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.
Click here to download the full size of the above Circuit.
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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