Knowing more is the other half of the battle ...

In my last post, I discussed the basic workings of the guitar, its electronics, and did my fair share of step exercises by climbing on and off my soapbox.  That's what boredom will do to a guy - or at least what happens when I don't have a real project to play with.  Happily, my parts should be in for my new amp build and then I can discuss real stuff.

Today, my plan is to unveil some of the mystery surrounding amps themselves and give another high-level view to these mythical beasts.

A little of the basics

There are probably as many amps and types of amps as there are types of guitars and players.  It's impossible to cover everything with any accuracy because there are just too many variables.  However, for this, I'm going to cover some concepts that have been present since the beginning with the Fender Bassman or the early Marshall amps.  These concepts do tend to be the common benchmark for many amps today, so it's a fair representation, as far as that goes.

I can't say this enough, this is just a high-level educational overview.  Some of the material covered will not be perfect or even completely technically accurate except to provide a teachable example of the concept.  This means, so go blowing yourself up if you decide to poke around in an amp because of something you thought I was telling you here.

Gain Stage 1

A Fender amp is a very cool thing.  I've owned many over the years and still have one now.  It's a classic and the early ones had a small bit of magic in them - probably from being such a prominent part of the dawn of rock and roll.  But, for as much as I worship at the altar of Leo Fender for many things, including his business acumen, his amp designs, although legendary, were not original or novel by any stretch.  Many of the concepts brought out were simply adaptations of earlier radio amplifiers.  This means that for the amp enthusiast, there are many authoritative works that can be explored that predate the 1959 bassman 410.

Here is a basic schematic of the Bassman 5F6, which is almost the archetype by which many amps model themselves.

But, let's start at the beginning.

Our signal has left our guitar and has hopefully traversed a very good quality cable.  Good quality is very important because a bad cable will let in noise, interference, and just poorly transfer signal from the guitar to the amp.  Now, this is important to recognize the variables we have here.  Our output signal from the guitar is somewhere between 20 and 750 millivolts.  This is a very small signal that is very sensitive to outside influence.  A good cable with good shielding is critical to ensuring that signal arrives at the input jack of the amp intact.

So, assuming our signal arrives in proper fashion, our amp has to convert that signal to between 5 watts and 150 watts to power our speakers.  The output wattage depends on many things, but usually we are running somewhere around 20 - 50 watts given a normal combo amp.  The trick of our amp is converting our say 100 millivolt guitar signal into the crowd pleasing roar of the speaker output.

Stage 1

Upon arriving at the input jack, out signal runs to the first preamp tube.  This is our first gain stage that amplifies our signal into something the amp can use in further stages.  Generally, this is a tube in the 12A_7 series.  It could be a 12AX7, a 12AY7, or even a 12AU7.  Each of these tubes are roughly of the same family and essentially convert signal to voltage at different levels.

As the signal enters the amp, it is pushed through various resistors and capacitors to modify the expected sound.  Think of the "Normal" and "Bright" or "Bass" inputs on a typical amp of this type.  The tonal differences of the inputs are achieved with similar but slightly different signal paths winding through various resistors and capacitors to "bleed off" certain higher frequencies to ground.  (For a more complete discussion of this concept, see the first part of this series that talks about the function of capacitors in guitar electronics)

Gain Stage 2

As the now amplified signal progresses through the preamp circuit, it encounters some variable resistors.  As we know, variable resistors are potentiometers or pots.  In this case, we encounter the volume pots for the channel.  Along with this are some additional signal modifying capacitors and resistors.  This helps to "shape" the raw tone both before and after the volume pots so that is it more reflective of the individual inputs.

Stage 2

For example, in the Bassman circuit, the signal for the "bright" channel progresses through a .02 picofarad capacitor into the 1M ohm volume pot and then onto the 12AX7.  The "Normal" channel goes through a .02 picofarad capacitor, hits a 1M ohm volume pot, and then the signal goes through another .0001 picofarad capacitor before progressing onto the 12AX7.  This simply means that the "bright" channel is actually more pure than the "normal".  The additional cap on the "normal" channel bleeds off very high frequencies (the same as it does on a guitar's tone pot when turned off).

At this point, the two channels' signals converge into the next stage.

Gain Stage 3

After leaving the second 12AX7, the now unified signal enters the tone stack.  In later Fenders, there are distinct tone stacks for the different channels, especially as one channel had reverb and vibrato while the other didn't.  In the case of the classic Bassman, there was one tone stack.

Funny enough, there isn't too much difference between the concept of the simple guitar tone control and this stage of the amp.  Pots adjust level while various values of capacitors bleed frequencies off to ground.  Put the different values together in a series and voila, you have tone controls that allow subtractive shaping of the tone by signal modification.

Stage 3

Subtractive Shaping? … What are you talking about?

When we look at the amp controls, we usually see our pots labeled from 1 - 10.  Naturally, we assume that if we start at 1 on the Bass control and twist it to 10 that we are adding bass into the mix.  However, in most cases, this is exactly the opposite.  Instead of adding anything, we are actually moving the signal closer to natural by simply removing any frequency attenuation.  That means that when all of our tone controls are on 10, that is really what the "raw" signal sounds like.  Obviously, "raw" after the internal modifications I talked about earlier.  Note that some amps do employ active tone controls where signal is added to the raw signal to boost frequencies, but it is more common for this subtractive shaping.

Gain Stage 4

Once our signal has passed through the tone section, it hits one last 12AX7 where the final signal is modified, balanced, and sent off to the power tubes.  I'll cover the power amp stage sometime later, but for now, let's recap quick.

Stage 1:  Input Jack to first preamp tube
Stage 2:  Preamp tube 1, through volume controls to preamp tube 2
Stage 3:  Preamp tube 2, through the tone stack to preamp tube 3
Stage 4:  Preamp tube 3 to the power amp

Stage 4

So where does preamp dirt come in?

One of the coolest things about tube amps is driving them hard.  Hard enough to create very pleasing distortion tones.  In general, this happens by driving successive stages harder and harder until they distort.

For example, if we have a very hot signal coming in to the amp, say from a very powerful pedal or active pickup system, we take the chance of overdriving our input stage.  This happens because we are feeding more power to the first stage than it was designed to expect.  Of course, the problem comes because the rest of the amp is designed to buffer distortion from early gain stages through the use of resistors and capacitors and signal routing.

However, there are techniques for driving successive stages harder and harder until they distort in very nice ways.  On Fender amps like this, one trick is to change out preamp tube 1 (V1) from a 12AX7 to something like a 12AU7.  The 12AU7, while a lower powered tube, actually tends to send a hotter signal down the line, which causes a harder signal to hit the second tube (V2).  This will cause an earlier break up because as the volume is turned up, more voltage passes through at a lower volume.  For an example of this, check out my posting concerning the mods I did to my Carvin MTS3200 or some of the changes I made to the tubes in my Fender Twin Reissue.

While some of that probably is clear as mud, the important thing to take away is that through creative use of voltage, we can drive the preamp harder as the signal winds its way through the circuit.  This will create preamp overdrive.

Next time around I'll discuss some of the relevant output stages including power tube distortion, negative feedback, and other fun stuff.  Or my parts will get here and I'll have something else to do instead of talking theory.  (The joy of waiting for custom wound transformers that are on back order)

Knowing is half the battle...

Like with many people who are looking for answers to questions in this day and age, I often find myself sifting through the pages of various articles, blogs, and message boards on the Internet.  Without a doubt, the hardest subject to find real, honest, and accurate information about is anything related to musical equipment.  Sometimes, it's humorous to read some of the most convincing arguments by people who have no idea, but they are selling it like snake oil to a rube.  Because of that, I wanted to take a little sojourn from my usual technical stuff (while I wait for my parts for my next amp build) and try to give some good, accurate information about this whole electric mess we know and love.  As I'm plunking this down all stream of consciousness like, I'm not sure if it will be a single post or if it will wind up with multiple, but either way it should be entertaining.

Please note that one of the biggest annoyances I have when dealing with much of this type of information is trying to make it too technical for the audience.  I know that most of the readers of this blog are not electrical engineers, nor do they aspire to be.  They are musicians who are looking for answers or just killing time.  Because of this, I'm keeping this simple and high-level.  I'm going to avoid the very involved math that makes this work.  I'm going to sacrifice pure technical accuracy for general understanding.  The descriptions and examples do not apply to 100% of the situations and circumstances out there, so if this isn't exactly your brand of mojo, it's ok, I understand.

Basics

There is no black magic involved in modern musical instrument design and amplification.  The concepts employed in making our Strats and Les Pauls loud are, at their heart, coming up on 100 years old.  This means that these were some of the absolute first things discovered in modern electronics.  I'm gonna burst your bubble at the right now, but if you think there's something special about this whole thing, you're mistaken.  I'm gonna tell you why…

The Guitar

The guitar is the key part of the chain.  It represents the user input and is a highly flexible input device at that.  It is essentially the analog of the touch screen on your smartphone or the keyboard and mouse on your computer.  Kinda sucks the fun out of music doesn't it?

On an acoustic guitar, we have an arrangement of strings.  Each of these strings are tuned to a different tension, which means they will vibrate at a different speed resulting in a change in pitch.  As we shorten or lengthen the string, the speed of vibration changes and the pitch changes accordingly.  When the string is halved in length, it vibrates exactly twice as fast and the resulting pitch is twice as high.  That's basic physics of a vibrating string and everything stems from this principle.  The acoustic guitar is designed to make things loud based on the design of the body and acoustic resonance.  Different wood types, body shapes, and construction all go into making the instrument more resonant providing volume and sustain, which is the property of allowing the string to vibrate longer.

An electric guitar (or bass) has most of these same concepts.  The designer still needs to allow the strings to vibrate and sustain is a concern as well.  Different designers have created different shapes and construction techniques to help everything move along.  However, the one big difference is that an electric guitar does not rely on the guitar itself for amplification.  Amplification is provided through a dedicated device such as an amp or PA or other mechanism.

So, the challenge comes in the question of how do we turn physical motion into electrical impulse to provide input into something to make it louder?  This is where things get pretty cool.  Years ago, some very observant people realized that if you take a magnet and wrap it in wire, you can create an electrical impulse which is known as induction.  However, if you take that inductor and modify the magnetic field, you will modify the electrical impulse generated.  Breaking this down, if we have a magnet or group of magnets and then wrap them with copper wire, we have a very basic pickup.  Using steel strings, we can vibrate those strings by playing them on the guitar, and create a changing electrical current.  Voila, we now have converted physical motion (vibrating guitar strings) into electrical impulses.

But Brad, why does my ______ wood guitar sound better than my ______ wood guitar?

This is where that all comes back to the instrument designer.  Is wood a conductor of electricity?  No, of course it isn't.  Simply by long held laws of electricity, we can logically conclude that your Mahogany, Ash, Maple, Pine, or whatever body has absolutely no direct bearing over your amplified sound.  Before the flames start, I said DIRECT bearing.  Wood does not get picked up by a magnetic inductor no matter how hard you try.  However, indirectly, the wood will lend itself to the resonance of the guitar itself and help the strings vibrate longer and decay in different ways.  This is how wood affects tone.  Unfortunately, not all guitars are created equal, therefore there's more than just wood involved, but that's the short answer.

Guitar Controls

Once we have a good transfer of energy from one form to another, we could very happily send it on its way to the amplification device.  There is nothing that stops us from wiring the negative and positive of the pickup to the jack and calling it a day.  But, there are things we really have grown accustomed to over the years, so we need to add them.

A basic inductor only needs two poles to function, positive and negative.  If we look at an old vintage single coil from the 50's, you will notice only two wires.  This does work, however we have grown used to certain things.  Ground wires.  We like ground wires. We probably remember from elementary school that electric likes to find its way to the ground as is the case with lightening and power lines.  In our case, not all electric impulses generated are good impulses.  Sometimes we get high frequency impulses that are introduced into the system, like the dreaded 60 cycle hum as well as others.  A good ground system will allow many of these undesirable frequencies to find their way out before reaching the amplifier and also getting louder.  The major drawback to our system is that we have a very self-contained electrical generation system surrounded by non-conductive wood held by a person above the ground.  There is no direct path … except there is - you.  We usually tie the ground back to the bridge or other metal piece that channels the path to the strings.  When you play the guitar and make signal, the ground path is complete through you.  Luckily, we're dealing with very small voltages, often less than 1 volt and usually closer to .5 or .75 volts at the very upper end of the spectrum.

With a pickup and a ground we have some level of silence, but this is pretty useful.  Something even the most ardent minimalists use is a volume control.  This is wired in between the pickup and the output jack and takes the form of 250k, 500k, or even a 1m potentiometer.  A pot is nothing more than a variable resistive component that, as it moves, increases or decreases the resistance to the signal from 0% all the way to 100%.  That's a mouthful, eh?  Think of it like a faucet - you turn the handle in one direction and the water is off while in the other direction it's fully open.  A volume pot works the same way just with a small difference.  Water can happily be held back unless there is steadily increasing pressure behind it.  Electricity needs somewhere to go.  In this case, the volume pot slowly redirects signal to ground.  When the volume is all the way "up," the signal is 100% (ish) going to the output jack.  When the pot is all the way "down," the signal is going all to ground.  Electricity follows the path of least resistance.

The above graphic is my miserable attempt at a free-hand drawing.  The three components shown are the pickup, the volume pot, and the output jack.  The red line is the hot from the pickup to the first lug of the volume pot.  The blue line is the output wire from the center lug or wiper of the pot to the tip if the output jack.  The green lines are grounds.  Notice that the pickup is grounded, the volume pot's third lug is grounded, and the sleeve of the output jack is grounded.  This is about as simple as things get.

Brad, I bought these super gee-whiz branded pots from expensivemusicsupply.com and they are the best!

This is one of my pet peeves.  I don't care where you bought your pot or how much you paid, a pot is a pot is a pot is a pot.  There are very few things that make a difference in electronic components.  One of these is construction quality, and unless you are buying mil spec parts that are manufactured under very tight tolerances, your pot probably came from china like all the rest.  But, is build quality the end all be all?  There's also component error.  If you buy a 500k tone pot, do you know the error on it?  Components are not manufactured to exact specs therefore they are built with tolerances in mind.  A pot may be a 10% tolerance.  That means it's acceptable for the 500k pot to be anywhere from 450k all the way to 550k (500k x 10% = 50k +/-).  However, it's just as common to find 20% tolerance in this type of component, which means your 500k could be anywhere from 400k to 600k.  This also means that a simple component swap with the same printed value could yield anywhere from 100k to 200k difference.   This is just the variance on a single pot.  If you add in a Tone pot to the mix, you could have a 40% variance on the two values.  In other words, although you might expect a 1000k total resistance across the two pots, you may find it's anywhere from 800k all the way to 1200k.  If you are playing a Gibson Les Paul style guitar with two volumes and two tones, your expected 2000k load could be anywhere from 1600k to 2400k.  These are significant differences that will have a noticeable effect.  My best advice is to look at your old 500k pot with a meter and then look at your new 500k pot with a meter and see the difference.  You will probably be surprised.  Many times when you hear other musicians rave about how a simple change of pots brought their guitar to life, it's just dumb luck.

Are high end "specialty" products worth while?  Nope - in most cases you can find better pricing with regular components.  Check out a place like Digikey that's not trying to bilk musicians out of money for snake oil and you will see what component prices are really like.

Tone Controls

Let me wrap up today's post with a little discussion on tone controls.  There are very few things that bring out the crazy like tone controls and what they do.

Very simply, a tone control is another variable resistor (pot) that will selectively add resistance to frequencies within the signal to change what gets sent to the output jack.  This means that through the use of a capacitor, the tone control can isolate a certain frequency threshold and send it to ground.  This is one of the cool features of capacitors that are used extensively in audio reproduction and amplification.

As we remember from the above discussion on volume pots, when we turn down the pot, we are actually adding resistance to the signal, which sends more and more of the electrical impulse to ground. This effectively shuts off the faucet.  A tone control works the same way except it doesn't affect the entire signal, it usually only works on the high frequency portion of the signal.  The capacitor is wired inline with the volume and tone and acts as a gate keeper.  The lower the value of the capacitor, the higher the frequency that gets affected.  A common value is either a .022 or a .047 mf or micro farad capacitor.  These values look at the whole signal and essentially say that anything past a certain frequency will get sent to ground.  This gives the effect of accentuating the bass tones more as the tone control is rolled off.

Here is drawing number two showing the simple innards of a guitar.  Notice the same basic scheme as the above simple volume only diagram.  Here, I've simply added a tone pot.  We can see that the red lead from the pickup goes to both the volume and the wiper (middle) lug of the tone pot.  This allows the circuit to utilize the resistance of both pots in tandem.  Additionally, notice that there is a capacitor on the other lug of the tone pot, which grounds it to the pot enclosure and then the pot itself goes to ground.  This allows the signal to "bleed" certain frequencies of the output to ground and reduce high frequencies as the pot is turned down.

So, back to an earlier discussion, does the capacitor brand make a difference?  Does it make sense to purchase a $10 or more specialty cap to improve your tone?  I will say yes and no.  Yes, only if it is a cosmetic change that will superficially help you obtain the desired aesthetics.  No in almost any other situation.

A capacitor is a device that, much like a resistor or pot, is manufactured to certain specifications.  It has a very well defined purpose in modern electronics.  One of which is to attenuate frequency as well as level out current inconsistencies.  Various capacitors have various designs and construction, but almost all of that is based on power handling and tolerance.  Whether you have a paper cap, ceramic cap, oil cap, or any of the others that are produced, they all do the same thing in the same way at the same value.  There is no tonal difference in capacitors of the same value.

However, once again, we get to tolerance.  Some caps have a 1% tolerance.  That means that a .022 mf cap will fluctuate between .02222 and .02178.  These are pretty inconsequential values.  The problem is using a cap with 5% or 10% tolerance, as is the case in some older designs.  A 10% tolerance gives a high end of .0242 and .0198.  These still aren't too bad, but then the .022 cap is not a huge tone difference.  On the other hand, if we look at the other common value of a .047, a variation of 10% is .0047 giving a high value of .0517 and a low of .0423.  This is a pretty significant swing.  This means that what could have been a perfect balance to your ear is just a little too much.  Although not always, many of the "vintage-like" products are designed and sold in their original state, which may have very high variances.  Not to mention, some of the antique components are actually lesser quality and add undesirable effects such as noise and hum and have a very finite lifespan.

Again, before just blindly falling for snake oil, get out your meter and measure these things.  Don't just change parts blindly, but do it with some sense of purpose.  The starting point it to know what you have now and change accordingly.  Just because you paid decent money for a cap at stewmac or all parts or wd or wherever, doesn't mean the part isn't crap.  Electronics is not black magic and it's a very well defined science with very reliable parts and components that are very cheap from real suppliers.  Don't buy the hype - get good information and know why you are doing things.

Hopefully this gave a little overview of the first part of the chain of tone and why things work the way they do.  In the next installment, I will cover the guitar output, the cable, the input jack, and the input stage of an amplifier.  Should be fun and exciting.

Making the Carvin MTS3200 suck less

A few months ago, I purchased a used Carvin MTS3200 from a friend who had it sitting in his music store.  It had been there for a while and he didn't really understand quite why it wasn't selling.  I played it and thought it was a cool tube head.  After awhile, my Fender Twin was getting a little long in the tooth for what I was playing and the voice in my head told me I needed something with some high gain.  So, after some horse-trading, I called the MTS3200 mine.

I have owned a ton of different amps from different manufacturers and brands over the years, but I didn't have a whole lot of experience with Carvin.  I had friends that owned some and my father had some of their equipment back in the day, but that was about it.  For me, this was really an unknown.

Superficially, the MTS3200 Master Tube Series (also known as the Metal Tone Series by some) has some pretty decent specifications.  It's a two channel, foot-swtichable head with master reverb and presence controls.  Each channel has its own independent tone stack consisting of Treble, Mid, and Bass.  Channel one, the lead channel, is the high gain side of the amp with a volume and drive control.  Channel two is clean-ish with a single volume.  There is no master volume on the amp.  It's a pretty standard design on the front panel.

The specs on the amp list it as a 100 watt model, which is switchable to 50 watts.  It has two speaker cabinet outputs with a 4, 8, or 16 ohm impedance selector.  It has output jacks for a FS22 footswitch, cabinet voiced line out, and an effects loop.  It has a bias adjustment switch to allow the use of either EL34 power tubes or 6L6GC.

Brad, you're selling this pretty good but the title of the post implies a problem …

For being a tube amp, the entire thing is pretty sterile.  It doesn't have a lot of life out of the box.  I know this isn't just mine because after some research online, it's a pretty common complaint and really explains why this amp didn't really fly off the shelf.

It has a pretty respectable clean channel that takes effects well when they are thrown right into the input.  It is not a Fender clean by any stretch of the imagination, but it's competent for most things.  The big problem however is that lead channel.  Although it is a tube head, Carvin made a decision in the mid-1990s to use clipping diodes to drive the high-gain aspects of the amp.  This is an 80's trick that was employed in some pretty big name amps at the time, but it's a technique that is kind of like spandex, leg warmers, and big hair - it's time went and passed and is now better left in the decade of Gordon Gekko.

What this does is takes our nice tube gain and as our volume climbs, which means more voltage passes through the circuit (in a high-level sense), we begin to feed the second gain stage and drive it higher and higher.  This is good and gives us that ooey gooey tube goodness we love so much.  But then the current gets to a fever pitch when our drive is fully saturating the circuit and BOOM - clipping diodes work their solid state magic to block our distorted climax.  What should be an awesome sample of audio orgasm really turns into a cheap dirt pedal in the chain.  The sound becomes brittle and thin and that's just no good.  So what can be done about this?

Mod Part One

Disclaimer:  I am a crazy fool that has no problem poking around inside tube amplifiers.  However, the voltage inside these bad boys can kill.  There are big capacitors that are designed to do nothing more than get, hold, and release filtered power to the rest of the circuit.  Because of this, they hold onto power for a long time.  If you don't know what you're doing - don't do it.  Do not say, "But Brad told me to do it!" or have your next of kin try that either.  I'm crazy, but I will only play around with cold amps that have been properly discharged of voltage - I recommend you do the same.

I did some research on other mods people have done and came to a few conclusions.  We can see, in the image above, the path from V1.A to V2.A is pretty straight forwards.  We have a 47k resistor at r11, a .0022 400v capacitor at C14, four 1n4745 diodes at D1 - D4 that are in the path, and then pin 7.  Some people have approached this by simply removing D1 - D4, which creates a direct path from C14 to pin 7.  The problem is that within this design, the diodes act to regulate voltage hitting pin 7.  Simply removing them isn't the best idea.  The goal is to maintain the design integrity of the amp design while getting rid of these diodes.

In the picture above, this is the section of the board being worked on.  In the center of the picture is socket V2, which houses both V2.A and V2.B.  I've circled and marked pin 7, which is easy enough to find simply by looking at where the "1" is on the board and counting around, clockwise, until you reach pin 7.  Directly below that is C14 and directly below that are the four diodes (D1 - D4).

A voltage divider to the rescue.  For this, I removed D1 - D4 and installed a 150k resistor from the pin 7 side of C14 to pin 7.  This wasn't too bad, however be warned that the board will have to be removed from the chassis to do this neatly - especially C14.  Doing this gave me one half of the divider, which created a resistance between the two points.  To complete the divider, I added a 150k resister from pin 7 to ground.  For simplicity, after desoldering the diodes, an astute reader may notice D4 and D2 connect to ground, which can be reused in this modification.

This design works pretty well and is clean.  It doesn't eliminate the diodes and the attendant design implications they have, but it replaces them with pure resistance.  Part one of the mod is done, but there's a second part.

Mod Part Two

Many people who have modded this amp have reported delay and various oscillations in the gain stage when trying to eliminate the diodes.  Honestly, there are a thousand and one things that can be affected in the process and it's not my place or inclination to track them down.  While I could do the math, quite frankly, I don't really want to.  That being said, the most reasonable fix is to stabilize this adjusted current in the next stage, which is really the other half of the V2 12AX7.

Here, we can see the next stage, which essentially runs from after our first voltage divider, through the tone stack of the lead channel and then into the next tube.  Don't get too confused, we are still working with the same physical tube, but it's the other half of it.  Our first mod removed the diodes and replaced them with a voltage divider.  This means that while we got rid of the solid state nasty, we did modify the amount of voltage winding its way through the tone stack and into the next tube.  Obviously, this will change the tone profile of this channel, so we need to compensate.

Another voltage divider to the rescue.  For this, I simply lifted the ends of C22 and R17 and put in a 220K resistor between them and pin 2.  Note that pin 2 and C22 are across the board from each other and will require some tracing of the PC board leads to really visually see the path.  There are several jumpers spanning the distance and it may require some creativity to get it taken of.  My specific PC board was Rev C while the schematic posted is Rev E.  My portions were mostly the same, but to the reader, be wary.

In line with this first resistor, I put in a 680pf Cap in parallel.  This serves to attenuate some of the higher frequency response and bleeds off some of the brittle-ness of the overall post-tone stack signal.  This is really a preference thing for me and can be changed up to dial in the response by simply changing out values on capacitors.

Much like the previous voltage divider, I then inserted another 220k resistor between pin 2 and ground. Again, while the schematic shows some distance between the stages, this is the same tube socket, so those convenient diode ground connections are available for use.  It makes things much cleaner.

And that was that.

Once I was finished with the soldering iron, I metered all of my connections to ensure the signal path was showing the expected resistance and then I reassembled.  Happily, Carvin used a whole series of quick connectors to connect the board to ground, other boards, transformers, and switches.  This prevented a lot of unsoldering or fighting with wires, however, I HIGHLY recommend labeling each wire prior to disconnection.  It makes reassembly that much easier.

After reconnecting everything and giving the pots a good cleaning, I reinstalled the tubes and gave her a test run.  The distortion was chunky with a very smooth sustain.  The pots were very responsive and went all the way from a quiet grumble on low settings to over the top screaming at 10.  The bass was a bit more boomy after the mod, so I rolled that back slightly, but all in all it was a success.  My only gripe is that the amp is very sensitive to the guitar running through it.  My PRS sounds like a dream but my Strat, while still rocking, creates a lot of noise on the lead channel.  But after 15 years, maybe it's time to revisit my strat.

I haven't really had time to get to know this amp yet now that it's been retrained.  I'll see how it goes and update as necessary.

Adjusting my 65 Fender Twin Reverb Reissue

Yesterday, I told the story of my runaway reverb unit and how I solved the problem.  Here's a little more about my '65 Fender Twin Reissue.

Since I was 16 years old I've lusted after a Fender Twin.  My father had one when I was a kid.  It was a Silverface 135.  He liked the amp well enough but its heft finally got him and he sold it around 1988 or so.  This was before I really started playing a lot or I would have begged him to let me "buy" it from him.  It took me probably around 24 years to finally get one of my own.  Unfortunately, like so many things I've lusted after in my life, once I had it I realized it wasn't quite what I wanted.

The Fender Twin Reissue is a 2-12 beast of an amplifier running at around 85 watts.  It has two channels, an amazing tube driven reverb unit and tube driven vibrato, and weighs as much as an economy line Ford car.  I learned this when I went down to Nashville for a week and carried that beast all over Broadway - up and down stairs, in and out of cars, on and off stage, etc.  I vowed that it definitely needed wheels … and a roadie.

But, I digress.

The problem I have with the amp is really one of concept.  It was envisioned and designed for a much different musical environment.  This amp is loud.  It will over-power an aggressive drummer and can easily compete with your most self-confident bass player.  All without even breaking a sweat or overdriving the power amp stage.  This made it a great amp back in '65 especially if you were playing a 200 - 400 person club or hall using a Shure Vocalmaster and pushing with stage volume.  Unfortunately, today it's significant overkill.

It's been a long time since I ran an amp without micing it and running through the front of house speakers.  Even the tiny clubs in Nashville all have sound people to mic the amps.  As much as I long for the days of palm rolling my high wattage amplifier and letting it scream like a banshee in heat, those days are gone.  This means that while I love the amp and what it is, to be useful in a modern context, it needs some tweaking from stock.

The first thing I decided to do was reduce the overall power output of the amp.  This is actually a very simple thing to do on this type of design, which is most things Fender from the 50s, 60s, and 70s with 4 power tubes (this also tends to apply to Marshalls of the day as well).  Yank out two of the power tubes either the inner or outers and you effectively half your power consumption.  This brings my twin from 85 watts down to roughly 40 watts (give or take).  The problem is that this circuit was designed to drive 85 watts into a 4 ohm load.  We now have 40-ish watts driven into a 4 ohm load, which is potentially taxing on the output transformer.  I say potentially because given the variance of component design and the tolerances built in, there is a very wide range of allowable values.

As an example, the Fender Twin uses two 8 ohm speakers wired to present a 4 ohm load to the output transformer  (8 ohms / 2 = 4 ohms).  At least it does on paper.  In reality, when verifying the values of my specific speakers, I had one at 6.5 and another at 7 ohms.  This would provide a combined load of 6.75 ohms.  Kind of an interesting play on tolerances, eh?

Getting back on track, something that's recommended as conventional wisdom is to disconnect one of the speakers to provide a proportional load given the reduced output.  In theory, this would shift the load from 4 ohms to 8 ohms.  However, as was seen in my personal example, if I disconnected the 7 ohm speaker, I would only increase my effective load by .25 ohms.  Definitely not the 4 ohm difference  we would expect.  My own opinion is to use common sense with it.  If you have the tools, evaluate your own numbers and make a determination.  Given the differences involved and the tolerances of components, your chances of hurting anything are minuscule (don't come back to me if you blow something up though).

So, I've halved my wattage and gotten closer to making my amp usable in a modern setting.  But, I'm not quite done yet.  My goal is to get a less than clean response out of it.  No, I'm not looking for JCM900 high gain madness.  All I want is a little growl when I push it.  Unfortunately, even at half wattage, the amp is still kind of clean for my tastes.

Here's a couple things I did to get it closer to my range of liking.  Some people may be comfortable with this and others may not.  It works for me and I'm happy with taking a risk.  Use your own judgement and discretion.

The cool part about the Fender Twin is that it has six preamp tubes.  Four of these tubes are 12AX7 while the other two are 12AT7.  While I'm not going to get into a whole dissertation on tube theory, I will say that these tubes are substantially similar.  They do different things and operate differently, but changing things out can make for some interesting combinations.

The first thing to understand is how the classic Fender circuit works and how there are subtle types of influences the two seemingly distinct Normal and Vibrato channels have on each other.  If you've ever seen a classic bassman or an old Plexi using the patch cord between the inputs, you kind of understand. The twin is very similar.  There is excess signal that is "bled" off into the other unused channel even when the volume is off.  This design allows the amp to stay as clean as possible for as long as possible. My goal is to drive through the preamp and hit the power amp hard to coax out some of that sweet, sweet overdrive.

Since I use the vibrato channel (I like reverb), the simple way of removing this bleed over into the normal channel is to remove the 12AX7 from socket V1.  No tube, no channel.  No place for the signal to go but forward.  So, what do we do with that AX7 I just pulled out?  Well, let's put it into V6.  This is the phase inverter.  Replacing the AT7 with an AX7 reduces the amount of headroom in the amp before it begins to break up.  Essentially, the AX7 is a tube that provides a lot of signal amplification therefore it absorbs current and increases it.  The AT7 does the same thing just not as much.  The AT7 will take the volts passed by the preamp and just keep taking them while providing a nice, steady current out at a moderate pace.  The AX7 takes the same current and hits everything downstream that much harder.  This means it takes less signal from the upstream stages to produce much more downstream current.

To recap, I've reduced my overall wattage, created a much stronger preamp signal on my preferred channel, and I've changed up a tube in my phase inverter to drive the amp harder at the same relative volumes.  All of this together means that my formerly very squeaky clean Twin is now a bit grittier and can get just a tad rough at a realistic stage volume.

Don't get me wrong, this amp is not, nor will it ever be a screaming dirt monster like a pegged Deluxe, Plexi, Mesa, or Soldano.  It is what it is and will always have that character, but it's nice to let it flex its wings a bit and let its hair down.

The case of the runaway reverb

Last year, I picked up a new Fender Reissue '65 Twin Reverb for a gig that I started with a band.  I was looking for that classic high-power, clean, make your ears bleed and empty out your sinus cavity Fender sound.  The twin was the ticket.  It served me well down to Nashville and back - I'm sure my chiropractor loved my decision as well because that beast is heavy.

Anyway, a few months ago, I discovered when I turned up the reverb control past 4 or so, I would get a high pitched whine.  I thought that it was funny but I chalked it up to a problem with tubes or whatever.  However, shortly after, I put the amp away and began playing with some of the other amps I own.  It was only recently I decided to put it up on the bench and figure out what was wrong.

In the course of troubleshooting, I went through a few different methods beginning with the easy and ending with the slightly more difficult, but still easy.

I first checked the tubes.  One of the problems that can often appear in a tube amplifier is a microphonic tube.  This is a condition where the tube actually becomes a sympathetic input source based on external vibrations.  In simple terms, it's a case where the tube vibrates because of something happening (noise from the guitar playing through the speakers, cabinet gets bumped, vibration in the floor, etc.) and the tube begins to inject its own noise into the circuit.  This can often be a bad thing if it gets out of control, but can also be manageable if it's not too bad.  The easiest way to check for a microphonic tube is to tap it with a pencil lightly while the amp is on and running.  It's pretty easy to tell if something is amiss.

Happily, the tube was ok.  In fact, all the tubes were fine in this regard.  So I moved on to a different test.

The Fender Twin Reissue is loaded from the factory with 4 - 6L6 power tubes, 4 - 12AX7 preamp tubes, and 2 - 12AT7 preamp tubes.  The 2 12AT7 tubes are used in V3, which is the Reverb driver / send position and in V6, which is the Phase Inverter.  A quick test I also performed was to swap the AT7 from V3 with the one in V6.  The logic is that if there was something else wrong with the tube, it would have changed the behavior of the amp.

After doing this, the reverb still whined in the exact same way.  So, I moved off of playing with tubes and tried the next thing before cracking the chassis and fiddling with scopes and meters and pc boards and all that fun stuff.  I unplugged the reverb tank from the chassis.  Problem stopped.  The new problem is that the Reverb also stopped, so while it stopped the symptom, it wasn't a cure.

While there are other things going on that I will not go into in a simple blog post, here is the troubleshooting chain of events.  I played with tubes that send the signal to the tank, which didn't do anything.  Next, I moved to the tank itself.  After that, we have to get into other components such as the reverb signal return (V4) and the controls, transformer, and everything else that deals with the circuit itself.

For those of you who haven't seen the workings of a reverb tank, it's a pretty simple device.  It basically gets a signal from the amp on one side, runs the signal through two springs (or more), which vibrate and modify the signal, and then return the modified signal back to the amp to run through the rest of the preamp and power amp and then on to the speakers for the enjoyment of the listener.  Similarly to the tubes, the reverb tank can become microphonic and vibrate sympathetically with external vibrations is it does not have enough isolation from the cabinet.

I pulled the reverb tank from the cabinet and set it down outside the tolex bag on the bottom of the cabinet.  This helped.  The control got up to around 7 before the whine started.  This was definitely a step in the right direction.  I then suspended the reverb tank above the cabinet using shock absorbing material.  Cranked the reverb to 10 and voila - no runaway whine.  Problem isolated and solved.

In the lean six-sigma world of Fender manufacturing, although they sell a reissue of a 1965 amplifier, it's fair to say that any resemblance to the original is simply cosmetic.  It has a black faceplate and a clone of an antique circuit, but it's heart is truly one of manufacturing efficiency, cost per unit, and maximized profit.  As such, its padding to isolate the reverb tank from the cabinet is a very simple piece of corrugated cardboard.  Over time, with travel, changes in temperature and humidity, and use the cardboard stiffens and loses its ability to buffer the external vibrations.

I've seen some people recommend placing the tank on a soft towel.  It accomplishes the same thing, which is isolating the tank from external vibration, but I chose to handle it slightly different.  I used two pieces of foam provided to me by two dollar store foam paint brushes.  I installed these at both ends of the tank by attaching them to the original cardboard.  The advantage is that the foam does not have the potential to interfere with the springs if the amp gets jostled or otherwise moved aggressively, as often happens when traveling.

I reassembled everything by setting the tank on its isolation board and sliding it back into the tolex bag. Probably the most difficult part of this operation was finding the screw holes under the tolex bag.  I then played it and made sure the problem didn't reappear.  Everything checked out so I reinstalled the back panels and once again have a happy Fender Twin Reissue.