HexeFX - Scream Machine

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.

It is a quite advanced project utilizing a potentially fatal high voltages. If you are in any way unfamiliar with high voltage circuits or are uncomfortable working around high voltages, PLEASE DO NOT RISK YOUR LIFE BY TRYING TO BUILD THE DEVICE.
USE AT YOUR OWN RISK: AS THE PROJECT AUTHOR I EXPRESSLY DISCLAIM ALL LIABILITY FOR INJURY OR PROPERTY DAMAGE RESULTING FROM THIS INFORMATION! ALL INFORMATION IS PROVIDED 'AS-IS' AND WITHOUT WARRANTY OF ANY KIND.

WARNING! HIGH VOLTAGE

Design:

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 decision was simple and quick. The Scream Machine was born.

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 transformer,
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.

All channel switching is done using DPDT relays

Transformers:

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

Controls:

Clean Volume - clean channel volume
Drive Gain - overdrive channel gain
Treble
Middle
Bass
Master Volume

I/O Front panel:

IN - main instrument input.

I/O Rear panel:

Send - FX loop send,
Return - FX loop return,
Footsw. - TS jack channel select footswitch,
Line Out, preamp line output,
OT out - power amp line output,
Speaker - 8R speaker output, dummy load switched on when not plugged.

Scream Machine