"Cigar Box" Amp

[galejt]

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.

Thank you.
Jim

[K O'Connor]

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

[galejt]

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

[K O'Connor]

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

[galejt]

Well. No luck. The board is the cleanest I have ever built, no slop, no visible solder bridges, cleaned every part with steel wool...and no sound.

I somehow lost a couple of caps (or my foster kitten removed), so I substituted film caps @ C8, C6, C1, a silver mica @ C26 (12p was all I could find) and an MLCC @ C5. Other than 12p vs. 15p all values match.


I get a faint click through the speaker when I plug it in loaded. The battery gets hot quickly. My meter wouldn't resolve any voltage across the 1ohm-1W resistor.

Any thoughts before I order one built?

Thanks. Jim

[K O'Connor]

Hi Jim

You have the output transistors swapped.

The numbers on the packages are very faint - as they do it now with lasers instead of ink - and it also looks like maybe you used flux remover that might have obscured some of the printing on them. Fortunately there is a C still visible which is the NPN device for position QN6; thew A is PNP for position QP7.

You can do a resistance check on each device from E to C and hopefully this will read high - at least not-zero. You may be able to swap them into their correct places and have it work provided you can extract and resolder them without overheating them. I can send you a new pair.

There is also a solder tail or slash on the bottom of the board in the upper right corner. Check what that is.

Have fun

[galejt]

I was going to ask, but hurry hurry is life recently...

I went by the small p and n recessed into the top corners of the devices. I shoulda got an EE instead of a CE.

I have a desolderer. I'll attempt it now. Thank you!

UPDATE: Yoiks! Small board de-soldering is nuts. They are out and both measure significantly non-zero. I had to cut them from below to clear the holes. Locally all I can source is ME105030/31 as NTE 54/55. I think I will pass on those.

I'll check with my usual on-line sources and get back to you. All of a sudden I'm excited again. This is to be a gift for my guitar teacher who bought solar off line housing. He has 12V plugs in his walls?.

Jim

[K O'Connor]

Hi Jim

The full part number is given in the parts list, including what we supply anmd what makes a suitable sub.

When desoldering something like this, it is easiest to add fresh solder, then run the soldering iron tip back and forth across the three pads while gently tugging on the part with pliers. Yes, a brutal method but it saves the holes in the board and the component leads are intact. You can do this with a nurly package like these BJTs have; smaller packages may be sacrificed for the board.

I will send you some new BJTs.

If the resistance is far from zero on any and all lead pairs in both directs, the devices are possibly still good. With an old-style VOM, you will see a low reading one way and a high reading with the meter leads reversed. This is also a way to determine the sex of the device.

[galejt]

Hi Jim

I will send you some new BJTs.

Wow! I guess you will have to. Those things are as rare as hen's teeth. Thank you!

Searches led basically nowhere, except China. Mouser has an MJF15030G/31G combo in the T-220 format (fullpak), but no explanation of the G suffix on the designator.

The intricate minutiae of electronics design!

[K O'Connor]

Hi Jim

Most semiconductors went through phases of production where suffixes were added to show Rohs compatibility and lead-free, so the G is common on ON-semi products and otherwise means nothing here. As long as it is the MJF and not MJE then it is fine. The MJE will work, too, but has an exposed tab that requires insulators both sides or the use of a nylon bolt and mica/thermopad.

Have fun

[galejt]

I did it! Rather, we did it, me and silent encouragement from Kevin.
I overcame my inbred fear of failure and put this together last night. Anticipation makes the day better somehow. Plugged it in this morning and Boom! disappointment, oh wait, the guitar volume was on zero. Fixed that and Boom! IT WORKS! Ha ha!

So far at only 12V it is loud enough to hurt. I turn it down for a nice tweedy clean with well rounded edges. Pairs with a reverb pedal nicely. Speaker is a WGS ET-90, 99dB in a Forte cab.

Thank you again Kevin.
Jim.