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Dumble Clone Oscillation Headaches


 
2/10/2004 5:37 PM
Dave B
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Dumble Clone Oscillation Headaches
After two years of gathering parts and weekend building, my Dumble Clone is complete. You know how some builds work perfect the first time and others, even with meticulous planning are prone to problems? Unfortunately the later is true this time.  
 
I'm pretty sure I've got an oscilation problem, posibly in the power amp. Right now I have the clean stage wired directly to the PI with the OD channel tube removed. I can barely hear the guitar at 1/4 volume. If I crank it up to 3/4 volume and strum, I get a very loud ugly distorted tone which dies quickly.  
 
So far my remedies are:  
all Grid wires shielded  
signal paths seperated as far as posible  
move OT to different positions  
 
So far nothing has made a difference.  
Points of note:  
I'm using an aluminum chassis  
(from Graydon Stuckley same as his Dumble Clone)  
 
coupling caps to power tubes are next to power tubes.  
(I have adjustable bias for each tube so there's a bias feed resistor, coupling cap, and grid stopper resistor next to pin 5 of each power tube mounted on a fiberglass board)  
 
Any Dumble Cloners/ Owners that have had similar problems? I have new respect for Mr. dumble and anyone who has sucessfully built one of these amps.  
 
Much Thanks if you can help,  
 
db
 
2/10/2004 6:48 PM
Matt
This is more of a bump for ya than anything. You probably are more experienced than I but based on your description and my limited experience, it seems that there is something else wrong. Something not wired properly or a bad connection or something.  
 
Since you've probably measured all the voltages and they are fine (you did do that, right ;)), check all the places where you wouldn't normally check a voltage like grid wires, grounds, etc.
 
2/10/2004 10:16 PM
Dave B
email

Yes all voltages on the plates and power supply are fine. The fact that I am getting an audio signal thru the amp tells me the signal path is good.  
 
I have repaired amps that have had this problem before so I'm sure it's oscillation. It's alway the most diffucult problem to solve, at least for me.  
 
db
 
2/11/2004 2:11 AM
Heiko Frost
email

Hey there,  
 
Not to sound like Mr. Know It All but I see problems like this here and on AX84 where people always jump to wacky explainations for problems that are hard to find but usually not too odd. I just fixed a similar problem in an amp design of mine that strays into the Dumble camp and it involved a ground not being hooked up in the reverb circuitry. My advice is trace the whole circuit from the preamp to the PI compared to your schematic. I'd venture to guess that something is not hooked up just right or hooked up at all. I hope you find your problem; I certainly feel for you. It's very frustrating to get a new build off the ground only to have it crash for sometime until a problem gets fixed. Good luck.  
 
Heiko
 
2/11/2004 5:43 AM
Stone
email

Hi  
 
I agree to Heiko.  
 
I had a very similiar problem with broken coupling caps (that ones you mentioned at the power tubes) and a broken ground connection another time.  
 
Hope this helps.  
 
Regards, Stone
 
2/11/2004 6:03 AM
Dave B
email

Hey I wish it could be so easy, I'm using a heavy copper wire for a ground buss and the grounds are rock solid. Hooking up a scope to the output of the clean preamp I'm getting signal thru it. From there I'm going straight into the PI and I am getting some audio out.  
 
I've heard oscillation before, trust me, its' oscillation. I'm hoping someone who has built Dumble style amps might chime in if they've encountered this problem too.  
 
I'll be very happy if I have to eat crow, but I don't think it'll be this time.  
 
db
 
2/11/2004 11:17 AM
Two Tone
Did you try reversing the primary leads of the OT,they might be out of phase
 
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