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Old 10th June 2006, 04:18 PM   #1
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Default Severe oscillation in 6L6 amp

Hi,

I've been building a 6L6 PP amp for the past few months. I finished it a few days ago, checked out all the points for correct voltages, etc. But as soon as I plug in the output tubes and turn it on, I hear a loud buzzing at the OPT's (Hammond 5k 30w). The frequency differs for each channel, and changes if I switch the tubes.

I went into the circuit and saw that with the output tubes in place, I measured an oscillation at the output tubes' first grids (after R9 in the schematic). So I thought that it must be coming from elsewhere in the circuit. I took the output tubes out and measured again at the same point. No oscillation.

Now I thought it must be bad output tubes/incorrect OPT wiring. Re-checked the wiring, and it's fine. Ran the output tubes without the 6SL7 inputs: no oscillation.

Now I'm thinking it must be R15, the feedback resistor. I measure at the output: no oscillation. I don't get it.

The schematic is attached.
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Old 10th June 2006, 04:25 PM   #2
Giaime is offline Giaime  Italy
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Hello!

Try to swap the output transformer secondary connections: swap the grounded wire with the one to the feedback resistor and see if it cures the oscillation.

Something that happenend to me at least once: check if you have connected each UL tap to the screen grid of the PROPER tube. I sometimes swap then and heavy oscillation occurs.

A note about the amp design: please note that C5 isn't absolutely needed at all
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Old 10th June 2006, 04:47 PM   #3
poobah is offline poobah  United States
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Remove R15 temporarily and run with reduced input to troubleshoot. If removing R15 solves the problem, then reverse your leads on the output of the OPT.

Giaime makes a very good point about the U.L. taps.

You need C5 when the amp changes from class A into class B operation.
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Old 10th June 2006, 04:50 PM   #4
Giaime is offline Giaime  Italy
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Quote:
Originally posted by poobah
You need C5 when the amp changes from class A into class B operation.
Just forgot of this. Too into "audiophile class A" kind of stuff.


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Old 10th June 2006, 04:57 PM   #5
poobah is offline poobah  United States
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I HATE oscillators... I have built many... still do!

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Old 10th June 2006, 05:04 PM   #6
Giaime is offline Giaime  Italy
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My last one was just a week ago... did you remember poobah? I swapped the anode caps of the two tubes of a push pull
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Old 10th June 2006, 05:08 PM   #7
poobah is offline poobah  United States
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Yes! And someday, you will try to build an oscillator... for real... and then it won't oscillate!



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Old 10th June 2006, 05:12 PM   #8
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Ok, trying to remove R15. It's very difficult to make changes now because I insulated everything very well (all covered in heat shrink and tape). My father decided to come to my aid and tried to measure the resistance between the cathode of VL1.1 and the hot output. Naturally, he got some strange number (768ohms), and told me to check R15 again.
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Old 10th June 2006, 05:21 PM   #9
Giaime is offline Giaime  Italy
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Quote:
Originally posted by poobah
Yes! And someday, you will try to build an oscillator... for real... and then it won't oscillate!



You read my mind. In fact some time ago I had some defective 555s that didn't oscillate
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Old 10th June 2006, 05:23 PM   #10
poobah is offline poobah  United States
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Well, removing R15 allows you to divide and conquer. Your feedback, through R15, could be negative (good thing) or positive (bad thing - oscillator). This all depends on the polarity of ypur OPT secondary... very easy to get it backwards!

With R15 removed you can rule out other causes of oscillation.

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