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First question for the new bbs

8/1/1999 8:24 PM
Carl Z
First question for the new bbs
Howdy folks glad to see the place is pretty much back to normal again. and now for the question....  
I've got a jcm 800 over here that's motorboating like crazy and i'm running out of places to look. here's what's happening.  
at lower preamp gain settings it's fine but as i turn the gain up past 7 or 8 it begins motorboating. If I roll off the bass to about 4 it stops. and runs great.  
I've checked all the p.s caps and they are fine as are the bypass cap and the coupling caps. I also tried swapping out tubes and it doesn't make any difference. I'm starting to run out of things to look for as everything looks spot on perfect in the circuit.  
Any comments or suggestions are more than welcomed.  
Carl Z  
Summit Amps
8/1/1999 10:00 PM
Benjamin Fargen

Did you check to make sure the neg. feedback resistor and NFB circuit are intact? Drifting NFB resistor value.....broken or cold solder joint?  
Good luck,  
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8/2/1999 1:37 AM

Oscillation always requires both gain and feedback. The gain gets over the threshold when you up the gain and bass response enough. Since we know that the volume and tone controls are involved, you need to find a low frequency path that involves the circuit earlier than the tone/volume controls. PS and coupling filters are always the first place to look, but if they're OK, you're going to have to track down whatever other path exists. Could be a shared ground, but it could also be a high-resistance shared path to the supply. Maybe the R in an RC decoupler drifted down enough to do it. Something like that.
8/2/1999 7:05 PM
Tim C.

Hi Carl,  
I have run into that before on two separate occasions and it was a wire placement problem. I turned the amp up until it started motorboating and found the culprit by carefully  
moving tone control and phase inverter wires with a set of good insulated long nose pliers. IIRC the volume and tone controls are hard wired on a JCM 800. I recently had a JCM 800 that came wired from the factory with the wires to the tube sockets cut way too long. They had actually stuffed the excess wire under the circuit board and I didn't notice the problem right away. For future reference I would be interested to know what the problem turns out to be.  
Cheers, Tim C., Mr. Tube Amp Repair  
8/3/1999 4:46 AM
Carl Z

The wires to the board aren't too bad. the only ones that are a little long are the ones going th the phase inverter. What's really driving me nuts now is that when I went at it again this afternoon it stopped doing it. However, the drive tone kinda sucks now. The bottom end is really mushy which is leading me to suspect that it's almost surely one of the cap cans. I've got a good mind to just shotgun the whole power supply and be done with it, but I would rather know exactly what the problem is for future reference in case something like this ever comes up again.  
Carl Z  
Summit Amps
8/8/1999 6:43 PM

I have an '85 Marshall 50 watt combo that I am rebuilding that was like that. It has about 8-10 inches of wire going to each tube connection. Really sloppy work and it was really obvious by the sound of the amp, which was terrible.  
8/2/1999 8:56 PM

Carl, I remember reading a thread about this on another board. the tech ended up contacting Marshall and the solution was rewiring the heaters with shielded wire grounded at one end. I think!! Mark
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