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Oscillation in tube amps
Oscillation in tube amps
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Old 16th May 2018, 01:01 AM   #161
gorgon53 is offline gorgon53  Finland
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Join Date: Nov 2006
Location: Finland
Originally Posted by DF96 View Post
Adding a small cap in parallel may create exactly the situation it is presumably intended to avoid: a high frequency peak in impedance. Fortunately most electrolytics have sufficiently high HF losses that the peak is not too big, so the unnecessary cap does less harm than it might.

The reality is that because electrolytics are not too great at HF they can damp down the resonance introduced by the extra cap. Normal quality electrolytics don't need bypassing; very low ESR electrolytics must not be bypassed!
Am really glad you pointed that out!
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Old 30th June 2018, 04:19 AM   #162
obseedian is offline obseedian  Canada
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Join Date: Apr 2006
Location: Ontario
I was chasing some spurious Barkhausen oscillation (while testing to near clipping at 4 ohms) in my new amps and discovered that the driver signal to the power tubes was not balanced at high frequencies, even though with the power tubes removed the signal was balanced. I haven't tracked down the reason for this, my guess is magnetic coupling from the output transformer since one side of the push-pull quad is closer than the other. This side showed much lower output at the power tube grids than the side farther away from the OPT.

Previously I had reduced the oscillations by fiddling with the Zobel values across the plate/B+ and CFB winding (which are returned to driver cathodes) and feedback capacitors across the feedback resistors but was not able to completely cure it. After I discovered the imbalance, I added 130pF total capacitance across the driver plate resistor that showed more gain and the imbalance disappeared, along with the oscillation.
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Old 7th August 2018, 06:02 AM   #163
dydek is offline dydek  Australia
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Join Date: Aug 2018
Location: Melbourne
By now I hope you have fixed the problem; from my experience a 100 ohm resistor in control grit of a tube (test each one) would work very well.
All the best ,
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Old 27th July 2019, 06:36 PM   #164
cbaldoni is offline cbaldoni
diyAudio Member
Join Date: Jun 2019
This is while McIntosh MC-60 has small chokes in each output tube plate, with different values, to reduce and/or eliminate the feedback that would occur if the resonance frequency were the same in both sides of a push-pull primary (with same value chokes). Also, if you pay attention, the valves are not physically aligned with the plates/ grids in the same orientation in the chassis, they are rotated 180 degrees to try to avoid the external coupling effect. This last one can be tricky to identify with a cold amp or running at civilized power level, but it can be clearly seen when you are prototyping your amplifier with all the wiring flying around and your output tubes are too close and somehow aligned to each other. Keep them at least 3" far from each other if you can. If you are running into problems using a single-ended, maybe the issue is the tube itself due to poor internal construction or it's worn after a long lifetime use.
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