This is all amazing, you guys!  Thanks for the schematics and tests; it all looks pretty promising.  I can't believe posting this thing on here got such a thorough and careful analysis. You guys are truly champions, and I really appreciate the response!

My main question at this point is how to integrate the preamp circuit with a TDA2030 or 2050.  Would it really be as simple as replacing the original opamp and use the same pinouts?  Is it still acceptable to use 9/18V power?  And will using a newer amp chip alter the tone in any way?  I'm assuming not, as the "preamp" stage has already been resolved, but I'm just curious.  Again, not as thorough an electronics engineering type as y'all, but I'm trying to learn!   :duh

Whoa, that's interesting.  Very similar indeed, give or take a few values.  Interesting that pin 7 goes through a cap to ground; on mine, 7 is deaded.  It also seems that pins 4 and 12 have been swapped.

I think the amp originally used 2 6V batteries in series; they were strangely-shaped terminals, like for a 9V but farther apart.  I haven't seen batteries like it.  There's no input for AC/wall wart.

So, I did a schematic today, but I'm still not sure how the transistor is functioning (e.g. PNP, NPN, FET...).  Can any of you infer from the rest of the schematic?  I'm also still unsure of the value of the larger green resistor; I have it as 220k, 10% right now, but there's another resistor with that value that looks way different...any thoughts?

I'm attaching the schematic (hand-drawn, so kind of sloppy, sorry!) and the key here.

Ah, it seems you're totally right; my mistake...again, haha.  Looking at the 9V battery adaptor, it looked as though the red lead was connected to (-), but after cracking one open, I saw that it doubled back onto the (+) lead. 

At this point, it seems I've been able to identify all the caps, but I still need a little help with the transistor, as well as finding a modern equivalent replacement for the IC.  I did find a datasheet for the TBA810, attached here.

Any thoughts?

Thanks for the taking the time, I really appreciate it!

The circuit works as is with the polarities as they are...why is that?

My mistake; the diode is actually a transistor, and at first glance it appears mostly circular, but has a very small flat edge to it.

Here are pictures of the caps (which I believe now are probably just old versions of ceramic equivalents today.

I'm also including the back of the PCB and a pic with the PCB reversed/superimposed with a few other notes.

Thanks!

Hey everybody - I'm a total newbie; I've had some experience with circuit boards and op amps, but it's been since college which is over a decade now...yikes.

Anyway - my bassist brought a little practice amp from the 50s called a Mighty Atom into our rehearsal space, and I actually love the way the little guy sounds, and I'd like to replicate the circuit.  Using a multimeter, I've been able to identify all the resistors in the circuit, and the axial caps are obvious (their farad rating and voltages are printed right on them, so that's easy), but I'm having trouble with 6 littler capacitors that aren't so obvious, and I haven't been able to find similar looking ones online anywhere.  They're little, thin, square and terra cotta colored, and I'm wondering what the modern equivalent of them would be.  I also don't really know how to measure them; my multimeter claims to be able to measure capacitance, and I was able to get accurate reads on the axials, but I can't really tell if these little guys are just too...well, little, or what.  There's also a little diode in the circuit with "CPMST101" printed on the side; from above, it's fully circular (not a half-circle like some other diodes).

Any ideas?  I'm attaching a picture of the circuit board here, with a couple of the caps I can't identify circled.  I think once I figure out what these little buggers are, I'll be able to reverse-engineer the circuit, and I'll certainly share the schematic once I do!

Thanks so much for any help!

br00klynbear