The Lightning Project
This project was essentially a clone of a Matchless Lightning. The Lightning
is a straight-forward amplifier. It is an all tube design including the
power supply rectifier. Cathode bias operation is utilized for the output
stage. This amp is somewhat similar to a Vox AC15, using a pair of EL84s
in a push-pull configuration. It is called a Class A design that uses
no "negative feedback". Still, the little sucker weighs in
over 40 pounds.
Be sure to give this amp a good audition. |
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Design
An all tube design, it uses 5 tubes:
- 12AX7 triode in the pre-amp that are paralleled input for more gain,
less noise.
- 12AX7 triode in the 2nd gain stage configured as a cathode follower.
The preamp operates at same rail voltages as output stage for cleaner headroom.
- 12AX7 phase inverter
- EL84 in the "Class A" output section. The output section tubes
are biased "hot" so you have to turn off the amp if it is to be
idle for long as there is no standby switch. It has an 8 ohm output jack
for external cab if desired. EL84s/6BQ5's are biased high with 100 ohm screen
resistors (approx. 12W bias dissipation). The output is designed to run into
4K impedance instead of 8K so the tubes operate in a different range of the
curve (load mismatch is a design intent). 4K impedance accentuates the treble
and provides higher output with more distortion.
- Tube rectifier with an iron core inductor choke to smooth out the power
supply. I used a 5UGB rectifier and not the 5AR4 in the schematic.
Schematic
The controls are basic (2) inputs, Power On, Volume, Bass, Treble, Master
Volume. The tone controls are highly interactive and capable of a wide variety
of tone and texture.
The design employs Active Tone Controls. This requires a second 12AX7 to the
preamp, adding a second tone control (for bass & treble), and moving the
tone stack from below the Preamp Volume (Gain) to just below the second preamp
valve. The tone control does not effects the amount of gain, the quality (and
quantity) of overdrive is improved as is the ability to dial in a wide variety
of tones.
The tone controls are interactive. i.e., cranking the bass up past a certain
point will cut treble and vice versa.
Bill
of Materials
Layout
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