Microcontrollers, ho!

One way or another, the next major project will probably involve microcontrollers. A few possibilities:

1. Reconfigurable tube guitar amplifier: An amplifier has a mutable topology that is be at least as flexible as a typical channel-switched amp, but with fewer signal-path components. The microcontroller is there to keep track of valid topological arrangements and operate relays and softswitches. It also mutes out switching sounds as appropriate and allows for timing sequences to prevent damage during switching. User favorites are assigned to a footswitch. It needs an entire amp designed and debugged with manual switching before the controller is brought into the mix.
2. Digitally-controlled analog stompbox: The device is a distortion, filter & vibrato effect pedal. The signal path is analog, but reconfigurable in a similar way to the amplifier above. The MCU controls timing of the vibrato, which can be simple or complex, and set to an actual BPM tempo to match the music. In addition, an envelope sense circuit allows the vibrato to be synchronized to the incoming signal.
3. Digital waveguide synthesizer: Based on the Karplus-Strong algorithm (US Pat 4,649,783), a standalone MIDI synthesizer module, though not necessarily simulating plucked or struck instruments. I'm thinking monophonic, bowed sounds with natural hammered-on slurs.
   

Projects 1 & 2 could use Arduino or one of the tiniest PICAXE controllers which are easy to program, but 3 would require a DSP, which could be integrated with the MCU (into a DSC). The usual microcontrollers don't have the RAM for a wavetable, and don't have the I/O speed to talk to a pro-audio-quality Digital-to-Analog converter. As far as I can tell, only PIC makes a DSC that is in-circuit programmable. I don't know if the PICAXE bootloader would work, or if another bootloader would be needed.

Teaser: Lama Kazu 12PP

It may be a while before concrete evidence of the next electronic project shows up here, but here's a little teaser and progress schematic for the next amplifier, the Lama Kazu 12PP. It will be a 15-ish watt push pull with 6CM6s for the output (they're 6V6s in noval bottles). The preamp will be:

Input -> 6AU6 -> Gain control -> 12AV7 gain stage (DC-coupled between sections) -> Tone stack (Probably BM-Π) -> Volume control -> Sunn/Dynaco style 6AN8 pentode recovery dc coupled to cathodyne.

I'm going to use a gain control that rolls off the bass as drive is increased to smooth out the distortion.

Coming soon: Repairing my friend Ray's Pearl King concertina.

Amplifier Tweaks: Final Lama Kazu 8SE

The tweaks to a simple single-ended amp could be endless. I think this will be the last adjustment to the Lama Kazu 8SE, however. (Note: It wasn't! Some edits to this post were made in October.)


I decided to try a 12BZ7 in the second gain stage/cathode follower. Of course the bias had to be adjusted for that. Not knowing how exactly the 12BZ7 would act at clipping, I decided to add a small (3.9kΩ) grid stopper on the socket.

After using the amp for a couple months with the 12BZ7 installed, I ended up switching back to a 12AX7, but kept the hotter bias caused by the 1.5kΩ cathode resistor I put in for the 12BZ7.

I added a 560kΩ resistor in parallel with the drive potentiometer. This serves to cut the bass response as the drive is increased, and really reduces the muddiness at high gains.

I also decided to go over the whole thing and check my original calculations; it appears I chose the wrong tap on the output transformer when I built the thing. I changed it over from the 2.5kΩ to the 5kΩ tap. It's now actually capable of producing the 8-or-so watts of power I originally expected!

Now with the 'BZ7 there's not a "normal" tube in the whole setup!

Coming soon: Sound samples of everything...

Eric Barbour or Fred Nachbaur?

Recently, I mentioned in on an online forum my idea for a 15-ish-watt amp based only on Compactrons with a 6AS11 preamp and a pair of 6LU8s for the power amp and phase-inverter (each tube contains a beam tetrode and a medium-μ triode). One response was: "You should get a job with Metasonix". I suppose I could have taken offense at the comparison: While I think there is a place (and probably a need) in the world for a guy (Eric Barbour) who makes an amp called the F*%@ing F*%@er [grawlixes mine], that's not me or my style. Also, amusing as it is, I don't think I could bear to build an amplifier that uses 800 watts of power for just 30 watts of output.

I think the Metasonix comparison really came from the idea of using new-old-stock tubes for other than their original purpose. It wasn't Eric Barbour that gave me the idea, though; it was the circuits and writings of the late Fred Nachbaur. Fred's pages were one of my key resources when I decided to move from repairing & tweaking amps (like a Kalamazoo model 1 and a "Blackface" Fender Bassman) to designing & building from scratch. His design philosophy was mostly rooted in classic methods, but he loved to use oddball tubes, especially ones with atypical heater voltages arranged into series/parallel combinations. That approach seemed to border on obsession in some cases: Just look at the power supply for his "Spunky" guitar amp. It's a pretty cool arrangement that eliminates the need for a transformer with a heater winding. A bit of goofiness though is seen in the two 10-watt power resistors needed to set the voltages right, and the fact that there's an entire separate transformer for the bias supply. I guess it works if you have the parts on hand.

In any case, the all-compactron amp is not going to happen, at least not right away... Since I'm on the waiting list for a new Danelectro reissued '63 Baritone guitar, which will need an amp, I'm going back to a previous idea for the 15-ish watt amp. The power section will be a pair of Electro-Harmonix 6973s. Just like when I chose the JJ 7591S as the power tube for the Lama Kazu 8SE, (7591s are rarely seen in single-ended amps— they mostly turn up in old Ampegs) I couldn't bear to use the typical EL84 or 6V6 power tubes that are more common in amps of this size. More to come on that project.

Treble Peaking Filter Online Calculator

A beta-test version of my Treble Peaking Filter Calculator is now on-line (graciously hosted by dianemariekloba.com)! What it does is calculate the resistor and capacitor values required to make a high-pass shelf filter with specified turnover frequencies.

If you use this in the early stages of an amp or distortion circuit, before any overdrive or clipping actually occurs, the concept (and circuit) is essentially that of preemphasis like the RIAA equalization used in record cutting. In amps & overdrive/distortion, the idea is to reduce the bass frequencies that tend to flood the distortion stages.

Somewhere later in the amplifier system there needs to be complementary deemphasis. This often happens in the tone stack, speaker and speaker enclosure.