Trainwreck Blues tube amplifier for Guitars

Posted on Jun 26, 2012

This revision reduced the signal level and treble response in the preamp for a smoother sound in both cathode and fixed bias modes. The power supply was also revised for lower B+ voltages to the EL84 plates. `This amp was built on the chassis of an Electar Tube 30 amp, which is advertised as a 20 watt Class A amp.

Trainwreck Blues tube amplifier for Guitars
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This variation uses a crossline 150k/1200pF RC network after the PI coupling caps to reduce the AC voltage on the EL84 grids and to cut some of the highs. This variation uses zeners across the EL84 grid stoppers to limit the AC voltage on the grids. The optional 100pF caps across the 100k * resistors add an LPF effect triggered by the signal level. This is the final version of the Blues Express amp. The zener limiting circuit was removed and a modified cross-line MV was installed in its place. The bias/grid load resistors were returned to their original value of 220k. The 2nd stage supply was rewired to B+5 instead of B+4 and the 0.22uF/630v poly cap from B+5 to ground was drawn in. A center-off DPDT switch was used to toggle the grid load of the 3rd stage from 56k to 82k in the two "blues" modes, while dropping to 45k to cut the gain a bit in the "Fat" mode. The switchable conjunctive filter was used to smooth out the response of the amp, making the 250pF caps across the 2nd and 3rd plate resistors unnecessary. With the .022uF and .033uF caps in series the net capacitance is .0132uF. The Smooth switch is a local feedback loop between the plate and grid of the 2nd stage. The traditional value of 22M allows for a compressed sound while the alternate value of 44M has a more subtle effect.

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