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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Any thoughts on my MC275's snap, crakle and pop? posted by R.B. on November 2, 2002 at 18:12:58:
Hey-Hey!!!,
I have a 275 behind me that did what you are describing. Sound form one channel, a bit like driping water, a bit of a sizzling sond to it. Tried swithing all the tubes to no avail. I found a bad cathode bypass cap on the 12AX7, something like a 3volt 100 uF 'lytic. Try measuring the resistance of these caps, as the voltage of an ohm meter is comparable to the actual woring voltage you might see the screwy behaviour. My HP multimeter would not settle down to a range on the bad one like it did with the good one. The 12 uF 250 V 'lytic coupling caps from the cathodes of the 12AT7 to the grid of the power tubes also failed on the other 275 I have. Suspect all 'lytics is the short story.
Short experiment, un-solder the bypass cap from the bad channel's 12AX7 cathode, less gain is all that will happen, and if it's bad you'll have your answer.
BTW, the 12AT7's section to section match is a bit critical for it controlls the actual grid voltage of the power tubes, if you have slightly mismatched power tubes and 12AT7's you can measure and match for better performance. just measure votlage at the power tube's cathodes after you measure the DCR of the cathode winding to determine current, Mc spec is .6 v
regards,
Douglas
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Follow Ups
- Re: Any thoughts on my MC275's snap, crakle and pop? - Sector-7G 12:21:04 11/03/02 (3)
- Thanks for the input - R.B. 05:21:15 11/04/02 (0)
- Good thought but can it be OPT problem? - koo 02:27:00 11/04/02 (1)
- Sounds and such - Sector-7G 04:56:06 11/04/02 (0)
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