Custom Amp Build

The Beginnings

I was recently contacted by a guitarist looking far a new amp.  He first asked if I have a Texas Tone 12 with reverb but not tremolo.  He’s gigged with that amp, and was looking for something specific.  I have a reverb amp in my line but it’s a totally different circuit with different characteristics.

Because he plays slide on a Gibson SG in a duo, sometimes playing small gigs, he was looking for an amp that has those Texas Tone 12 cleans and that breaks up nicely when he pushes it a little (Princeton-esque), and that has a sweet reverb.  All the other amps he tried “sound great but the volume I have to get to push the breakup creates too much stage volume for the shows we’re playing,” and that “all my sound guys keep telling me to turn it down.”

We corresponded back and forth, with him telling me what he wanted, sonically, physically, and cosmetically.

  • Ivory/cream like a Texas Tone 12, with black grille, chrome trim
  • A shrunken down Super Reverb
  • Texas Tone 12 was a bit too much volume
  • 3-band EQ
  • Tube rectifier sag
  • Single 10″ or 12″ speaker
  • Princeton style cab
  • Fender brownface knobs, with number on the faceplate

The speaker selection process was interesting.  When browsing speaker websites and specs, they say things such as, speaker ‘A’ has “vintage American tone with punchy lows and warm, smooth, bluesy mids and highs, while speaker ‘B’ has “Fatter tone with more depth,” but the response graphs tell a different story.  Speaker ‘A’ has a much louder treble peak, while speaker ‘B’ was more balanced, with a pinch more bass.  He chose speaker ‘B’, based on the charts – balance, and presence – and I concurred.  We also chose a 10 rather than a 12, due to his choice of thick tone from the guitar.

I took his input, and went to work designing an amp that would fit his needs.

Starting with a Princeton Reverb cab and chassis, and our chosen speaker, I went with the same cosmetics as the “Honeybeard” Texas Tone 12 – Ivory Tolex, bluesbreaker style grille cloth, chrome corners and handle, and added a black faceplate with white numbers and cream colored knobs.  That was the easy part.

The Circuit

Because he wanted reverb and no tremolo in a Princeton Reverb package, I chose an AA1164 “reverb-no-trem” layout.  However, I made numerous major and minor changes and tweaks to the AA1164 circuit, all keeping in mind the desired outcome.

  • Re-voice the 1st gain stage for his Gibson SG.
  • Add a Mid-range control to the Treble and Bass tone stack, comparing calculations and frequency response.
    • Less of a mid scoop than a typical blackface amp
    • Shifting the base frequency of the mid scoop.
    • Shifting the frequency of the Bass control
  • Re-voice the split-load phase splitter for smoother breakup and less raspy distortion.
  • Changes to the output section with grid stoppers and a half-power switch to allow the power tubes to break up at lower volumes.
  • 5U4 tube rectifier.   This, of course, changes the high voltage levels, requiring requisite changes throughout.
  • Other changes non-specified.

I was pleased with the result.  The half power switch along with High and Low gain inputs, allow plenty of flexibility.  Using the Low input and half power yields plenty of warm tube breakup at low volumes, great for more intimate shows  Using the High input and full power allows for opening up the amp for more volume for larger shows with a full band.

The Result

AhmansonSpecial

The design and resulting amplifier seems to have fit the bill!  We all like the amp.  After his first gig, which was with a full band,  he commented, “Tone for days!”.  I simultaneously breathe a sigh of relief and nod a satisfied smile.

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Video of on-stage Low Volume Growl at an intimate concert at The Groove Nashville.

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Low Noise

I’m often asked how it is that my Texas Tone amps have so little hum and hiss compared to many other tube amps, and yet have such sweet tone. It’s by design.

So many tube amps today are descended from amps in the mid-1950s to the mid-1960s. Many builders today make tube amps that are either clones or replicas of those amps. Texas Tone tube guitar amplifiers are not clones or replicas. They are new designs that are inspired by those iconic amps of the past. In those rare cases where we make a traditional amp design, it’s updated for low noise and safe operation.

Why not clones or replicas?

Safety

In some cases, the choice is clear – safety. Guitar amplifiers of the past had two-pronged non-polarized AC plugs. Turning the plug upside down reversed the polarity, which was interesting because there was no way to know which end was up. The danger of this is that the so-called “ground” side of the amp chassis could become “hot” and vice-versa. Tube amp builders made compromises and concessions to this. Two of these were the Ground switch and what became known as the “Death Cap.”

The Ground switch reversed the polarity of the plug (equivalent to turning the plug over, when you didn’t want to turn off your tube amp). I recall that the trick was to get all the amps on stage to the same polarity.  How did we do that?  Hold your guitar strings and tap a microphone. If you got shocked, you needed to flip your Ground switch or turn the AC plug over.  Ouch!

The Death Cap was a capacitor wired to one side of the AC power, or to the Ground polarity switch. It was intended to get rid of hum if your polarity was reversed. The reason it’s called a Death Cap is that if the capacitor fails, then your amp chassis, and by extension your electric guitar, could become hot with 120 Volts AC, a most unpleasant situation. Today, all amplifiers are built with a three-prong AC plug and a power ground, or at least they should be.

Noise

The noise floor of many vintage amps (new ones, too) is quite high. Many guitarists just get used to the noise and hum. I know one who doesn’t like tube amps because they hum and hiss.  I told him, “mine don’t.”  Some guitarists buy noise gates or noise filters or line filters to help reduce unwanted noise and hum. This noise is due to two things – component selection and wire lead dress. A little background may help.

Grounding

Many amps are built with poor grounding techniques, with multiple ground points throughout. This is a recipe for noise, as multiple ground points cause ground loops, a main contributor to noise. Texas Tone amps are built with solid electrical engineering ground scheme to eliminate ground loops and ensure low noise operation.  Well-respected electrical engineer and amp builder Randall Aiken has a nine-point plan for proper grounding.  As he states, “Grounding isn’t rocket science.”  Makes me wonder why some amp builders don’t follow basic grounding rules.

Lead Dress

In the mid-1960s Leo Fender sold his company to a large conglomerate. They immediately instituted cost-savings, partially due backorders of items sold but not yet produced. The Fender General Manager, Forrest White, had implemented a quality bonus program to ensure high quality work, but this was one of the first things scrapped to increase production numbers. Due to the rush to push product out the door, and without the quality incentives, amplifiers were built without proper attention to wire lead routing, called “lead dress”. Poor lead dress can lead to hum, hiss, oscillations, squealing, and generally negative impacts. Rather than fix this, the company changed their circuits to add anti-oscillation components, and changed component types and values to save money. The end results was that guitarists began to seek out the older amps that sounded better.

Components, Wire, and Lead Dress

Texas Tone tube guitar amplifiers are built exclusively with aerospace grade military specification wire. It’s flexible and yet holds its shape, even with right-angle bends, is conservatively rated at 150C (300F) and 600 working volts. I’ve seen some tube amps use 105C or even 85C wire rated as low as 300V. Since most tube amps produce between 300 and 500 Volts DC, using 300V wire is asking for trouble.

I make sure that the wiring inside Texas Tone amps does not induce noise, by ensuring that high voltage wires are kept away from signal wires.  Any time wire paths cross, they do so at right angles and sufficient air space.  Many sensitive signal wires are shielded.  There are lots of current spike and noise in transformer center-tap wires and choke wires.  These are kept away from sensitive preamp nodes.  I’ve seen some amp builders, in the name of neatness, bundle wires together with tie wraps – a sure recipe for noise.

Amps of the 1950s used carbon composition resistors. Why? Because that’s what was available and cheap, not because of some “sound mojo”. Allegedly, and subjectively, carbon comp resistors are said to be somehow warmer sounding.  Huh?  Many of these resistors aren’t even in the signal path!  Carbon comp resistors have notoriously loose tolerances, poor drift and noise characteristics, and are responsible for most of the crackling and frying sounds heard in most old tube amps.  I’ve also tested 220k carbon comp resistors that read anywhere from 185k to 240k!  Not good when the circuit calls for two 220k resistors tied together, or using one as your supply voltage for a gain stage.  Rest assured, the gain and distortion characteristics of a preamp with a 190k versus a 240k are quite different.

Most of these carbon comp resistors are rated at 350 Volts, not a happy situation in tube amps that routinely see more that 400V!  The resistors used in Texas Tone tube guitar amplifiers are known for their stability, high temperature and voltage rating, and extreme low noise, and they don’t drift like carbon comp resistors. Here’s a comparison:

Type Tolerance Temperature Voltage Low-Noise
 Carbon Comp 1/2 Watt  ±10% 125C 350V NO
Texas Tone USA ±1% 175C 500V 0.10 μV/V
Texas Tone 1 Watt ±5% 155C 500V -10x Carbon Comp

Not only are these components low-noise, high voltage, and high temperature, they have tighter tolerances and they don’t drift, so they maintain your sound.  You want your sound to be consistent, night after night, gig after gig, session after session. You want a reliable musical instrument, not one that crackles, pops, hums, and is unreliable.

When you hear a Texas Tone amp, you’re not hearing hum and hiss, you’re hearing yourself at your best!  When you sound your best, you play your best.  Your tone matters.

What makes the Texas Tone 12™ different from a Princeton amp?

Recently, I was showing a new amp build to a friend of mine.  After telling him a bit about the amp, a 14W tweed style combo with tremolo, he rightly asked, “What makes your Texas Tone™ amp different from a Fender Princeton amp?”  It’s a good question.

The Princeton wasn’t in mind when I designed this amp, although I used to play through a mid ’70s Princeton (AB1270 with a 5U4 rectifier), in a band with this friend.  I used the Princeton both in the studio and on live gigs. I eventually sold that amp because I couldn’t get loud enough unless I had my Tele wide open on the bridge pickup, and couldn’t get good distortion unless I was using the neck pickup. Now the Tele bridge pickup wide open is not a bad sound, it’s just not appropriate for every song and every situation.

I often play my Tele with both the bridge and neck pickups together, and I often roll the tone control off a bit, no matter what pickup I’m using.  I like the flexibility.  I couldn’t get either the volume or crunch that I wanted. So, I went home, pulled up a Princeton schematic and did a comparison.  What I found is are some interesting thoughts about amp design.

Leo Fender intended the Princeton to be a student amplifier, using in teaching studios and bedrooms.  Of course, I’ve known many people who have used them on gigs and recording sessions, myself included.  In my opinion, it’s fine for not only its intended application, but for use on small gigs, such as coffee houses, and recording, as long as what you want is clean and bright.

So, here’s a synopsis of the differences between my Texas Tone 12™ and the AA964 black-faced Princeton amp. Compared to the AA964 black-faced Princeton, the Texas Tone 12™ has:

  • Cathode biased power tubes vs. fixed bias. The cathode bias I use is somewhat cooler than a 5E3 (the reference for tweed, low wattage, single 12” combo amps for blues and rock), as I find the tone to be much richer that way, and rich tone is what these amps are all about.  Better living through better tone, I say.
  • No negative feedback vs. negative feedback on the Princeton. NFB reduces distortion and gain, while slightly enhancing treble response by attenuating low frequencies. A feedback loop helps create a cleaner signal that goes into cutoff distortion at a much higher volume setting (which ties in with why I wanted to move on from the Princeton). The cleaner sound of NFB is good for clean headroom (a la black-faced Fender amps), great for country, jazz, and Hi-Fi. I find amp headroom in other ways.
  • A 12AU7 cathodyne phase splitter vs. a 12AX7. The single-triode cathodyne phase splitter is slightly-less-than unity gain (about 0.9), meaning there is no voltage gain (actually a slight loss). The 12AX7 tube triode is a voltage amplifier, while the 12AU7 is a current amplifier, and therefore more suitable to cathode follower/cathodyne applications, and can drive more current to the output tubes.
  • The Texas Tone 12™ operates at a higher B+ voltage, and has a beefier output transformer.  Since cathode biased amps tend to have less output power than fixed bias amps, all other things being equal (which they rarely are), the higher voltage level gives me more flexibility to help recover some of that loss while operating at a more reasonable bias than the hot-biased Fender tweed amps. Besides richer tone, the slightly cooler bias leads to longer power tube life. The beefier output transformer has a richer tonal response and more power handling capability.
  • A Jensen speaker that is both higher end and larger (C12Q vs. C10R) than the one on the Princeton. The result is a louder, fuller tone.
  • A subtle and full range discrete Tone control and independent Volume control vs. interactive Volume, Treble, and Bass controls. Easy, full range of tonal coloration and less insertion loss.  It’s not quite as “bright” as a tweed 5E3, although brightness is not lacking.  I get it back in other ways.

Both amps use a grid bias tremolo, although the component values are different and the Princeton acts on fixed bias power tubes while the Texas Tone 12™ acts on cathode biased power tubes. As far as the tremolo goes, everyone who has played or heard this amp raves about the tremolo.

The combination of higher voltages, a 12AU7 PI, reasonable cathode bias, no negative feedback, a beefier output transformer, and a larger, louder, full-range speaker enable the Texas Tone 12™ to be amazingly dynamic and touch responsive, while the single Tone control allows the simple freedom of varying the amount of warmth or sparkle. When the University of Illinois Physics department did an analysis and upgrade of a Weber 5E3 kit, they found that, after their many mods, “the sound of the amp was great, but in many instances the amp without the feedback loop sounded more interesting.” They go on to elaborate. I concur.

Better living through better tone.