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7/8/1999 8:24 PM
I just added 1/2 a 12AX7A to the input of the differential amplifier on my power amp to act as the 2nd of a two stage preamp. The volume control is between the two stages of the preamp. Without the preamp... no hum. With the preamp... it's hummin' like the dickens... The hum, of course, gets louder as I turn up the volume. Also, no signal source is required for the hum (in fact, it gets louder if I pull out the guitar cord from the amp completely!). I've got my filement wires wound. Do I need to use shielded wire all throughout my preamp or just from the input jack to the first control grid? Are there any other suggestions as to the possible cause of the hum?  
PS: I have my grounds running to a psuedo-"bus" that then goes, with a single wire, to the chassis. Does this constitute as "star" grounding, or do the wires have to PHYSICALLY all join at the same place?
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