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<blockquote data-quote="Ampdog" data-source="post: 41608" data-attributes="member: 144"><p>No, it is not normal - a little disturbing. Is this erratic, or periodic? It might be difficult to see that with a DVM because of its finite update time - an oscilloscope would be better to see the exact nature of the variations. If the circuit is the same as the Mullard 5-20 (I did not re-read all previous posts), you could pull out the phase inverter tube, after which the current should be dead-constant (do it with the amplifier switched off!). With NFB disconnected? I would hesitate to blame this on incorrect ac balance, depending. First try with the phase inverter tube (a 12AX7/ECC83?) unplugged, then no NFB connected and let us know.</p></blockquote><p></p>
[QUOTE="Ampdog, post: 41608, member: 144"] No, it is not normal - a little disturbing. Is this erratic, or periodic? It might be difficult to see that with a DVM because of its finite update time - an oscilloscope would be better to see the exact nature of the variations. If the circuit is the same as the Mullard 5-20 (I did not re-read all previous posts), you could pull out the phase inverter tube, after which the current should be dead-constant (do it with the amplifier switched off!). With NFB disconnected? I would hesitate to blame this on incorrect ac balance, depending. First try with the phase inverter tube (a 12AX7/ECC83?) unplugged, then no NFB connected and let us know. [/QUOTE]
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DIY & Tutorials
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