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Thread: Best way to cure output osscilation

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    Best way to cure output osscilation

    Hello, I just recorded some pretty high amplitude 300 kcps osscilation at the OT output with speaker connected. This are not present with a resistor, and also are suprimed with speaker and resistor in parallel. Sometimes are suprimed just connected one of my voltmeter over outputs, sometimes just disconnected one of preamp stages. It is not a well trusted osscilation but when appear is pretty consistent. Does it worth to bother to found its source into circuits or just to suprimed snubbing it over the OT winding, please?

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    Last edited by catalin gramada; 02-14-2019 at 01:21 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    Please post the actual schematic involved

    Unless you want the classic "it must be some thingie in there" generic answer, that is.

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    Juan Manuel Fahey

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    No,no, I like more that one
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    I.ll post one, Thanks JMF

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    Just to be clear, it's some thingie in there causing it.






















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    Hi, this is what I did

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    Last edited by catalin gramada; 02-14-2019 at 04:26 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    i told you,that was it

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    First all I am not surprized that the oscillation stops when you wire a load resistor in parallel with the speaker. Amp stability is influenced by open loop frequency/phase response, NFB ratio and open loop gain. All three of these influences depend on load impedance. A speaker has an impedance of over 100 Ohm above 100kHz and turns the phase because it is inductive.

    Also your PI circuit has a much wider HF bandwidth than typical designs, which makes things worse. The main reason is the low impedance cathode follower driving the PI input. As a consequence the Miller effect limiting HF response at the PI input grid in standard high impedance designs is essentially eliminated.

    To reduce HF gain of the PI, you can try to wire a capacitor of 100p or more between the PI plates.
    To compensate for the reactive speaker impedance, a suitable Zobel network across the OT primary is recommendable.

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    Are the power tube 1k and 10k grid resistor bodies mounted at the tube socket terminals?
    Is the feedback resistor 470 ohms?
    That seems a lot of NFB (16ohm output, 470 ohm/100 ohm feedback network).

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    Thank You gents,
    Yes 1k and 10k resistors are over the tubes socket, 470 ohm resistor on nfb ( ca 15 db) Weird thing I had before the pi, biased by cathode bias resistor and did not noticed any problems. And just now is osscilate by a margin, sometimes did sometimes not, but when appear is pretty consistent and around 2v pp level.
    Also, when osscilation appear get some buzzy ,sort of hum noise in speaker but not found any components to justify those noise ,other than this 300khz osc. I can suspect a bad component like 12at7 tubes, which from my curiosity I toasted hk insulation at 150v for a while, but it measured very good so I think are out of question.
    This osscilation appear not suddenly at startup. It takes 10 sec around after the stb switch is engaged, of course with tubes all ready heated

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    Last edited by catalin gramada; 02-14-2019 at 08:19 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    Is it stable when open loop?
    OT plate wires, and LTP output wires, twisted? As these wire pairs carry balanced signals.

    With using that tap and those feedback network resistor values, you are applying a lot more NFB than a regular BF Fender.

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    Last edited by pdf64; 02-14-2019 at 08:23 PM.

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    Sometimes it helps to ground speaker chassis.

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    I will try, thank you

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    Quote Originally Posted by pdf64 View Post
    Is it stable when open loop?
    OT plate wires, and LTP output wires, twisted? As these wire pairs carry balanced signals.

    With using that tap and those feedback network resistor values, you are applying a lot more NFB than a regular BF Fender.

    Plates wires are not, there are a lot of building issues I have to cover but just for now it is a platform for experiment. It was excellent stable before to try this c.f. direct coupled with pi stage. Maybe is a building issue and becomes more vulnerable to osscilate due to rised hf sensitivity as Helmholtz states

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    +1 on too much feedback and check whether problems disappear with NO NFB applied.
    Just try it.

    Then use something like 1k5 to 2k2 instead of the 470 ohm one.

    And above suggested 100pF plate to plate at the PI

    I cleaned it a little:

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    the original drawing made my eyes bleed.

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    Quote Originally Posted by J M Fahey View Post
    +1 on too much feedback and check whether problems disappear with NO NFB applied.
    Just try it.

    Then use something like 1k5 to 2k2 instead of the 470 ohm one.

    And above suggested 100pF plate to plate at the PI

    I cleaned it a little:

    Click image for larger version. 

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    the original drawing made my eyes bleed.
    Thanks JMF. I found the source of osscilation in front of c.f. so I need to have care about that. Anyhow followed you advice and changed nfb resistor to 1.5k for 8db nfb around. Found crossover distorsions appears as amplification goes up.this was not present with 15db nfb before. There is a way to get rid of it? I tried to bias power tubes as hot can be around 75percent at iddle. PI outputs are pretty well balanced. I get 495 plate, 490 screens with -44 bias at iddle and 433 plates with 396 screens -42.3 bias below onset clip for 64 vpp into 4.2ohm load for 1.9k OT. Thanks

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    Last edited by catalin gramada; 02-17-2019 at 02:27 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    Are you experiencing crossover distortion prior to clipping? More important,.. You mentioned that gain increased with reduced NFB, so, are you experiencing crossover distortion at the previous maximum gain level? It's possible the increased gain is allowing the tubes to attempt drawing some grid current and the coupling caps are charging. You could try decreasing the coupling cap value.

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    Last edited by Chuck H; 02-17-2019 at 03:25 PM.
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    Quote Originally Posted by Chuck H View Post
    Are you experiencing crossover distortion prior to clipping?
    Yes, that I talking about, there are not severe but still. Sending more feedback into loop cure it, otherwise running power tubes hotter did not help. Thanks

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    Quote Originally Posted by catalin gramada View Post
    Yes, that I talking about, there are not severe but still. Sending more feedback into loop cure it, otherwise running power tubes hotter did not help. Thanks
    Is the crossover distortion occurring above or below your previous gain level when you used more NFB?

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    Click image for larger version. 

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    Trying to post a pic, found is much better today, but still can see. The mains voltage get lower with 10v around from 232v yesterday to 223v now.....
    I measured everything slightly before clipping in respect with the bias voltage was set
    Sorry for images was not pretty accurate, it is still pretty noisy, I have a spider on the table

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    I don't think it's likely to be crossover distortion in the sense of 'tubes being too near cut off at zero crossing', rather it's probably 'gm doubling' distortion, as discussed https://music-electronics-forum.com/...ad.php?t=41210
    Due to that, the transfer function is nowhere near straight, so waveforms will get distorted; try plugging the numbers into nickb's interactive valve data sheet page http://bmamps.com/ivds.html
    I suggest not to worry about it, it's part and parcel of class AB tube amps, so don't lose sleep and tube life by attempting to run the tubes so hot that the transfer function straightens out.

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    Is the crossover distortion (or transfer function) occurring above or below your previous gain level when you used more NFB?

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    I never get before clipping before, that was with 15 db nfb instead 8db now that means top of semialternances was squashed first then crossover distortion appears. Now get crossover dist before peaks to clip

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    I understand. But does the amp currently produce the same gain without crossover distortion as before when you were using more NFB. Or, let me ask this a different way...

    What was the maximum gain before clipping when you were using more NFB?

    And

    What is maximum gain before crossover distortion now that you are using less NFB?

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    Quote Originally Posted by Chuck H View Post
    I understand. But does the amp currently produce the same gain without crossover distortion as before when you were using more NFB. Or, let me ask this a different way...

    What was the maximum gain before clipping when you were using more NFB?

    And

    What is maximum gain before crossover distortion now that you are using less NFB?
    Understood too, I didn.t recorded, give me couple of minutes to measure, please. Thx

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    Ok, let me show you some numbers:
    With 470 ohm nfb resistor: 26vpp PI input -77vpp PI output -58vpp OT (meant no trace of clipping, it clips at 64vpp arround)
    With 1.5k nfb resistor: 10vpp PI input - 56vpp PI output- 45vpp OT (the output is far away from clipping but at this voltage I can see first trace of crossover distortion on the scope)

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    Quote Originally Posted by catalin gramada View Post
    Ok, let me show you some numbers:
    With 470 ohm nfb resistor: 26vpp PI input -77vpp PI output -58vpp OT (meant no trace of clipping, it clips at 64vpp arround)
    With 1.5k nfb resistor: 10vpp PI input - 56vpp PI output- 45vpp OT (the output is far away from clipping but at this voltage I can see first trace of crossover distortion on the scope)
    Thank you. That answers my question. I don't actually have a suggestion at this time, but I thought the information could be beneficial to the thread. I had thought it might be that your maximum undistorted gain might not have changed, but it seems it did.

    I would ask though, is this amplifier intended for sound reinforcement or as a guitar amplifier?

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    Last edited by Chuck H; 02-17-2019 at 07:00 PM.
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    Quote Originally Posted by pdf64 View Post
    I don't think it's likely to be crossover distortion in the sense of 'tubes being too near cut off at zero crossing', rather it's probably 'gm doubling' distortion, as discussed https://music-electronics-forum.com/...ad.php?t=41210
    Due to that, the transfer function is nowhere near straight, so waveforms will get distorted; try plugging the numbers into nickb's interactive valve data sheet page http://bmamps.com/ivds.html
    I suggest not to worry about it, it's part and parcel of class AB tube amps, so don't lose sleep and tube life by attempting to run the tubes so hot that the transfer function straightens out.
    The signal waveshape shows (slightly) reduced slope near zero-crossing. That's just the opposite of what you would expect from "gm doubling" and is typical for dominating tube non-linearity near cutoff (reduced gm).

    Of course "gm doubling" doesn't mean actual increase in gm. Rather voltage gain increases in class A region as the load current is shared between 2 tubes, making each tube see "doubled" load impedance.

    But what's wrong with a little distortion in a guitar amp?

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    Last edited by Helmholtz; 02-17-2019 at 06:50 PM.
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    Quote Originally Posted by Helmholtz View Post
    The signal waveshape shows (slightly) reduced slope near zero-crossing. That's just the opposite of what you would expect from "gm doubling" and is typical for dominating tube non-linearity near cutoff (reduced gm)...
    Yes, I agree, I posted before I'd seen the scope pics; I'd been expecting the kink to further along from the zero crossing.
    As the issue is only reported to manifest at very high signal levels, I wonder if there's some degree of grid rectification going on, increasing the effective bias voltage.

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    Quote Originally Posted by pdf64 View Post
    Yes, I agree, I posted before I'd seen the scope pics; I'd been expecting the kink to further along from the zero crossing.
    As the issue is only reported to manifest at very high signal levels, I wonder if there's some degree of grid rectification going on, increasing the effective bias voltage.
    Maybe smaller coupling caps? You could probably go as low as .033uf without noticing.

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    Found crossover distorsions appears as amplification goes up.this was not present with 15db nfb before. There is a way to get rid of it?
    If itīs not present at lower volumes then it is NOT crossover distortion by definition, although waveshape looks *somewhat* similar.

    It is bias shifting which appears at high power, a very diffeent thing, in my way is "natural", it was "always there since day one" , is an important part of tube sound (so much so that Transtube and others create it on purpose, go figure) and people startted worryoing about it *only* after they started scoping amplifiers , misinterpret it and, of course, "listen with their eyes" so itīs now "unbearable"
    Snowflakes.
    [/rant mode]

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    Quote Originally Posted by J M Fahey View Post
    If itīs not present at lower volumes then it is NOT crossover distortion by definition, although waveshape looks *somewhat* similar.

    It is bias shifting which appears at high power, a very diffeent thing, in my way is "natural", it was "always there since day one" , is an important part of tube sound (so much so that Transtube and others create it on purpose, go figure) and people startted worryoing about it *only* after they started scoping amplifiers , misinterpret it and, of course, "listen with their eyes" so itīs now "unbearable"
    Snowflakes.
    [/rant mode]
    So, JMF this is a consequence of rail supply sag? If bias voltage sag proportionally in respect with voltage rail at momentary current draw ensuring new bias point in respect with voltage plate at high power this artefact still appear, please ? Or in case of stiff power supply and regulated bias, for instance?

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    No, the suggested issue is unrelated to power supply sag, but rather it's due to the signal at the power tube grids getting rectified and increasing the effective bias voltage, see https://www.ampbooks.com/mobile/ampl...ias-excursion/
    It's most noticeable during overdrive, but, especially with tubes, such effects as grid conduction may not be a 'brick wall', eg zero grid conduction below Vg1-k<0 and then full on grid conduction for Vg1-k=>0; rather more of a gradual curve.
    Of course, sag at the HT nodes (especially g2) will exacerbate this.
    The point is not to worry about it, and definitely not to decrease bias voltage in an attempt to mitigate for it.
    As Chuck notes, lowering the value of the coupling caps to the power tube grids may help.

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