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.