RT 66 Jr. and the Eminence Platinum L12-40A: A Boutique Tube Amp Built Around Tone

Great guitar amplifiers don’t always begin with a schematic. Sometimes they begin with a sound so inspiring that everything else must be built around it. That is exactly how the RT 66 Jr. came to life—a boutique tube amplifier intentionally designed around the tonal character of the Eminence Platinum L12-40A, a premium alnico guitar speaker known for its warmth, balance, and musical response.

This unique speaker-first approach resulted in an amp that delivers rich harmonics, true Class A dynamics, and exceptional versatility for stage, studio, worship environments, and home playing.

Players attending the NAMM Show 2026 can hear this exact pairing in person this week at the Eminence booth, where both the RT 66 Jr. amplifier and the Platinum L12-40A speaker will be available for live demos on the show floor.

Below is the original interview with DR. Z, shared in full and unedited, detailing the design philosophy behind the RT 66 Jr. and why the L12-40A became the foundation of this amplifier.

“The design of the RT 66 Jr. took a different twist from my typical combo amp development.
I normally design and build a chassis install it in a combo cab then test with a variety of speakers till I find the one that sounds special.

In this case I found an amazing speaker the Eminence Platinum L12 - 40A and designed an amp around this special sounding Alnico speaker.
I thought what output tube would best drive this warm, rich, smooth, and balanced speaker, the KT 66 immediately came to mind.
The KT 66 is the featured output tube used in McIntosh high end stereo’s to the defining tones of the Marshall Bluesbreaker amp.

The basis of this design is from my award winning RT 66 amp, this amp received the first Editors Pick award from Guitar Player Magazine in January 1999.
So I reviewed this very popular design and decided to update it to match and enhance the L12-40A speaker.
I used the exact EF 86 pre-amp design, but altered the output section to be True Class ‘A’ operation by utilizing a single ended KT 66 tube.
Single ended amps impart a full unique harmonic response that is unequaled in any Class A/B push pull amp, and worked so well with the L12-40A.

I enhanced the new RT 66 Jr. by adding a build in Attenuator that varies the amps output from full 12 Z Watts, to 6 watts, and down to 3 watts effectively producing three amps in one.
I also added a great buffered Effects Loop which players now find to be a standard feature on amps not an option as in the past.

The RT 66 Jr. will work on stage, in Worship applications or studio recording, as well as at home for low volume rocking.
The combination of the RT 66 Jr. and the Eminence L12-40A speaker is a match made in heaven.

Stay Well,
DR. Z”

The RT 66 Jr. stands as a rare example of intentional amplifier design—where circuit topology, tube selection, and speaker choice work together as a complete system. From its single-ended KT66 output stage to its built-in attenuation and modern effects loop, every detail serves musicality first.

For players, builders, and tone enthusiasts attending NAMM 2026, the opportunity to hear this amp and speaker combination live at the Eminence booth offers something recordings simply can’t replicate: the feel, response, and harmonic complexity that only a true Class A, alnico-driven design can deliver.

If you’re on the NAMM show floor this week, this is one demo worth plugging into.