Method to track down hum and tube crackle

OTL Asylum

OTL, Output Transformerless Amplifier User Group.

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I am a new owner of a used pair of MA-2's (MkII) that I just received from a factory check-up to make sure the amplifiers were operating up to spec. Out of one amplifier I am experiencing a significant 60 cycle hum and tube crackle. My questions to the group are: (1) am I right to assume this is likely a tube that has gone and (2) if so, what is the best method to sort out which tube or tubes need to be replaced? Is the source likely to be one of the driver tubes or one or more of the output tubes?
To give a bit more information, the hum and crackle have been intermittant, but are getting more persistent. The problem exits whether the interconnect to the preamp is connected or not. I am using the RCA input, and pins 1 and 3 of the XLR input are shorted. When the hum and crackle occur, checking the DC offset shows the meter needle flicking about erratically. I have tried reversing polarity on the AC plugs and making sure the AC cables and speaker cables are separated. The other amp is operating fine. Any guidance provided on the orderly way to sort out the solution would be appreciated.

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Kimber Kable

Topic - Method to track down hum and tube crackle - Rush Paul 18:59:35 02/03/01 (7)

Re: Method to track down hum and tube crackle - mark 07:20:49 02/04/01 (6)

Re: Method to track down hum and tube crackle - Rush Paul 09:23:14 02/04/01 (5)

Re: Method to track down hum and tube crackle - Rush Paul 10:20:47 02/04/01 (4)

Re: Method to track down hum and tube crackle - Rush Paul 11:06:34 02/04/01 (3)

Re: Mechanical buzz from transformers - Rush Paul 18:09:10 02/05/01 (2)

Re: Mechanical buzz from transformers - Lew 08:15:35 02/06/01 (1)

Re: Mechanical buzz from transformers - Rush Paul 11:07:28 02/06/01 (0)

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