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RE: Fisher SA-16 With Burned Resistors

Posted by FlaCharlie (A) on September 23, 2016 at 07:55:02

In Reply to: RE: Fisher SA-16 With Burned Resistors posted by Michael Samra on September 23, 2016 at 00:03:54:

I understand that the resistors are used to create a center tap and also fulfill the same role as a hum pot, though not adjustable. I didn't think they caused the two output tubes to lose vacuum, I was speculating that it might be the other way around - that the tubes lost vacuum and perhaps their heaters were shorted somehow causing the resistors to burn.

I suppose temperature could have been an issue regarding the tube losses. I recall when I pulled these that the amp was mounted in a fairly small, and completely enclosed, section of the cabinet which seemed odd to me.

I assume there was either excessive current or excessive voltage going through those resistors. I'd just like to figure out what caused it, if possible, so any related issue can be dealt with.

The other thing that occurred to me just now is that the so-called "death cap" (a .1uf 600v poly) went bad and was allowing too much current or voltage to the chassis. Would that affect those resistors or not?

The fuse is original - a 2A Slo-Blo. The output tubes (6BQ5 Amperex Bugle Boys) may have been replacements installed in the very distant past but if I had to guess I'd say the console was never serviced in any way during its lifetime.

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