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unintentional dist.....

3/5/2003 7:03 PM
unintentional dist.....
I've got a problem w/ my latest project.  
Its a standard blackface preamp and poweramp built into an old silverfcae bassman.  
ALL parts are new w/ the exception on the trannies and the OT is salvaged from another amp.\  
The problem is, it sounds ok unless you hit the strings w/ a little intensity the signal distorts kinda nasty. Mostly on the lower frequencies. Its not good sounding either.  
I *think* its coming from the PI. The voltages are kinda different from any schematics I've seen.  
On the plates I have around 285VDC, cathodes 30VDC and the grids 9VDC.  
The OT I am using also came from a 4x6L6 amp and I am only using 2x6L6. I know your suppose to be able to do this as long as you double your load impedance, but I've heard it's still not a good idea because of different inductances and stuff...  
Could this be the prob??  
Anyways, if anyone knows anything, lemme know.  
3/6/2003 2:25 AM
Carl/Zwengel Amps

You've probably got a parasitic oscillation above audio. Is is a scritchy, scratchy kind of distortion that rides on top of the fundamental note? I'd also kind of doubt it's the phase inverter but I wouldn't write it off completely. Most likely it's the power tubes. That's why they put those little ceramics from grid to ground in the silverfaces. If you put those back in or preferably replace them with fresh components I'll be willing to bet your problem will dissapear.  
Zwengel Amplifiers
3/6/2003 2:41 AM
I've had the same problem twice on BF builds. Both times it was an oscillation in the preamp due to bad lead dress. At least, thats how I fixed the problem. Try moving leads around inside, esp. the lead from the volume control ctr tab.  
3/8/2003 7:30 PM

What is exactly "bad lead dress" ? it's the wiring around the potentiometer? Are you talking of the screen of a coaxial wire?
3/9/2003 12:10 AM
Lead dress, as "I" am referring to it, is how the different componant leads and wires are arranged in relation to each other. Some wires and leads don't perform well when they are too close to others. When that happens, you can get all kinds of bad stuff happening.
3/6/2003 3:42 AM
John Culp

Sounds like you need to poke a 'scope around in there.
3/13/2003 8:22 AM
problem solved
I looked over my work one more time and notice that I had the choke before the first cap/resistor filter on the power supply. I fixed this and now it works great!  

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