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Old 30th November 2012, 09:03 PM   #31
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DeathRex: I'm using JJ GZ34, and if I hook up the feedback then the left channel gets crazy loud hum (even without music) and the right channel is very quiet.

DF96: I did, the bode plot looks good up until 17 kHz, then it got kinda funky. However, I don't understand how this could be the problem since it appears to happen randomly and the left channel doesn't exhibit this problem.
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Old 30th November 2012, 09:10 PM   #32
DF96 is offline DF96  England
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I am wondering whether the 'hum' you get on the left channel with feedback is actually LF loop instability. Have you checked that it is actually at exactly 60/120Hz? Sorry if this seems a daft question. Also, not accidental positive feedback.
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Old 30th November 2012, 09:16 PM   #33
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I haven't been able to check anything when the feedback is connected for that channel. It is so loud I have to turn it off and I don't have a resistor box handy to load the output.

Also, I'm worried something might get damaged since the only measurement I was able to make was at the grids and the voltages were huge. Something is causing that feedback to not work, this is my first time working with F.B. so I don't really know what to do. I calculated those resistor values using Morgan's book, but he didn't explain very well that math. Finally, why would one channel exhibit a problem and now the other? They are (supposed to be) identical.

And it wasn't wiring, since I completely re did it a couple weeks ago. I swapped tubes from sides and nothing.... No idea what to do here.

WITH FEEDBACK:
-Left Channel: LOUD hum/distortion. (All grid voltages are huge)
-Right Channel: Quiet music

WITHOUT FEEDBACK:
-Left Channel: Works perfectly.
-Right Channel: Works but at high volumes you get random violent sputter/distortion/spitting.
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Old 30th November 2012, 09:41 PM   #34
DF96 is offline DF96  England
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Have you checked and double-checked that the left channel has negative feedback and not positive? Try greatly increasing the value of the feedback resistor. At some point you should find that the left channel problem disappears, as you approach the 'no feedback' state. Now does the (small amount of) feedback increase or decrease gain, when compared with no feedback?
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Old 30th November 2012, 09:46 PM   #35
jrenkin is offline jrenkin  United States
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Is the feedback wrong (positive) on the left?
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Old 30th November 2012, 09:57 PM   #36
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How can I check if the feedback is positive? AC Voltage measurement from the positive terminal going to the speaker to ground? Or to the cathode of the pre-amp, (the voltage across the feedback resistor)?
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Old 30th November 2012, 10:16 PM   #37
DF96 is offline DF96  England
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See my post 34. A simple wiring error is the most likely cause of the left channel problem.
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Old 30th November 2012, 10:27 PM   #38
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I'm not sure wiring could be the issue, I redid all my wiring already.

I guess tomorrow what I'll do is:

- Hook back the feedback and increase that resistor value.
- Put grid stoppers for the KT88s.
- Measure if the feedback is positive or negative (still not sure how to do this).

I'll report back tomorrow.
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Old 30th November 2012, 10:35 PM   #39
DF96 is offline DF96  England
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One of the problems with debugging your own circuit/build is that any misconceptions you have can propagate. Checking a mistake against your own misunderstanding when the misunderstanding is the cause of the mistake can lead to the situation where you are sure everything is right, yet it still doesn't work: clear evidence that something is not right.

I have told you how to check whether the feedback is positive or negative. Reduce the feedback, then see if it raises or lowers gain. The normal amount of feedback will either greatly reduce gain (if the loop is stable), or cause oscillation. If the loop is unstable then both positive and negative feedback will cause oscillation, but at different frequencies.
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Old 30th November 2012, 10:48 PM   #40
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Had this happen at diytube: hum when feedback connected, he didn't have a ground reference to the output secondary.

Last edited by DeathRex; 30th November 2012 at 10:51 PM.
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