Why Re-invent the Wheel?

When discussion turns to new amps designs, the phrase, “Why re-invent the wheel?” often comes up in the conversation.  I heard it twice at the 2015 4 Amigos Guitar Show in Arlington.  This is usually accompanied by a discussion of the iconic, Holy Grail amps of the mid-1950s to mid-1960s, especially the Fender blackface “Reverb-Amp” designs, the Fender tweed Deluxe, and the tweed Bassman, a design which was also the basis for the iconic Marshall JTM45 amp.  A well-known website for finding amplifier schematics actually links to the Bassman schematic as the JTM45 schematic.

With these three amps being considered the epitome of tube guitar amp design and implementation, do we really need new or even different amp designs?  Do we need to re-invent the wheel?

Leo Fender’s preamp.

Leo Fender’s guitar amplifiers set the standard for guitar amps.  Not only are his tweed Deluxe and Bassman amps considered all-time classics, his black-faced Deluxe Reverb and Twin Reverb amps are also classics, the amps by which others are judged.  All of these amps, including clones and reissues, are widely used today.  The same can be said of the famous Marshall Amp designs.

Although today’s guitarists appreciate those tweed amps for their sweet sustain and creamy, compressed distortion, Mr. Fender didn’t intend for them to be distortion machines.  Looking at the evolution from tweed to blackface amps, one can see that Leo Fender was on a quest for more clean headroom, which is what the Twin Reverb is all about – clean and loud.

1959FenderTweedDeluxe-058

Throughout his changes in circuit designs, power, voltage, and headroom, one thing remained fairly constant over 15 years – his 1st stage preamp design.  Leo certainly didn’t re-invent the wheel.  He used stock Western Electric circuits right out of tube manuals, and even paid licensing fees to Ma Bell (AT&T).  In fact, the component values for the little-to-no clean headroom tweed Deluxe and the lots-of-clean-headroom Twin, are essentially identical.  The 100kΩ load/plate resistor, the 25µF cathode bypass capacitor, and a 1.5kΩ to 1.6kΩ cathode resistor.  How does that work?

1st_stage_preamp

The only difference between the 1st stage preamp design on a tweed Deluxe and a blackface Deluxe or Twin is that the tweed amps used a lower gain 12AY7 and the blackface amps used a higher gain 12AX7.  Oh, and one more thing.  Higher DC supply voltage, especially on the Twin.  A tweed Pro preamp had a supply voltage of 250V, while the Twin had 410V!  All other things being equal, more supply voltage lends to more clean headroom.  For example, the 1st 12AX7 in a blackface Deluxe Reverb idles at about -1.3V bias voltage, while in a blackface Twin, this same tube idles at about -2V bias.  That extra -0.7 Volts is an indicator of how much more signal the tube can take before it saturates, 1.4 Volts peak-to-peak (-0.7 x 2) – that’s the extra clean headroom.

This higher voltage rule of thumb also applies to the power tubes, by the way.  All up to a point, of course.  There is such a thing as too much voltage or current, and the key here is “all other things being equal.”  When the values of Ra, Ck, and Rk are the same in a tweed Deluxe and a black faced Twin, the supply voltage becomes the big difference.

So, why reinvent the wheel?

Some people don’t want the crunchy distortion of a tweed Deluxe, or the clean and loud of the Twin Reverb. They may not like the higher noise floor of the tweed amps for when they’re recording.  There are any number of compromises, cons, and drawbacks to any amp.  There are other ways to get clean headroom and smooth distortion characteristics besides higher voltages, including the choice of cathode resistors. Even though the 5F6a tweed Bassman and the JTM45 Marshall share the same circuit diagram, we find that Marshall Amp voltages, and preamp cathode resistor and capacitor values, are quite different than Fender’s, as are the tubes used, and the tone stack component values, all of which help account for the different sound of Marshall amps compared to Fenders.

When I designed the Texas 2:10 special, it was for a friend who played a tweed Bassman clone in small clubs.  It was too loud for him; he couldn’t hit that tube amp “sweet spot” at the low volumes required in these clubs, so he resorted to using boost pedals to get the tone he wanted.  What did I accomplish with the design?  I built an amp with almost no circuit noise that can go from clean to dirty at reasonable volume levels.  An amp that can go from the “scooped Mid” feel of blackface clean to a punchy midrange grit, all without being too loud for small clubs or studio use.

It’s not the end all of guitar amps.  It’s just really good at what it does.  I’m pleased by all the positive reviews I’m getting on it, words such as “amazingly impressive”, “the best amp at the show”, “coolest amps I have ever seen” and “the crisp cleans, and crunchy punch and pop that I’m always looking for.”

Thanks for all the kind words.

-Bruce

Advertisements

Guitarlington 2015 – The 4 Amigos Guitar Show in Arlington Texas

Last weekend, I displayed my amps at my first guitar show.  Besides the obvious marketing aspects, I met some great people there.  Although lots of vendors had amps, especially vintage amps, I was surprised that there were only a few other amp vendors there.  There were many choice pieces there, including some ’57-59 tweed Fender Bassman amps.

Daniel manning the Texas Tone booth

Daniel manning the Texas Tone booth

One of the amp vendors was Brown Amplification from McKinney, Texas.  They make heads and speaker cabinets, but not combos.  Since I only make combos, I sent them folks looking for heads and they sent folks to me looking for combos.

The first folks I met were the team from Wathen Audiophile. They make some really choice speakers and amps, but they were mainly displaying their select line of preamp and power amp tubes, all cryogenically treated with their proprietary treatment and selected to their own very strict specifications.  Each individually serialized tube includes its own laboratory test report for Heater vol, Plate vol, Screen vol, Grid vol, Transconductance, Grid Leakage, Plate Current, Plate Resistance and Gain. They were kind enough to supply me with a 12AX7-WCM that I used Sunday afternoon in my Texas 2:10 Special in the V1 preamp stage.  That amp got rave reviews (more on that later).

In the booth next to ours, between us and Warthen, was RBi Music, featuring FRET-King guitars by Trev Wilkinson . I use the Wilkinson compensating brass saddle Tele bridge on my guitar.  I really like those guitars, especially their JD Duncan Jerry Donahue model (my personal favorite) and a beautiful Elise semi-hollow model.  Rick Taylor and I got along well, and I let them test drive guitars on my amps, to our mutual benefit.

I had an interesting conversation about guitar amps and the music scene with Mark Daven of the Guitar Radio Show. Mark’s a great guy, and was nice enough to give me a shout out is his blog about Guitarlington 2015. Lauraine O’Toole from Avalon Multimedia dropped by for a visit and had some nice words for the Texas 2:10 Special.

I also got to meet Kevin Butts at Killer B Guitars.  I had to do this after Mark Daven brought a beautiful Killer B lefty T style guitar over to play through my amps. This guitar not only sounded great, but it was a work of art.  I had to tell the builder how impressed I was with his guitar.

The Texas 2:10 Special

I had three amps at the show – The original Texas Tone 12, the tweed Texas 2-Step, and the Texas 2:10 Special.

Daniel manning the Texas Tone booth

Daniel manning the Texas Tone booth

Although the tube tremolo on the Texas Tone 12 always warms my heart, and gets good response, on Saturday the Texas 2-Step got the most attention.  The ability to go from single-ended Champ to push-pull Deluxe circuits, and easily moving from clean to saturation, brought lots of positive response from listeners.

On Sunday, however, the Texas 2:10 Special was the star of the show.  Two incidents stand out.  A nice lady with a Tone Forge T-Shirt came by to get some brochures, and told me that the Texas 2:10 Special was the best amp in the show.  Late in the day, a very talented Nashville guitarist named Nathan came by, looking for the star amp.  He said he had heard about the amp (Texas 2:10 Special) and had come by to test it out.  He played and played on that amp, covering a variety of styles quite handily, all the while raving about the tone and responsiveness of the Texas 2:10 Special.  Rick from RBi let him play the JDD and Elise, but most of his playing was on my personal Telecaster.  He was most impressed with the amp, and I hope to get in touch with him again when he comes back to Austin.

All in all, it was a great time, and a great showing for my amps.  It’s always good to get feedback from musicians, and it’s extra special to get such positive response from people for whom music is their livelihood.

Bruce