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Bizarre hum problem from tone stack

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  • #31
    Hey Alex,

    Actually, I've seen this happen at least a couple of time - it explained mystery "red plating" with 'scope determinination. But it's still more or less a BS as a hum explanation - "mikey-poo" musta picked it up on some obscure web site as it's pretty rare and there was no excess hum noted, just "runaway" output tubes.

    Rob

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    • #32
      Why wouldn't turning the treble control all the way down (temporarily) cure this oscillation? If the oscillation is out of the signal path, where's it coming from?

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      • #33
        Actually, looking at this:

        http://www.univalve.net/forums_new/v...er=asc&start=0

        My valves aren't "red-plating" at all, and are fine. Which is a relief.

        Comment


        • #34
          Originally posted by mykey View Post
          now we have looked at the cause of 60 cycle ingress
          1. A. coming from cheaper parts such as disk capacitors
          B. or carbon comp inductive resistors
          C. or unshielded audio paths
          which induct 60 cycle from the power supply and heaters, then the gain stages amplify the 60 cycle along with the guitar audio...
          Let's play a game. We'll call it "Produce The Facts".
          1.A. (hum comes from cheaper parts like disk capacitors); I call. Show me the facts. Produce even one external reference that says that disk capacitors cause hum.
          1.B. (carbon comp inductive resistors); Having called, I owe you a fact. Have a look at http://www.welwyn-tt.co.uk/pdf/appli...s/CCR_AN_A.pdf and check out the note " However, these wirewound and thick film alternatives do not fully match the pulse performance and low inductance of Carbon Comp."

          And this one: http://www.electronicproducts.com/Sh...l.apr2005.HTML
          "Carbon-composition resistors are non-inductive and provide an adequate solution at low average power, but they can be unstable because of moisture or high voltage."
          Those are just the first two I found. It seems like these resistors pros vote with me that carbon composition is non-inductive.

          We agree on unshielded audio paths. So you owe me an explanation of how cheap components cause 60 cycle hum. Your turn.

          Originally posted by mykey View Post
          and we have looked at those who try to compensate for inferiority complex by proclaiming themselves superior to all others...
          A. by demanding theoretical explanations and mathematical formulas
          B. by proclaiming that certain solutions offered CAN'T possibly solve the problems described.
          C. or by persistent general heckling of others who join the discussion with differing viewpoints.
          I'm just guessing that you're pointing that finger at me. Let's look at cases.
          A. Formulas and theoretical? Far from it. I asked for any single independently verifiable fact that would hold up your contention. I'm of the opinion that if it works in the real world, the math will uphold it once you really understand the math. The old saw about equations proving a bumblebee can't fly is based on an idiocy - they equations they used were based on fixed wing aircraft models. When you set up the model based on a moving wing, the math says bumblebees have plenty of lift. But this is a favorite resort of people who want to not get into the math. Dealing with the real world is complicated. You need experience AND the math to really know what you're doing.
          B. Claiming certain solutions CAN'T solve... That's because the proposed solution is so far outside the realm of possibilities that we can just play the odds on it. There kinda needs to be a reason why your proposed solution MIGHT solve the problem. That involves both knowing the basis of the problem and some clue about how the proposed solution would fix the underlying cause. You could as well have proposed painting the amp purple as a solution. You've given exactly as much support to the purple paint fix as the solution I didn't believe. I'll believe in that fix when you can show me some plausible and independently verifiable (that is, not just you saying "Does so work!") basis for it working.
          C. Persistent heckling... Actually, I'm trying to get some information here. I have been doing electronics professionally for a long time, both theoretical and practical. The stuff you're proposing does not match my internal test of reason and your continuing mode of trying to avoid explaining yourself have raised my BS indicator level. It appears you're putting out stuff you don't have any basis for. I feel like it's unfair to the others who are here trying to learn to present that kind of non-information to them as the truth. So like I've said - you want to state a fact, prove it. I'll be happy to do the same with any assertion I make.

          Then there's that "inferiority complex" and superior to all others" stuff. First, you're the guy tossing around inferiority complex. I don't have that problem. And I think you'll find that you're the one talking about "superiority". I have a pretty simple issue here. You want to assert that something is a fact? Prove it. Show me some reason to believe. I'll do the same.

          Originally posted by mykey View Post
          then third we must also consider the possibility of this hum being caused by high frequency oscillation, it seems to be in the tone stack because when the tone control is turned the oscillation occurs.(hence the "bizarre" classification) REALITY IS THAT the problem is not really the tone stack at all, but the tone stack ALLOWING high frequency to pass thereby TRIGGERING the oscillation.
          I was going to write up a response to this, but it turns out that it's not needed. The guy found that he had speaker problems and a faulty lead wire. Seems like it was not RF oscillation after all. Mother Nature has spoken.

          You owe me an answer from 1.A. up there. How do cheap components cause hum? With some back up please.
          Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

          Oh, wait! That sounds familiar, somehow.

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          • #35
            Good. I think the guys will now find the answer in the absence of that worrisome old oscillation issue...

            Comment


            • #36
              Rob, thanks for the info, electronic and, what shall we say, sociological?

              I think, he added tentatively, that it was me who first mentioned the possibility that the treble cap might be picking up a bit of noise and might be better replaced with a new silver mica. You know, I think that just for a little while I'm going to keep quiet about why I thought that. The wonderful word 'inducting' was definitely not in my mind however.

              Out of interest, how does oscillation-driven overheating happen? How come the preamp doesn't clip it? I can't theorise my way to it. There are in truth few things I can theorise my way to. Mind you I have a working hypothesis about mykey.

              Comment


              • #37
                Noise or hum? While hum is indeed noise - unwanted signal information - but we tend to reserve hum for 60/120Hz and noise for anything else. At least in my head.

                My working hypothesis is someone looked inside an amp at 15, putzed around with them, and eventually bumped into the Peter Principle, all without the aid of a mentor. Then with the addition of manure, the resume blossomed.

                Of course, I could be wrong.

                On the other hand, I am quite familiar with inducting. Back when I was younger, the army was doing a lot of it. I managed to avoid them. Some of my friends have their names on a wall...
                Education is what you're left with after you have forgotten what you have learned.

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                • #38
                  Noise, hum, induct - what a difference a word makes. We all use them to communicate, and we all use them for other purposes too. I think the BS meter starts to twitch when the square of the other purposes gets bigger than the square of the communication value.

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                  • #39
                    Alex, an amplifier and oscillator are identical - the oscillator is just self referenced. So, "something" that couples the output of output stage (no, I dddddid nnnnot sssttttuttter!) back to the input can create an ultrasonic oscillator within that stage - or perhaps including the PI stage - which with no level controls winds up drawing a much power as the PS will supply. Others such as R.G. can probably provide some likely frequency bounding for such an unintentional oscillators but I suspect that the ultimate frequency is below the -12dB OT bandwidth point or such. In my case when I encountered this the symptom was red plating output tubes and hot OT with no apparent output signal - in at least one case one tube was much hotter than the other. The scope I was using at the time (I've never owned a new scope - my "newest" is a late 1970s dual trace B&K SS job) was an old early 1960s EICO 1 mHz single channel tube job with an uncalibrated time base (still really like that scope for ease of use) so I know the frequency was above the speaker's response and above the 24 kHz that my then audio oscillator was capable of - I unsuccessfully attempted to guesstimate the freq. using lissajous figures but there were too many unstable elements in the set up (scope timebase, audio osc limits, and probably some frequency changes within the "oscillator" as I attached various probes, etc.) - but I only tried for about 1/2 hour.

                    Anyhoo in both cases the replacement of output tubes, a thorough cleaning of the tube sockets, and general maintanence seemed to help but I had to replace the screen bypass cap on one amp (an old Bell hifi w 5881s) and restore the clipped out "zoebel network" on the second "cheapie" 1960s Japanese tube amp (forget the name, one of those "Elk" type jobes with the gray oil filled caps and 6973 outputs - I may still have it in the attic.).

                    Rob

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                    • #40
                      That does happen, Rob. The frequency tends to be down near AM radio with audio pentodes. The tough ones are in amps where the outputs are MOSFETs. Those things can oscillate at VHF or UHF. Very few of us - me included! - have scopes that can even see that stuff.

                      When a tube amp oscillates over the audio range, it tends to make a hiss that sounds angrier, for want of a better word, than normal thermal hiss. Not always, but often.

                      In most cases putting a grid stopper resistor in the grid circuit of the power tubes and possibly PI tubes quenches it. That is in fact why grid stopper resistors are called stoppers - they stop Nyquist oscillation in circuits where the layout is bad enough to let some stray feedback get oscillation going. It does this by forming an RC lowpass network with the Cgk and Cgp/Miller capacitance of the tube and kills the RF gain of the tube. Most output tubes include grid stopper resistors these days. But they have to be right on the tube pin to be effective.

                      An amp that's oscillating may not produce any sound at all if the amplitude of the oscillation is full swing. This is the kind of oscillation that will eat up output tubes in a sudden death meltdown. Nyquist (gain-phase, or phase shift) oscillation builds up until something limits its amplitude. If there is a limiter of some sort, like the oscillation pushes a tube into positive-grid territory, the oscillation will be limited there even if it's not full swing on the amp output. In this case, you can have audio plus the oscillation.

                      Tube amps are at least honest about this. Some solid state amps have NPN/PNP outputs where the PNPs are slower than the NPNs and you will get oscillation on one half of the output swing but not the other. It's common enough to have a name - "bottom side fuzzies". Tracking one of those down and killing it is fun... if you're a masochist. 8-)
                      Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

                      Oh, wait! That sounds familiar, somehow.

                      Comment


                      • #41
                        Originally posted by R.G. View Post
                        When a tube amp oscillates over the audio range, it tends to make a hiss that sounds angrier, for want of a better word, than normal thermal hiss. Not always, but often.
                        This hiss wouldn't be taken out with turning down the treble control would it?

                        I replaced the filter caps, and the hum's gone (also I found a lot of treble and volume that I'd been missing). I've got a bit of a hiss now. It goes when I turn down the treble control. What's thermal hiss?

                        Comment


                        • #42
                          Originally posted by R.G. View Post
                          This is a VERY high impedance tone stack. The impedance is so high that it is probably picking up hum. It's likely that constructing it using vanishingly short component leads and shielded cable might reduce or eliminate the hum, but because of the high impedance levels, everything becomes critical. For instance, there is some hum coming in if the control pot bushings are not fully grounded to chassis. Building it inside a shielded cage might help.

                          A more robust approach would be to put followers on the 12AX7 plates and drop the impedance of every single part by 10X: resistors become 1/10 resistance, caps become 10X capacitance. That should help a lot. Good construction will still matter, but it will be much less critical.

                          Did this today, put in some mullard tropical fish caps. The hum is completely gone. There is just some thermal noise now. I can't believe what a difference it made!

                          I am now starting to really dig this amplifier. Thank you for all the help and advice.

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                          • #43
                            Hey! Great! I'm glad that worked out for you.

                            Now go make LOUD noises!! 8-)
                            Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

                            Oh, wait! That sounds familiar, somehow.

                            Comment

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