Saturday, May 3, 2008

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.

No comments: