1940's vintage amp - Photos of guts added

aja · 2010-07-04T21:35:48+00:00

I recently bought this amp. It was made by the "International Projector Corporation" Model NP-20 and is built into an old school wooden suitcase with leather corners and metal clasps with a 12" magnavox speaker complete with transformer bolted to the speaker chassis and weird copper mesh speaker grill. Now, according to the owner from whom I bought the amp, there were a pair of these amps at a local cinema for many years. One day, one of them was stolen and the cinema owner decided to upgrade the sound system altogether and sell the remaining amp. It was then shuffled around/used as a P.A./stored/used by muso's and finally traded to a music and english teacher who sold it to me, as well as two nylon guitars to Keira. It sounds fantastic: surprisingly loud and articulate, yielding huge tonal changes with small adjustments of any of the knobs. It just oozes character. Strangely, the bass knob controls bass, but the treble knob seems to control the treble and midrange at the same time. The more you crank the volume, the more bass you get, until the whole room vibrates. Really amazing considering it is such an old speaker. First question: What the hell is this thing? Second question: What should I look at replacing/checking? Apparently it dates to the 40's. It looks like the tubes are the only things that have ever been replaced. Everything is full of gunk and dust. Reason I ask - the pots crackle like Guy Fawkes day and when I switched it on after leaving it plugged in overnight (but turned off) it made some really loud and strange unstable squeaking feedback type sounds when turned on that only went away after I unplugged the amp. Also, it sounds like a fridge (hummmmm) and I hear something like cracking glass when I switch it off ??? However, I like the unique sound - will the amp's tone change if I change caps? Help ? P.S. - what is that ugly little transformer on the right and what on earth is the black stuff that seems to be growing on it? ??? ?

arjunmenon

singemonkey wrote That's not to say the person's necessarily going to be a shredder. You can't tell what area of music someone like that will come from (except that it won't be from classic rock ?). It could be a Jazz player, or a classical player, or a pop guitarist even. But someone out there is going to make a big leap of intuition about how to use the new technology - and that means doing a lot more with it than simply emulating other guitars in a possibly more convenient package.

Case in point, Steve Howe using his variax, Adrian Belew with his signature Parker and John McLaughlin with his Godin + laptop setup.

Karel-Mars

Hey Aja

That is such a nice find! I see the schematic on the bottom plate in one of your photos. How about giving us a scan of it?

I have all the tubes to do an amp like this - I'm thinking of doing a similar amp for myself. It has some parallels with the MASCO MA18 circuit that some people like so much.

Any way; my guess is this is a 6J7 - volume/tone - 6C5 split load phase splitter - push-pull cathode biased 6L6. Those transformers inside is probably chokes. In the old days chokes were cheaper than caps (and caps had little capacitance).

This would make a great amp for harp too.

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