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.

Amplifier Tweaks: Lama Kazu 8SE

The Lama Kazu 8SE was revised last fall; I never got around to mentioning it here until now. The revised schematic is at right. Most of the changes were to reduce the gain, smooth out the overdrive character and clear up some blocking distortion.

• V2 was changed to an ECC832, (JJ's equivalent to 12Dw7) with the high-μ triode at the input and low-μ triode at the driver stage.
• V1 was changed to an ECC81/12AT7.
• The volume potentiometer was changed to 1MΩ and the attenuator resistor was removed.
  
• Coupling capacitors into the tone stack and power amp were reduced to 22nF to drop some of the "mega bass" muddiness.
  
• Cathode bias resistors were changed to 4.7kΩ at the second stage and 3.9kΩ at the driver stage.
• Load resistor on the driver stage was reduced to 39kΩ. This, combined with coupling and valve changes really reduces the output impedance so that grid clipping at the power stage is more graceful.
  
• The 68kΩ dropping resistor in the power supply was reduced to 39kΩ, since the second preamp stage is using more current with the ECC81.

Sound files will be posted eventually. This amp works really well with my Rangemaster clone, known as the "Buffalo Box" and based on an old B175 PNP Germanium transistor I found in my junk bin.

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.