negative feedback and it's a favorite in the Blues circles.  The Fender Twin is an example of an amp that has a TON of negative feedback and that amp, no matter how loud you turn it up, sounds the same, it just gets louder because that feedback loop is suppressing every little bit of distortion the power amp would normally generate.

The job of the "splitter" is to "feed" the top and bottom tubes in the P-P power amp section.   The splitter has two outputs each of them 180 degrees out of phase.  So, let's say the signal entering the splitter is going toward some positive value, the plate of the splitter will be outputing a negative going value (180 degrees out of phase) and the cathode of the splitter will be outputting a positive going waveform.   BOTH SHOULD BE at EXACTLY the same voltage.  Note:  "should be".   Again, component variations didn't allow for "exactly", just close, so we ended up with an "unbalanced" signal entering the power amp pretty much all the time - which makes one wonder why buy "matched" power tubes if the signal isn't matched to begin with?  Plus, the original Fender amps of yore would NEVER have had "matched sets" anyway...  

There are a lot of different splitters used in Fender amps, too many, really to chat about here... they all perform the same function.

Well, my number of characters is running out...

Dar

Say a guy had a 100 Watt amp that he wanted to try with just one power tube.  Which tube should be left in?  Does it matter?

Oh, and if you want to expand on the "You'll Put Yer Eye Out"!!!!! (Burn the transformer) tale, I'll certainly listen :D

DM;

Doesn't matter which tube you leave in the amp, at all, if you're going to do this.  They're all hooked up to the same place in the end.  :-)   It's all good.   I will explain, here, why this works and why you won't burn up your OT if you do this.   I know there are a lot of "guru's" out there that say you can/will burn up the transformer by doing this. I'll put a bit of the technical explanation as to why it CAN'T happen.

Let's look at the way a tube works - briefly.   The plate is "charged" with a positive voltage from the power supply.  The Cathode is "charged" with a negative voltage from the power supply.  The grid is "charged" somewhere between the two of them and this is what is called "bias".   The cathode emits electrons - it's made of a metal that emits electrons when it's heated.   So, given the rule that opposites attract, electrons are negative... the electrons come off the cathode and head on toward their positive counterpart(s) the grid and the larger, much more attractive plate.   Because the grid is biased to some slightly negative value not all of the available electrons get to fulfill their life's wish and head to the plate and thus bias actually "restricts" current flow.   In short it adds some amount of "resistance" to the flow of electrons from cathode to plate.    Keep this in mind.

When the signal hits the grid and goes from 0V toward some positive value it begins to remove the negative charge, from the grid, that was repelling/resisting electron flow in the static, or quiescent, state.   As the grid becomes more "nuetral" more and more electrons fulfill their wishes and head toward that big beautiful plate at the top of the tube.    At some point, as the grid becomes increasingly positive, it's going to "look" more and more like the plate and the electrons will no longer flow to the plate, they will flow to the grid.   The point just before this starts to occur is what is commonly referred to as "saturation".  It is at at this point that the tube, theoretically, supplies 0 resistance to current flow and looks like a short from cathode to plate.   Beyond this value positive grid current starts to flow and an "avalanch" occurs in which the tube destroys itself internally.

When the signal hits the grid and goes from 0V toward some negative value, it begins to increase the amount of negativity produced by the grid.  As with humans when another human is very negative, the electrons wishing to travel past the grid to obtain enlightenment at the plate are repelled in greater and greater numbers.   The more negative the signal becomes the greater the number of electrons that wish to give up their purpose in life and just hang around at the cathode.   At some point, the grid will eventually repel every electron and no current can flow in the tube.   In this instance it "looks" and "acts" like a resistor of infinite value - this is commonly called "cutoff".  

Between saturation (0 resistance) and cutoff (infinite resistance) is a median value of resistance.  This is called the "operating point" or "quiescent" point.   This is what a designer sets up all those resistors on the outside of the tube to obtain.   In reality, a tube cannot produce either 0 resistance or infinite resistance.  The actual amount of resistance produced by the tube, at all times, is called the "Plate Resistance" in tube literature.    This value is, actually, quite high (80,000 ohms average for a 12AX7, and about 10,000 ohms, or so, for power tubes).  

This becomes important, here, as we move to the next topic and that's a law created by George Simon Ohm - Ohms Law.  This defines the relationship between Voltage, Current and Resistance.   Voltage is the POTENTIAL for current flow.  Current is the actual flow of electricity and resistance is exactly that - resistance to the flow of current in a circuit.   Higher value resistors = low current, and vice-versa.   The flow of current generates heat.   I can have a voltage of 10,000 volts and no current flow and, consequently no heat.  Start current flowing and heat cranks up as a function of the amount of current that flows.

When resistances, or loads, are connected in parallel, you must sum the currents of each load/resistance to find the total load on the power supply.   When resistances, or loads, are connected in series you simply take the power supply voltage, divide the total series resistance into it and get the current value.

Current = Voltage / Resistance

Let's say we have a power supply of 100V and three 100 ohm resistors.   If I put all resistors in series (one after the other) and connect them to the power supply, I have a total resistance of 300 ohms.   100V/300ohms = .333 amps.  That's not bad.  When I connect these SAME three resistors in parallel (side-by-side), I have 100V / 100 ohms * 3 = 3 AMPS.   Same power supply voltage, same 3 resistors but the current increased by a factor of 10.   That is a LOT!    

Power = Voltage x Current

So, in the first example, each 100 ohm resistor drops roughly 33 volts at .33 amps.  That's 11 watts in the resistor.

In the second example, each 100 ohm resistor drops 100V at 1 AMP = 100 watts!

Watts = heat... more watts = more heat.  You can see the resistors in the parallel circuit will be equivalent to a furnace and the ones in the first example will be equivalent to a small space heater.  LOTS of difference.

OK... so, tubes = resistors... resistors in parallel = more current = more heat.   That's what we've learned, thus far.

ALL the power tubes, in your amp, are fed through the output transformer (OT), primary (tube) side, center tap (middle winding).   All 4 (if you have 4), or 2 (if you have two) are fed from the same point, thus they are all in parallel.  Four tubes installed will require 4 times the idle current.  Remove two of the tubes and you reduce current consumption by 1/2.  Remove 3 of the 4 tubes and you've dropped current by 75%.  

ALL CURRENT flows through the output transformer, thus removing tubes will actually decrease current demand through the transformer making it run cooler... There are several that say this is not the case... another interesting "phenomenon", according to ohm, is that when resistances are placed in parallel, the SUM of the resistances is less than the value of the smallest resistor in the parallel circuit.  To make the math easy, if there are 4 resistances of 100 ohms each, the final result of them added together is 25 ohms.   Do the math with a 100V power supply... 100V / 25 ohms = 4 amps.   4 amps x 100 v = 400 watts.   Remove all but one of the resistors and you have 100V / 100 ohms = 1 amp.   1 amp x 100V = 100 watts!    You can see which circuit runs cooler and demands less of the power supply and the components feeding it.

Anyway... there's the technical explanation of how this works.  Truthfully, if the OT burns up when you do this it's NOT the tubes, it was sheer dumb luck and coincidance together, for the most part.  One thing that can occur, however, and this is something to keep in mind especially with lower quality products.  When you remove the power tubes, the lack of current draw is going to bring up the voltage feeding the power stage, and other stages in the amp.

To be continued....

SOMETIMES the OT's have a voltage rating BELOW this new value of voltage and poorly insulated transformer windings will arc to other poorly insulated windings burning away the insulation and causing shorts.  If you've got a good quality product, again, that just shouldn't happen. I've been running my Hot Rod Deluxe (Fender) and my Peavey JSX in a single tube configuration for a long time (several years) without issues.

And, finally, you can buy THD Yellow Jackets, or Groove Tubes Power Reducers to put in place of your output tubes.  They convert 6V6 and EL34 tubes to EL84, self-biased tubes.  Those EL84's draw SIGNIFICANTLY less current than the original tubes.  If you're going to burn up the amp OT using a single EL34 or 6V6 you'll burn it up with a complement of PR's or Yellow Jackets, too!   Both companies ENSURE you that it's safe to run those tubes in the amp YET will turn around and tell you how dangerous it is to run a single power tube.   A full quartet of Yellow Jackets and/or Power Reducers will draw about as much current as a single tube in the socket!   You can actually measure that!  

Hope that helps.

Dar

Very interesting!  Again, I appreciate the response.  So, Tubes, or Valves ( as the foreigners prefer :D) are big goofy looking resistors.  Didn't know that.  I will certainly see what happens with this info.  It also explains a little how a Univalve or Bi-valve can run various tubes, or the Bi-Valve with one or two tubes in the Power section.

I'll give it a whirl with the Laney AOR.  I rarely play the thing, but if I can drop it down to a 25 or 30 watter, rather than a 100 or 50 watter,  that may make it a better choice for fiddlin" :D