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...

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.

First Amplifier Project, The Lama Kazu 8SE

It started as an attempt to create something like the Kalamazoo Model 1, but with more gain, better tone control and a master volume.

Summary:

• Single-ended, ~8W RMS (Clean); ~11W (Full distortion).
• Preamp tubes: 2×ECC83. Power tube: 1×7591.
• Speaker: Jensen C8R

Cabinet is FSC-certified pine, finished with Tried & True wood finish (hand-rubbed Linseed & Beeswax product). Grill cloth is Marshall "cane".

(Partly-visible bottom shielding is 6" wide aluminum flashing tacked to cabinet.)

Built in a Hammmond 13.5" × 5" × 2" aluminum chassis box. I forgot to take a picture of the circuit before final assembly, so here is an early image with just the power supply underway, and heatsink hemostats protecting a diode: