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    Dynaco Mk. III's faulty KT88

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    benji_w40k

    Posts : 2
    Join date : 2012-09-24

    Dynaco Mk. III's faulty KT88

    Post by benji_w40k on Mon Sep 24, 2012 5:57 pm

    Dear Dynaco user community,

    I recovered 2 mono blocks Dynaco Mk. III from a family member. The right channel's fuse was blown, I replaced it and soon understood why the fuse blew in the first place : one of the KT88 tube is behaving eratically : it does not light up (but still let current through, as the connected speaker sounds normal), and sporadically pop up 3 or 4 times causing strong sputtering in the speaker. It then rest again and continue this cycle. When changed for another tube, everything is fine. Is the faulty tube definitely dead ? If yes, where is the best place to look for a replacement ?

    Thank you for your answers !

    Jim McShane

    Posts : 154
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: Dynaco Mk. III's faulty KT88

    Post by Jim McShane on Mon Sep 24, 2012 6:37 pm

    There is a fundamental error in your thinking here. The KT-88 CANNOT work unless the cathode is heated at least close to operating temperature. The fact that you can't see the glowing heater is irrelevant - it may just be that the heater wire is completely hidden inside the cathode sleeve. If the tube is hot to the touch (be careful!) the heater is working.

    Also, it is not unusual for a push-pull amp like the Mk III to have one tube not functioning at all yet sound normal. This is because at lower outputs the single tube will often have the entire signal waveform applied to it and will drive the speakers with the full waveform - only when the tube is driven hard(er) can you tell something is wrong. It's a long story as to why, but it has to do with how a "push-pull" output stage and the phase inverter work.

    You could have a tube with a problem - or it could be bad socket contact. Clean the tube pins on the "suspect" KT-88 and clean inside the socket it was installed in. DeOxit D-5 and some plain old pipe cleaners work great. Be sure the tube pins are gripped snugly by the socket!

    Sometimes just the act of removing and reinstalling tubes from a dirty socket restores enough of a connection that the new tube works - and because it does and the old one didn't many will assume the original tube was bad. Not necessarily so!

    If you do need a new tube(s) I guess it's okay to mention that Bob recommends me, so drop a note if I can help. But you may find if you follow my suggestions that you don't need a tube!

    Best of luck to you...

    benji_w40k

    Posts : 2
    Join date : 2012-09-24

    Re: Dynaco Mk. III's faulty KT88

    Post by benji_w40k on Sun Sep 30, 2012 5:59 am

    Thanks Jim for your kind explanations.

    I definitely have a faulty KT-88, as it now does not even heat up. I did not know about this particularity of push-pull circuit you describe.

    So I do need a new tube... Check your PM

    GP49

    Posts : 735
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Dynaco Mk. III's faulty KT88

    Post by GP49 on Mon Oct 01, 2012 10:58 am

    Jim McShane wrote:
    Also, it is not unusual for a push-pull amp like the Mk III to have one tube not functioning at all yet sound normal. This is because at lower outputs the single tube will often have the entire signal waveform applied to it and will drive the speakers with the full waveform - only when the tube is driven hard(er) can you tell something is wrong. It's a long story as to why, but it has to do with how a "push-pull" output stage and the phase inverter work.


    Depends on the class of the push-pull output stage.

    Class B: each half of the push-pull circuit is designed to handle half the output waveform. There is (virtually) no sharing between the halves of the signal, which is split in the driver stage and "reassembled" into a whole for listening; in a transformer-output amp, this happens in the output transformer; in an output transformerless amp, the two halves are connected together at the output to the loudspeaker. Most transistor output stages are virtually Class B; there is a TINY bit of sharing only to extremely low power levels, because if the "hand-off" between the two halves is discontinuous, there is a particularly annoying form of distorion that occurs, and it's mostly audible at low volume where you actually listen most of the time and the sound isn't loud enough to mask it. This distortion was why many transistor amplifiers sounded bad.

    Class A: each half of the push-pull circuit handles the entire output waveform. All the signal is amplified by both halves of the output stage. The advantage is that there is no chance of any discontinuity at "hand-off" of the signal between the halves of the circuit, because there is no hand-off. Class A amplifiers are inefficient, however. They consume a lot of electrical current for their power output, wasting a lot of that power, which is dissipated as heat.

    Class AB: As the name implies, this is partway between Class A and Class B, and it's the mode in which Dynaco tube amplifiers operate. Each half of the push-pull circuit handles somewhat more than half the output waveform but less than the complete waveform. There is a significant overlap between them, where both are amplifying the signal. This minimizes the audible effect when the signal is "handed-off" because when it does occur, it is happening at higher power than in a Class B amplifier, and the relatively small amount of discontinuity can be masked by the louder signal. A Class AB amplifier uses less current and dissipates less heat than Class A, but more current and dissipates more heat than a Class B. The point at which the two halves of the circuit stop sharing the signal is varied by the bias adjustment; the higher the bias current, the more "sharing" there is of the output waveform, but the more current is used and the more heat is dissipated (more sharing: the Class AB amplifier goes farther toward Class A).

    There are subclasses: Class AB1, AB2, etc., that have to do with whether the output tube grid draws power from the preceding driver stage.

    Most hi-fi power amplifiers are Class AB, a compromise between low distortion and efficiency/heat dissipation (most preamps are Class A). A single EL34 (which would be in Class A by definition; no hand-off of the output signal) at its maximum rated dissipation is capable of about 10 watts output at 5% distortion. A push-pull pair of EL34s in Class A, about 20 watts. A pair of EL34s in Class B can produce 100 watts at 5% distortion but probably would sound bad. The compromise Class AB is good for 35 to 60 watts at 5% distortion, depending on how the output stage is biased.

    5% distortion sounds high, and for hi-fi specification purposes it is. But you may be surprised as to how little effect it really has on the sound, since it's only at maximum output and is mostly low harmonics (the distortion created by improper hand-off in a Class B amplifier is high-harmonics...very annoying). It is also an open-loop distortion rating; in a real life amplifier low-harmonic distortion is reduced substantially by negative feedback...which is another topic.

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