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Oscillation in tube amps
Oscillation in tube amps
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Old 13th January 2015, 02:18 AM   #31
JamesG is offline JamesG  United States
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I usually ground the shield at one end only at the jack.
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Old 13th January 2015, 02:30 AM   #32
lordearl is offline lordearl  Australia
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Ok - how do you seal off the shield at the other end, just with some heatshrink?

The amp I am modifying had the shield connected to the grid but separated from the signal via a 100k resistor. Since I've removed this the amp is oscillating rather wildly!
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Old 13th January 2015, 02:34 AM   #33
TonyTecson is offline TonyTecson  Philippines
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Oscillation in tube amps
grid stopper can help here, install the resistor within 1/4 of the tube pin...
what i do in my amp is install a grid leak, about 1meg right at the socket
pins to nearest ground...
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Old 19th January 2015, 03:28 AM   #34
ampexperts is offline ampexperts  United States
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In my experience, an otherwise apparently stable amplifier can be made to oscillate with the use of excessive amounts of negative feedback. Due to the fact that phase shift occurs in real world amplifiers, at extremes of frequency, the uncompensated feedback loop will carry everything from the output to the input, including what was supposed to be out of phase, but at extreme frequencies, due to phase shift, are now enough in-phase to make the loop positive feedback at those phase-shifted frequencies.
Some tube types, such as 6BG6 and 807, are more prone to oscillation than more common types used such as 6L6 or 6550. Such amplifiers require phase compensated feedback loops (to reduce the feedback amount as the amplifier's phase response starts to shift) and may even require Zobel networks on the outputs to make them completely stable with inductive loads.
Lack of a ground common reference between the output transformer secondary in a negative feedback amplifier will cause the amplifier to oscillate at a wide range of frequencies, usually in the audio band where it is most efficient, when connected to a floating load, such as a loudspeaker.
Of course, before we get to all these issues, we must start with good layout practices. Keep power tubes, output transformer wiring and driver stage signal wiring away from input wiring and put as much distance as possible between these tubes to limit cross coupling. Use of shielded coax cabling for long runs within the chassis may be necessary to ensure good s/n and stability. Use of a common grounding point is also preferred.
I find the best results come from using good RF design practices in audio designs. This makes an inherently stable layout, where audio and power leads are kept short or shielded when longer runs are necessary.
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Old 11th February 2015, 09:27 PM   #35
65 DegN is offline 65 DegN  United States
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I'm thinking that surely hf or vhf oscillations are not created or being sustained by resonance in the OPT which would likely present megohms at those frequencies.
More likely caused by inductive or capacitive coupling in the wiring correct?

Last edited by 65 DegN; 11th February 2015 at 09:29 PM.
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Old 11th February 2015, 11:19 PM   #36
ampexperts is offline ampexperts  United States
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Quote:
Originally Posted by 65 DegN View Post
I'm thinking that surely hf or vhf oscillations are not created or being sustained by resonance in the OPT which would likely present megohms at those frequencies.
More likely caused by inductive or capacitive coupling in the wiring correct?
The bandwith of output transformers at small signal levels can be surprisingly wide. For instance, on an 807 amplifier with more than 20dB of inverse feedback, driving the amp near clipping results in a bubble appearing as it comes out of clipping, on the sinewave output. This bubble contains spurious oscillations as high as 1 mhz when viewed on spectrum analyzer. Some of that could be capacitively coupled between pri and sec windings.
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Old 12th February 2015, 07:13 AM   #37
65 DegN is offline 65 DegN  United States
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Interesting. Hadn't considered capacitive coupling within the OPT. Well this topic has had my interest lately since I modified a SE 300B for a friend using a 3 pin regulator and filtering on the filaments. The mod really quieted down the hum but now the amp ended up motorboating at clipping. I could see it on the scope too, flagging just below the peak of the falling waveform.
Tonight I went looking for it, put some latex exam gloves on for a bit of extra protection and began carefully moving wires around looking for impact on the scope. I found that the wire coming off the negative side of the filament DC supply definitely changed the waveform. Moving the part of the wire that ran parallel to the 300B socket made a huge increase in the spur. Moving it closer made a big spike. I tried using RG58U and grounding the shield but that didn't completely eliminate it. So I made the wire just a tad longer and re-positioned it to make a hard 90 degree turn as it approached the tube socket instead of running parallel. That completely eliminated the spur. Now it clips very nicely with a somewhat rounded waveform peak.
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Old 12th February 2015, 07:28 AM   #38
65 DegN is offline 65 DegN  United States
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For anyone interested, I run my home on an Outback VFX3648 inverter. Well, I bought some Eton silk dome tweeters and installed them on my stereo. They only lasted a couple days and one opened up. Brought the scope home and a Dale 8 ohm non-inductive load resistor(NH250) .I found that there was a ~20khz waveform on the load. If the amp was really cranked (Hafler Pro5000) the waveform got to ~75VPP.

I found it was coming from the inverter. Looking at the inverter waveform there was a little ~20khz oscillation or possibly some kind of computer noise riding on the very top of the 60hz waveform.

It was entering the amp through a defective audio cable between the DVD player and the preamp input. One end of the cable was ungrounded which apparently created a high enough impedance for 20khz to develop across the wire.

I had another tech confirm that he is seeing this waveform on his inverter as well. Replacing the audio cable eliminated the signal but I am going to build a low pass filter and put it in the AC line. Ideally it should go at the inverter output but that would probably require a huge choke.
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Old 10th March 2015, 03:57 AM   #39
grbullets is offline grbullets  United States
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Just a wild guess, but have you tried changing the snub resistors value?
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Old 17th March 2015, 06:02 PM   #40
Jeff Yourison is offline Jeff Yourison  United States
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Quote:
For instance, on an 807 amplifier with more than 20dB of inverse feedback, driving the amp near clipping results in a bubble appearing as it comes out of clipping, on the sinewave output.
I may have run into this recently. At 1 KHz, pushing the input to the point of clipping, on a breadboarded amp that is rather poorly laid out, I got these "bubbles" when I backed off the test input level. This was located right after the phase inverter, and before the final output tubes (not 807s, but pentodes). There is a high level of feedback in this situation. The image shows both "legs" of the inverter output.

Are these the same kind of bubbles that are described for the 807s?

I hope these bubbles will disappear in the final build using better layout and much shorter wires. . . . I also noticed that touching an AC balance pot in the PI section affected the bubbles (in one case well below clipping causing it to appear, and then disappear when not touching the pot), but the bubbles appear on their own at just below clipping. I hope this is just RFI and not due to the OPTs.
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File Type: jpg 1 kHz near clipping.jpg (40.4 KB, 773 views)
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