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previous: pumpstein I just thought of another possibili... -- 8/31/2000 10:22 PM View Thread

Re: Help with motorboating?

9/1/2000 8:08 PM
Re: Help with motorboating?
Well, last night I did a little troubleshooting…unfortunately when I checked here for responses last night at around 8pm (out here in Seattle), you guys hadn’t posted yet, so I was on my own. : )  
Thanks for the help, though – always appreciated!  
So here’s what I tried…  
First I tried re-routing the OPT pri leads; no luck.  
I unsoldered one of the filament leads and jumpered it in again, turned on the amp, let it hum, then disconnected it. The hum faded slowly, which apparently means the hum is not in the filament circuit. So I checked out the filter caps, and they all seemed fine. I jumpered in a known-good electrolytic in parallel across each filter cap, but no reduction in hum. I de-soldered all the (+) terminals and verified resistance was similar to that of the known-good cap. I checked for shorted and open caps with my little old Heathkit cap tester. The DC voltages I’m getting are very stable, FWIW.  
Next I got a .1 uf @1600v cap and connected one end to ground via a jumper wire. I went through all the signal nodes, seeing which ones would reduce the hum, and found that the interstage transformer (both pri & sec) and the output tubes were all separate sources of hum. Touching each of these nodes in succession revealed that there were different-sounding types of hum being generated.  
This got me thinking that maybe the interstage PI needs to be better shielded. I took a look at the schematic & layout for the Hammond Chord Organ that I salvaged the PI from, and it was mounted on the outside of the chassis, whereas I mounted mine on the inside, between the preamp and power amp. It’s very near to both the tone stack and the output tubes, sandwiched in between. I’m going to try moving it to the outside of the chassis, and see if the hum disappears.  
Hey Steve – re: the 6BQ5 voltages…I was wondering about that, having read here and other places many times that 250-300vdc on the plates is desirable. I have the cathode at 12.6vdc w/a 125-ohm resistor bypassed by 50uf.  
Could you describe in more detail, or point me to a link that might give more details, about the zener method? I seem to remember reading that I would just put as many as necessary/desired in series between PT CT and ground? Is that right?  
Re: the 100uf in the 1st filter position, I’ll try swapping in a lower value and see what I hear. Thanks. BTW, how does that work? Why would this much filtering w/a SS rectifier cause motor-boating?  
Thanks again, Steve and Winnie –  
- P

Wild Bill I wouldn't worry about those voltag... -- 9/2/2000 7:26 PM