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hum/motorboating?


 
12/9/1997 6:19 AM
Ed
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hum/motorboating?
Oh, boy. I thought this was going to be easy.  
 
I'm building a Champ. I thought everything was fine; I finished soldering and checked and rechecked the connections. I plugged the thing in and got no sound at all. The rectifier tube glowed but the other two didn't. (I had the filament wire going from pin 7 on the 6v6 to pin 4 (and then to 5) on the 12AX7. I found another schematic that had the wire going to the 6v6 pin 2 then to the 4 and 5 on the 12ax7. It also had a wire running from the 7 to ground on the 6v6. I tried this, plugged the beast in, and got only a loud hum/vibration. No sound at all when I plugged in the guitar.  
 
I checked the filter caps and they all seem to be going in the right direction. ALL the parts in the amp are new, so I don't really suspect that they are faulty.  
 
Is this a grounding problem?  
 
Am I about to electrocute myself?  
 
Thanks in advance fro saving me from chucking this monster out the window...  
 
Ed
 
12/9/1997 9:24 AM
R.G.
Rule 1 with tubes - if they don't glow inside on the filaments, they don't work.  
 
The schematics with filament voltages going to ground were using the chassis as a filament return - guaranteed to give you hum problems. The filament voltages should NOT be connected to the chassis; instead, the CT of the filament windings should be grounded. If there is no CT, use two identical 100 ohm reisistors to make a fake "centertap" across the filaments and ground the center of those.  
 
Get your meter out. Pull all the tubes out, then use the meter on AC volts and measure for AC volts from the top side of the socket to be sure that there really is filament voltage on every tube. On the 12AX7 verify that there is 6.3VAC from pin 9 to BOTH pins 4 and 5.  
 
Once you verify filament voltages on all tubes, power it up again with only the rectifier tube in. Set your meter to DC high voltage, preferably over 500V full scale. Verify high voltage on the plate pins of all tubes. Shut it down and allow the HV to drain away.  
 
Add in the output tube. You should hear a faint hum or hiss, an "aliveness" from the speaker.  
 
Add in the 12AX7. Should be sound.  
 
Fix the section that you find the problem with.
 
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