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Old 20th September 2013, 12:46 PM   #11
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Quote:
Originally Posted by luixssj3 View Post
I'll try 100k+100pF from anode to anode but this is a guitar amp arrangement...
The amplifier itself doesn't know about its function, it simply amplifies. Guitar, bass, microphone, phono. The input is the same. The design is not too different from another amp.
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Old 20th September 2013, 12:54 PM   #12
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yess but an hi-fi amp is optimized for the highest bandwidth and lowest distortion, this solution can decrese bandwidth...
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Old 20th September 2013, 01:00 PM   #13
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Sure, Ok, but if your amplifier is oscillating at 700KHz, reducing BW to say, 100KHz won't affect audio contents substantially, but it may stop the oscillations and becoming it stable. It is necessary to try some values for R and C, the highest R values and the lower C as possible.
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Old 20th September 2013, 01:32 PM   #14
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no way!!! I've added a RC between anode of 12AX7 but oscillation is still there....

I need an exorcist.
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Old 20th September 2013, 01:44 PM   #15
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As a first crude test, short both anodes. No gain, but to see if the oscillation is in the first stage or after it.
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Old 20th September 2013, 02:09 PM   #16
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Can you just confirm, does the oscillation take place even if the GNFB-loop is disconnected ?
How many dB's is the GNFB ?
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Old 20th September 2013, 02:13 PM   #17
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its the ccs or one of the fets in the first stage.

that is if the problem goes away when the first tube is removed.
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Old 20th September 2013, 02:15 PM   #18
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Perhaps yes, perhaps no. You also broke the loop, so any component after of before the cut can be the trouble.
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Old 20th September 2013, 02:30 PM   #19
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1.- Grid stoppers on 12AX7s , 12AT7s and Gate stoppers on JFETs.

2.- R1 in series with C1 between cascode anodes, start with R1=RL/10, C1=100pF, Variable cap and pot isn't a bad idea.

3.- R2 in series with C2 instead of feedback loop C, start with R2=Rfbk/10, the value of C2 is critical.

4.- Inject a 10KHz square wave and optimize values, add C of about 220nf on output to verify stability.

Good luck !
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Last edited by popilin; 20th September 2013 at 02:43 PM.
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Old 20th September 2013, 02:39 PM   #20
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most likely because the op did not state he made sure the fets match as a differential pair.



but he could see if the oscillation goes away when the ccs and fets is replaced by a 2K resistors to ground but it might be a little difficult cause of the pc board.

substitution method would be better.
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