Dear pak Alex, Saya refer kepada pembicaraan kami (a.l. tabung 26) kemarin tentang info anda bahwa " bila tabung itu microphonic, maka umurnya sudah dekat alias sd masuk masa "twilight", sebentar lagi akan mati" Saya tergelitik atas statement anda dan dibawah adalah apa yg saya temui semalam dari: a) RCA Radiotron Designers Handbook b) Sylvania Electronic Application Handbook.
Summary: a) Statement anda tidak benar, bahwa bila tabung itu microfonic, maka umurnya sd mendekati ajal. b) Low level-high gain tubes selalu microphonic seperti 12AX7 (gain 100) dll. c) Mechanical interference seperti getaran/induksi juga akan kontribusi terhadap terjadinya microfonic, contoh silahkan dekatkan tangan anda pada tabung 26, dan dia akan microfonic (dengung) kr induksi badan kita. Kepekaan tabung 26 itu yg menjadikan tabung 26 microfonic, kepekaan tersebut juga merupakan kekuatan 26 dalam mereproduksi signal secara jauh lebih afdol, makanya anda sendiri menamakannya the "angels voice" . d) Saya yakin bahwa produsen tabung , terutama produsen NOS , dulunya sangat patuh terhadat standar yg disepakati. Pabrik2 tabung bukan pabrik pembuat mainan a la abal2 tanpa memperhatikan kualitas, terbukti bahwa tabung2 tersebut tetap dipilih/pakai oleh a.l. military - yg biasanya menuntut spek jauh diatas standar yg ditetapkan. Khusus tabung 26, memang sifat kepekaan tabung tersebut bergejala microfonic. Tentu saja "usia tabung" 26 tersebut "biasanya" sd masuk usia senja sebab produksi sekitar th 1920-1940, tidak ada tabung DHT26 yg NOS (masih perawan) , itupun bila ada - perawan sd tidur suri sangat lama sehingga bisa saja dia "kejang2" (tapi bukan orgasm signal lho....) bila dikenai anode load mendadak . Tapi sifat microfonic bukan gejala bahwa usianya sd dekat kuburan pak. Wassalam PJ http://www.freewebs.com/komkris2000 Quote : Here are some excerpts on microphonics from RCA's Radiotron Designers Handbook (4th edition) and Sylvania's "Electron Tube Application notes". A quick look at actual tube datasheets shows two trends. Some types claim to be low in microphonism, but were simply tested for shock (rigidity). Of course rigidity minimizes microphonism. Other types were subjected to specific vibration tests, but the specific test conditions (frequencies used, duration, acceptance limits) seem to be specific to the particular tube type, and were probably driven from specific end-user requirements. The vibration tests usually specify an allowable maximum RMS voltage in millivolts at a specific excitation level. Radiotron Designers Handbook, fourth edition Page 84 (Chapter 3, Section 1, Part v) (f) Microphony Small variations of electrode spacing cause corresponding variations in the output of the valve, and it is desirable to ensure that little or no vibration reaches the valve. Such vibration may reach the valve by way of the socket or acoustically, and it should be noted that microphonic trouble can originate in the converter or I-f stages through modulation of the signal or I-f carrier at audio frequencies. Page 107 (Chapter 3, Section 3, Part iv) (K) Microphony There is no published standard test for microphony outside of Service or manufacturer’s specifications. However, an indication of the microphony of valves may be obtained using the same conditions as the R.M.A. standard audio frequency noise test (see below). It is reasonable to adjust the amplifier gain to be of the same order as the gain of the subsequent portion of the amplifier in which the valve is to operate. Nevertheless, valves unless specifically designed fro low-level high-gain pre-amplifier use should not be expected to be non-microphonic and free from noise at low levels when tested under such conditions. (L) Audio frequency noise The R.M.A. standard for audio frequency noise is for normal voltages applied to all electrodes of the valve under test; the plate is coupled to the input of an amplifier with a frequency range from 100 to 2000 c/s (plus/minus 5 db of the response at 400 c/s). A dynamic loudspeaker with a rating of at least 2.5 watts is connected to the output of the amplifier. The minimum external grid resistance is 100,000 ohms. The gain of the amplifier and the plate load resistance of the valve under test are to be as specified. The valve under test may be tapped lightly with a felt or cork mallet weighing not over 1/2 ounce. Any objectionable noise heard in the loudspeaker is cause for rejection. Sylvania Electron Tube Application Notes pages 21 and 22 Microphonism Microphonism is caused by mechanical excitation and consequent relative movement of the tube elements. Assume that a speaker is mounted adjacent to the tube that drives it. Speaker sounds strike the tube, causing vibration of the tube elements and displacement of the tube parts relative to each other. As the elements move back and forth, they cause variations in plate current that are proportional to the mechanical excitation which, in turn, appears as more sound output from the speaker to further excite the tube. Microphonism can generally be broken into two categories; sustained and dampened. (1) Sustained microphonism is typified by the above example whereing the mechanical feedback is sufficient to start the oscillatory condition. It is a function of the physical tightness of the tube, the electrical gain of the tube, and the amplitude of the mechanical feedback. Thus, a high gain system requires relatively little feedback excitation to establish sustained microphonism, whereas a system having low electrical gain will require a more intense mechanical excitation. Microphonism of this description is generally associated with audio systems. (2)Damped microphonism is a phenomena that is not self-sustaining; i.e. mechanical excitation causes an undesirable output which ceases with the disappearance of the exciting force. Thus, jarring the cabinet can cause undesirable bongs, crashes, pings, etc., in the audio output or cause the picture in a television to break up momentarily. It differs from sustained microphonism in that there is insufficient feedback for sustained micro, but is more or less a s simple cause-and-effect relationship wherein a mechanical excitation causes an undesirable effect on the output for the duration of such excitation. The parts of a hypothetically perfect vacuum tube would be completely rigid with relation to each other; hence, it would be impossible for them to move and microphonism would not be a problem. The tube manufacturer’s ability to eliminate microphonism from vacuum tubes is limited to the mechanical tolerances which can be applied to the manufacture of the mica, ceramic, and metal parts. Thus, microphonism is minimized through rigid quality control procedures. [Paragraphs about chassis design to minimize microphics omitted] Microphonism is one of the most difficult tube properties to measure. It is known that any tube will exhibit microphonism if sufficient mechanical force is applied, hence, the mechanical excitation of the test must be closely specified. It is further known that the output level is dependent on: the gain of the tube under test; subsequent stages; the specific application; and will be a function of the particular tube type. Figure 20 illustrates the standard Sweep Microphonism test. The test equipment consists of a special adaptor (including a tube socket) which is vibrated by means of an electromagnetic transducer. This test permits analysis of the individual frequencies at which the tube structure teds to resonate and introduce microphonism. In edition, the described test serves as a basic reference point for comparing the circuit demands of a particular design with the capabilities of the tube type in question. [Figure 20 shows a block diagram of the equipment. Block 1 is a sweep generator, which is connected to a transducer (block 2), which is connected to the tube under test (block three), which is connected to a scope (block four). The diagram also shows “power supplies” connected to the tube.] In production testing of finished electronic equipment, the aforementioned basic reference tests would be extremely unwieldy. It would be most desirable to test the overall system with the tubes “in application”. In such cases, it is recommended that judicious choices of mechanical exciters be used. First and foremost, acceptance or rejection should not be based on tapping the tubes. In field use, this is certainly not typical of the way in which mechanical excitation would be delivered to the tube. Preferably, a light blow should be delivered to the chassis with a small rubber hammer, or better yet, turn up the volume and listen for ringing. Unquote. ------------------------------------ Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/ELPOP-Audio/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/ELPOP-Audio/join (Yahoo! ID required) <*> To change settings via email: mailto:[EMAIL PROTECTED] mailto:[EMAIL PROTECTED] <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
