Lunchbox – 1W Push/Pull 12AU7 Amp from Adam’s Amps


Internal L-Pad

One more small addition – I added a switchable 6 dB of attenuation to the internal speaker. The L-pad is built right onto the switch which enables it, on the rear panel.

Hearter Switching

The Lunchbox has one more feature which doesn’t appear on the main schematic, and that’s a DPDT switch to switch the heater wiring to support either the standard 9A basing (for 12AU7, 12BH7, etc…) or 9AJ, which is the basing used for the 6CG7 (9-pin version of the octal 6SN7) and the Russian 6N1P tube. 9A and 9AJ are identical except for the heater. 9AJ supports only a 6.3V heater (pins 4 & 5), with pin 9 connecting to an internal shield between the two triode sections.
With this switch & adjustable cathode bias, the output section supports quite a range of output tubes. Here are some of the tubes which could be used as output tubes:

Revision 2

Revision 2 came about because of my natural inability to leave well enough alone. I love tremolo, but it’s an effect which, to my ear, sounds best with a good clean tone. And while this amp has a decent clean tone, it just doesn’t have much headroom. My Moonlight, on the other hand, has great clean tone. And with my Moonlight having reverb too, it was just calling out for the addition of tremolo (that’s a project awaiting some time for planning). I figured it would suit each amp better if I added tremolo to the Moonverb and gave the LunchBox another raucous crunch option.
So revision 2 has the following changes:
Tremolo removed
Bright switch removed (to reuse the switch for the channels, below)
6AU6 converted to a gain stage, as a second channel in parallel with the 5879 channel. The amplification factor of the 6AU6 is about twice that of the 5879, so this channel should get pretty raucous.
The bright switch was converted to a switch to use both channels in parallel through either input jack. This leaves the inputs flexible — with the switch open you can use each channel independently, or pan between them if you happen to have one of them cool Ernie Ball panning volume pedals. With the switch closed, you get both in parallel.
The power supply connections for nodes V2 and V3 have been swapped. In R1, V3 fed the PI and V2 fed the 6AU6. With moving the 6AU6 to an input stage, I wanted for filtering on its B+ supply, so I swapped it with V3.

For the 6AU6 channel I’m using the Marshall 18Watt tone control.

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