DIY Champ amp oscillation problem - diyAudio
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Old 16th December 2011, 03:59 AM   #1
dcline is offline dcline  United States
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Default DIY Champ amp oscillation problem

I've just built my first amp from one of Tino Zottola books. A 764 Champ.
I carefully made sure all wiring was correct. He says in the book that the circuit is known to oscillate. And boy does it! I can change the pitch with the volume knob. He talks about supersonic oscillation. This is not supersonic. It is clearly audible. I've tried reversing the speaker wires and that did not do the trick.
He says to put a .005 uF cap across the output transformer and I'm waiting for one to come in. Any thoughts on the matter would be greatly appreciated. I have a scope but still learning how to use it.
Thanks in advance

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Old 16th December 2011, 04:35 AM   #2
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You could try reversing the output transformer primary leads.
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Old 16th December 2011, 04:57 AM   #3
ke4mcl is offline ke4mcl  United States
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you can probably pick up a .005uF ceramic at you local radio shack. if its just acting as a filter theres no need to go fancy.
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Old 16th December 2011, 06:16 AM   #4
Serge66 is offline Serge66  Switzerland
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Any resistor on the control grid of the tubes? The resistor has to be mounted very, very close to the grid tag.
Good luck.
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Old 25th December 2011, 03:24 AM   #5
dcline is offline dcline  United States
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Changing the primaries on the output trans did the trick. Thank you! The little amp sounds really good! Very low noise and has a really nice bite to it!
I'm noticing that when I turn the treble knob up all the way it starts to distort really bad. About 3/4's up to max. Not getting a great deal of change out of the bass or treble. The book calls for mylar tone caps and I used polypropylene orange drops because I had them laying around. I wonder if this could be the issue? Gonna have to read up on tone caps. It also wants a mylar cap for the coupling cap going to pin 5 of the Power tube. Can anyone tell me why this is? Thanks again. Once I get this amp all tweaked, I can not wait to start a bigger one.
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Old 25th December 2011, 04:18 AM   #6
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here you go man, rock-n-roll

3 - 5pf caps please if you have to go a little bit more aggressive 22pF

.005Uf is really hard into the audio range as 220 pf is the biggest I've ever installed as a rejection cap (across the output)
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Last edited by DavesNotHere; 25th December 2011 at 04:29 AM.
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Old 25th December 2011, 05:10 AM   #7
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I want to congratulate you attempting one of the most difficult amps to work nicely.

This amp will sharpen your construction skills.

Pointers that will help you with this build and others tube Pt-Pt stuff

1 . twisty common powers coming of your power transformer. not only you can get to and identify, twisting help lower the electromagnetic interference radiating off the wire.

2. always try to keep lead length at a minimal. same reason as above.

3. try to keep the signal runs perpendicular to the heaters as possible.
lay out the tubes in such a fashion that there is little to no parallel heater/signal runs.


4. try to run power wires (b+, heater, bias) close to the chassis and if possible down the corner same reason above.



5. try to jacket leads of components with scrap stripped wire jackets to prevent short circuits. this amp is going to travel, and its like the old saying "an ounce of prevention is worth more than a pound of cure".

6. use "star" grounding method. jacks, everything that has a ground (even though its mounted to the chassis) run a wire to the one point and that point is bonded ground.

7. establish a bonded ground so that star grounding can turn the chassis as a shield. sand metal chassis clean approximately 1/4" around the circumference of the drilled hole for star ground. clean with rubbing alcohol.

8. when soldering, try to clench leads by wrapping them 3-5 times around terminal eyelets and 2-3 times around socket eyelets. this prevents parts from breaking off their termination during transport.

9. I strongly recommend using solid wire. it will give you the best workability with routing and soldering. Some people have claims of stranded wire, but there lack of proof and solid was and still the standard. The only wire that is stranded should be transformer wire leads if that. One person said to me about black jacket wire is no good. And I told him the only stuff I wire with is clothbound and the wire jacket I strip is white regardless of cloth covering color. So I can not prove/disprove this.

Last edited by DavesNotHere; 25th December 2011 at 05:16 AM.
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Old 25th December 2011, 02:24 PM   #8
dcline is offline dcline  United States
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Dave, I'm not having the oscillating issue anymore. Reversing the output primaries worked. The amp sounds great! Didn't need the .005 cap. Just having issues in the tone section as described. What is the purpose of the 3 5pf caps? I can only assume to block out frequencies? Or is there more to it?
Thanks
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Old 29th December 2011, 04:28 PM   #9
dcline is offline dcline  United States
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Default Still tweaking

I've been re-studying about caps and I am lead to believe that the kind of caps I'm using is not the problem. As I said in an earlier post, everything is now sounding great until I crank the treble 3/4's the way up to max. At that point it starts to distort and break up really bad. Any ideas as what to look for?
I would really like to get this amp tweaked and all the bugs out before moving on to building a bigger one. Thanks in advance.
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Old 29th December 2011, 06:27 PM   #10
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Grid stoppers on the triode and 6V6. Try in the 50k range for the triode and 5k for the 6V6. Adjust up as needed.
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