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SOS...oscillations...please


 
5/5/1999 7:42 AM
jason
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SOS...oscillations...please
I just finished modding a SF princeton reverb to a circuit I got from a fellow ampager. Basically the eyelett board was stripped of all components. All new components were used in construction (with the exception of transformers). The circuit consists of a vox top boost pre amp, a marshall PI, and 6V6's ran cathode biased.  
Now, the problem...  
It has a bad oscillation that is at a frequency of 100Hz. I measured it with a scope and also injected a signal into the amp from a sig gen and when the sig gens freq is set to 100Hz it matches the oscillation freq. With all knobs (vol,treble ,bass, and presence) set to 0 I have no problem. As soon as I turn the presence knob the oscillation starts. It actually seems to change freq a small amount as the presence knob is rotated from one extreme to the other. Once the oscillations have started I can turn my treble or bass pot and cause it to stop, and of course rotating the presence knob again starts it up all over.  
I can pull the pre amp tubes a just leave the PI and power tubes in and the problem is still there. I have checked my grounds, and circuit hook up 1000 times (and will probably do it a 1000 more). I'm going crazy!  
I have searched the archives and found no cure yet. I have taken a new cap (20µF 450v) and clipped it in parallel with all my filter caps one at a time and no luck. Though I did notice that upon power up I read over 500v on my first filter cap(rated at 450v) for a few seconds then the voltage drops to 390 or so.  
Does anyone have any suggestions?  
Thank you,  
jason
 
5/5/1999 8:32 AM
Doc
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Try reversing the output transformer lead connections, either the Brown & Blue primary leads at the 6V6 sockets, OR (but not both) the two secondary leads going to the speaker.  
 
Since you reworked the signal circuit topology, there is probably one more phases reversal than the stock circuit, causing your negative feedback to actually become positive feedback (the makings of an oscillator). The oscillations become prominent above some threshold where there is no longer enough damping due to component & wiring capacitance losses.  
 
That's my guess. Just reverse whichever pair of leads is easiest to get at.
 
 
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5/5/1999 8:56 AM
jason
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Doc,  
Thank you so much for the speedy reply. I have been tinkering with this problem for a few days now with no luck. You suggestion did fix my oscillation problem.  
You are the man!!  
Thank you,thank you, thank you!  
jason
 
5/5/1999 1:44 PM
Mark Buckingham
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What does it sound like?
Now that you've got the oscillation fixed, what does the thing sound like?
 
5/6/1999 8:55 PM
Steve A.
Website
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Re: SOS...oscillations...please
Doc:  
 
    Just a quick question... if a feedback loop is not used, it shouldn't matter how the OT is hooked up- right? (I.e., it is when the expected negative feedback is inverted into positive feedback that you get the oscillations, etc.)  
 
Thanks!  
 
Steve Ahola  
 
P.S. In most BF/SF Bassman heads made after 1964 the feedback loop is connected to the INPUT of the PI (not to the cathode circuit). When converting such a head to a more traditional long-tailed pair be sure to switch the polarity of the feedback loop as outlined in your post.
 
5/5/1999 9:44 AM
Bruce
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Jason are you using negative feedback from the OT with this amp?  
Bruce
 
5/5/1999 11:30 AM
jason
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Bruce,  
yes, I am using negative feedback on this amp. I got the info on the mod from your buddy Bob S.(the vox guy). He suggested a 56K feedback resistor. Does that sound like a problem? After changing the two wires on the 6V6 plates as Doc suggested everything seems to be okay except the presence control is not doing anything. I'm still checking the circuit over and everything seems to be connected correctly. I'm still a little concerned about the 500v on the 450 volt caps at power up.  
Any thoughts?  
Thanks,  
jason
 
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