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"Cigar Box" Amp
#1
Hello again mes amis.

This cigar box amp is a little jewel! It looks for all the world like it will fit into a Hammond 1590BB enclosure with room to spare.

A few questions come to mind when reading through the instructions:

If I substitute a J201 for the 2N5458, would this work, and, would the resultant character be more gain overall?

I ask because it would seem like two of these in a small enclosure with appropriate switching would make a nice little two channel pedalboard mounted low watt amplifier.

This what I get from reading your books...ideas. Idea

Thank you.
Jim
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#2
Hi Jim

MICRO already has bags of gain available - probably too much for most players - and it is very important to use the control on your guitar called 'Volume' - hehe. Most guys leave it at '10' but here is a situation where using it is important.

The input stage has a small gain of about 3, which itself may be too high already for some guitar pickups like those on a Les Paul. This is why the notes mention changing R17 from 10k to 3k3. The Edge control can boost the gain of the stage to quite high levels, either broadband or just for treble depending on the value used for C8.

The J201 has a transconductance of one-third what the 2N5485 has, so i would expect the J201 to have lower maximum gain but there may be little difference in the MICRO application.

You can reduce the maximum gain of the PA by slashing either trace going to the Blast control and jumpering the break with a resistor. Blast plus the added resistor work against R44 (10k) to set the gain as A=1+(R44/Rnew). For example, if the new R=1k, then maximum gain is reduced to 1+(10k/1k)=11, which is quite a drop from >300. But try it first and hear the sounds possible.

MICRO has a pretty compact layout, so be careful soldering to avoid solder bridges.

Have fun
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#3
Aye on being careful. I am hoping my recent pedal constructions will serve me well.

Question. The kit came with one A25K and two B25K. Is the A just an upgrade to the Retro potentiometer?
Also, rather than cutting a trace, if I mount the Blast pot off board I can wire a switch on one of the two active legs, yah?

Thank you.
Jim
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#4
Hi Jim

Yes, the 25kA should be in the Retro position.

Unless you intend to use the output transistors to partially- or fully-support the PCB, I would not mount Blast off the board. Slashing the trace is pretty easy to do if you try that.

I would suggest just assembling it as stock to play with it and assess what changes you might want to make.Otherwise, there is no foundation or reference to work from.

Have fun
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