Flot-A-Tone Spy amp mysteries

[Packerswin14]

Okay, it wasn't actually used by spies, but it does have a mysterious box inside marked "U.S. Army Signal Corps".  The amp is a Flot-A-Tone 600, which as far as I can find in my research was built in Milwaukee by some guys associated with Eko.  They built a bunch of these, and none of the cabinets or circuits are quite the same.  It's like they were built with whatever Lenny and Squiggy could get out of a parts bin (note Milwaukee reference).  I haven't been able to find a schematic.  The cabinet looks like a 1959 kitchen cabinet matched with 3/4" plywood, and the chassis is a metal plate that screws down onto plywood.  I have two issues:  One is that it sounds terrible, harsh, and makes the speaker 'fart out'.  The other is that I want to know what the secret mystery box's purpose is.

The Signal Corps box was some sort of mic preamp used in Army radio, I found it on a website of military history, but it's been repurposed-- where the selector knob should be, there's a 12AX7, and it appears that wasn't the only thing changed.  The company has a reputation of building things from odd parts, bet they got this at a military surplus store?  I think the company made a whole bunch of faceplates, but them used them with different designs, as the face area where the two pots stick out of the mystery box has a patch plate thing on it that doesn't match.  Even the knobs are different, maybe that's later, or maybe this box wasn't original to the amp?  I'm going to post pictures of the box, note that it's missing one component-- a 40uf 450v elec. cap went from the terminal strip on top to ground.  I'm not sure which terminal, as the cap was detached (and rattling and shorting-- I hoped that this was the only cause of trouble, but apparently not).

So, anyone have any ideas?  Here's the photo album of the amp: 
http://s875.photobucket.com/albums/ab312/Packerswin14/

[jojokeo]

You may have it correct in that it's a mic preamp box. It's very common to find that these needed shielding to keep stay RF/interferences from affecting the early preamp stage(s) and being amplified further in the following stages. Also, could be for protection depending on it's use/placement?

As for your "harsh/farting" descriptions, do have the ability to "follow the circuit" from input to following stages to visualize what's going on? If so, you don't need a schematic and can make any or all adjustments needed. Going by your description, it's a little too much to give suggestions on what exactly you're hearing. However, the farting out part is usually do to too much bass in the signal and can be minimized easily by changing out a coupling cap or two to smaller values. Also, look at your plate/load and cathode resistors and bypass cap w/ them if they have any? You can fine tune some of these parts and get you more of what you're looking for. You may also likely have to do a power supply filter cap job, as if these aren't good (if they're original then they definitely should be replaced), this should be the first thing to do before the other fine tuning stuff. Lastly, are you sure the speaker's okay too?

[Packerswin14]

Thanks for the quick reply.  I did try a couple of different speakers, and tested tubes and tried new ones, and I changed the filter caps (you can see where I put new ones in with terminal strips to replace the old can caps).  I'll try the rest of your suggestions next.  I wasn't thinking of it being too much bass, that could be it.  I'll do some investigating regarding cathode resistors and coupling and bypass caps.

I'd still like a schematic, so I can determine if someone previously changed something, or if the weird box was original.  Anyone else have any ideas?

[dmckenzie]

I served in the Army Signal Corp and trained at Fort Monmouth N.J. in the early 70's.
There was a Signal Corp Museum there and you might be able to access old tech manuals if you can find a model number of some kind.
Fort Monmouth is located in Eatontown, NJ
Hope you can find something.

Doug

[sluckey]

None of those pictures resemble any military gear I've ever worked on.  I don't think some military gear was modified. Rather, I think someone just stripped a chassis and built something else in it. It should be fairly easy to draw a schematic and then at least you'll have an idea what it's supposed to be today.

[PRR]

The sub-chassis is die-cast, a very odd thing.

It has that cross-shield which suggests VHF radio more than audio.



As for a schematic.... I'd say these guys were too busy building to document anything, and I doubt there's enough of these out there to have many reverse-engineered drawings (you could get lucky).

I would assume it is some dead-classic row-of-12AX7 and a 6L6-pair. Old-hat even in 1959. Look at plate and cathode voltages, grid R-C ratios.

[Packerswin14]

I did find the weird box on a webpage of a military museum, with the correct part number.  It's clearly been repurposed-- there are markings around the tube socket that indicate what the settings were for the original device-- there used to be a rotary switch there, which was seen in the museum picture.  Additionally, the hole with grommet the wires are going through was  I think an indicator lamp if I remember correctly.  As for the cross shield, it does look original, I say that because there's a hole drilled in it with a resistor stuck halfway through, and it seems like it was well-designed, not something someone did with a tin snips.  I'll follow above advice and let you all know if I make any more sense out of it.  My other wonder is if someone added something to the amp later on, and built the additional circuit in the weird box.  Is there a tankless reverb circuit or something I'm not familiar with that someone could have added?  Probably have to give up on the schematic, I've now seen the same faceplate being used at least a decade earlier, as well as somewhat later than this amp.  I've seen old posts of people who said there was a schematic glued inside their cabinet, but I think this operation was too haphazard to expect that those schems will look anything like this amp.

[Packerswin14]

Military-oriented guys, people who used to do tech stuff in the military, the mystery box is "Jackbox BC-366" (or at least that's what it was built as).

[PRR]

The "mystery spy box" is not even a preamp. A large WWII aircraft had an intercom, several radios, and many crewmen. Each crewman could hear intercom or radio, at desired volume, or speak on intercom. Just a switch and a pot.

It was common mil-surplus into the 1960s, someone musta made far too many of them. I recall seeing them, but my surplus shop had better carcasses. The curve on the corner is distinctive.

Looking back, I suspect it is really a commercial thermostat or similar die-cast cover, re-purposed for the war effort.