Bogen PA--->Guitar Amp, few questions

[Tom Bavis]

The 6550 needs more drive - it will work, but may not produce full output without distortion (from the driver). Grid resistors should be lowered to 100K or less, and bais supply may not have enough range to bias them correctly. Other than that, it's a drop-in.

8417s will COST you though... The Sylvania (Philips) ones are supposed to be best... lower the grid resistors to 100K if you have GEs.

There's no Sams folder for CHB-50. The schematic you have should be close for the front end. Power amp section is simple enough.

[DigitalJunkie]

I once had a MO-100A (quad 8417) on the bench. For giggles I measured the primary impedance of the output transformer and found it to be about 4.5k.

It looks like the M120 used a quad of 8417's also.

[Turbo7MN]

Guess what came today!

Got some more pics:

[Turbo7MN]

Pics of the GE 8417s. They look to be in good physical shape, haven't tested them yet though.

What's the best way to test the amp? Should I put it somewhere where it won't harm anything, plug it in (non-polarized two-prong plug by the way), switch it on, and see if the tubes light up? Do I have to have a load on the output terminals if I don't put any input signal through it?

[Turbo7MN]

Another angle

[Turbo7MN]

I got a bit of a surprise when I pulled off the covers on the preamp tubes to discover a pair of USA-made Raytheon 6EU7s. Very rare tubes.

[Turbo7MN]

The Raytheon fun doesn't stop there. I then proceeded to discover an even rarer Raytheon 7247, even a google sarch turns up practically nothing on these.

[Turbo7MN]

Lastly, a top view of the whole thing. I'd get a picture of the wiring but after looking in through the sides I'm not all that anxious to open up that mess until I know exactly what I'm doing.

[rickl]

Originally posted by Turbo7MN

What's the best way to test the amp? Should I put it somewhere where it won't harm anything, plug it in (non-polarized two-prong plug by the way), switch it on, and see if the tubes light up? Do I have to have a load on the output terminals if I don't put any input signal through it?

What I've done to bring up old, unknown amps is to:
search for schematic, if not found, trace out and write it down
put an old car speaker on the outputs
pop the lid and connect a dvm to B+
short input
bring up *slowly* on a varaic (say 1-2 hours to get to 115vac)

measure voltages at *all pins*.

make sure input (grids) are at 0vdc, cathodes are biased correctly, power on tubes (plate to cathode) isn't too far out of spec.

Once I feel good about these, plug in a "disposable" mp3 player and see how it sounds.

rick

[Turbo7MN]

Wow, haven't updated this in a while. It did fire up nicely, I did have a problem to begin with that turned out to be a bad speaker wire (yeah, ran it with no load and it's still fine ) but I've been playing through it for most of the week with the insides still original for now. I ghetto-rigged some conectors to plug in the guitar and speaker cab.

I'm not sure I even want to mod it. I was expecting something that would be pretty clean until I modded it, but the disortion this thing is capable of is insane. I can't really get a clean tone out of it, but I can get pretty nice heavy metal tones without any sort of pedal.

The thing that concerns me is that it seems unnaturally quiet. I'm playing it through a Peavey cab with a 12" and 10", and it's about loud enough for a practice amp in my room, but if I were to take it on stage I would almost certainly have to mic it. I know it can't possibly be putting out the full 60W. Anyone have any ideas?

-Darren