0.5W 2 channel guitar amp.

After using the famous Cream Machine by Hughes&Kettner for over a year i decided to completely rebuild it and make a full featured two channel mini guitar amplifier out of it. The only original parts i used are the transformers (power and output) and a few axial 15uF/450V electrolytic caps.

Scream Machine

The original circuit is hybrid solid state / tube using NE5532 dual opamp, one ECC83 and a half of ECC82 tube in the preamp section. The power amp is made as Single Ended triode stage using the 2nd half of the ECC82 tube. The lack of any kind of EQ, except one fixed Speaker Simulator output was really  limiting the number of possible uses. There's enough space for more knobs on the front panel, so, the descision was simple and quick. The Scream Machine was born.

Cream Machine


Design goals:

  • 2 channels: Clean and Drive,
  • separate Gain control for both channels,
  • common 3 band tone stack,
  • serial effect loop,
  • dummy load for the power amp,
  • channels can be changed using external footswitch,
  • Line Output 1 - the path includes the preamp, power amp and output transfomer,
  • Line Output 2 - preamp line out.

Because there was not enough space for another tube and adding one would exceed the power transformer capabilities i decided to use a solid state stages for a few secondary tasks. A MOSFET and FET transistors are used as tone stack diver and linear gain stage after the effects loop. The overall gain of the effects loop recovery stage is set to let the circuit work in his linear region. One dual opamp TL072 works in the effects loop, too. The second one is used to buffer the Line Outputs.
The channel switching is using DPDT relays.


Transfomers:

Output transformer technical data measured on my unit:

  • voltage ratio: 51.1
  • impedance ratio: 21.2k : 8R
  • primary resistance: 848R
  • secondary resistance 2R.

If you are building this amp from scratch, the power transformer should be equipped with the following secondaries:

  • anode voltage: 250-280V AC, 20mA
  • heater voltage: 6.3V AC, 600mA...1A
  • solid state supply: 2x17V AC, 2x100mA

Scream Machine Scream Machine Scream Machine


Schematic (pdf file)


Demo videos / Sound Samples: