Ben Gray
Guest
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 Posted:
Wed Oct 05, 2005 5:20 pm Post subject:
Re: FIXED |
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Ian Iveson wrote:
| Quote: | If the anode voltages are quite close to each other, connect the
preamp with *no signal* and measure again. If the anode voltages
become very different again, the preamp and absence of C12 are to
blame.
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12ax7 anode tests:
preamp disconnected:
pin 1: 255v
pin 6: 246v
occasional crackle would send pin 1 to 270v
preamp connected no signal:
pin 1: 385v
pin 6: 245v
fail mode would become apparent and pin 6 would drop to 215v. pin 1
steady.
Swapped the resistor living in C12 for .22uf cap of unknown voltage.
Problems went away!
pin 1: 261v (540vac with signal)
pin 6: 246v (515vac with signal)
B+ at C3: 398v
That resistor has been occupying C12 for a very long time.
So I still don't understand what made the amp go bad.
Anyway, as soon as I acquire a rated cap, my sons can get back to
rehearsal.
Thanks to everyone for your input and patience.
Ian - you nailed that sucker. Cheers.
...now to track down that hum
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Ian Iveson
Guest
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| Posted:
Wed Oct 05, 2005 8:24 pm Post subject:
Re: FIXED |
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"Ben Gray" wrote
| Quote: | ...That resistor has been occupying C12 for a very long time.
So I still don't understand what made the amp go bad.
Anyway, as soon as I acquire a rated cap, my sons can get back to
rehearsal.
Thanks to everyone for your input and patience.
Ian - you nailed that sucker. Cheers.
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Good.
If you can look inside the preamp and find an output capacitor, it
may be worth replacing. It is possible that the DC voltage from the
amp input has slowly killed it.
Thanks for reporting back.
cheers, Ian
| Quote: | Ian Iveson wrote:
If the anode voltages are quite close to each other, connect the
preamp with *no signal* and measure again. If the anode voltages
become very different again, the preamp and absence of C12 are to
blame.
12ax7 anode tests:
preamp disconnected:
pin 1: 255v
pin 6: 246v
occasional crackle would send pin 1 to 270v
preamp connected no signal:
pin 1: 385v
pin 6: 245v
fail mode would become apparent and pin 6 would drop to 215v. pin
1
steady.
Swapped the resistor living in C12 for .22uf cap of unknown
voltage.
Problems went away!
pin 1: 261v (540vac with signal)
pin 6: 246v (515vac with signal)
B+ at C3: 398v
..now to track down that hum
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