In article , Fred Nachbaur
wrote:

Brian wrote:

I have a push-pull 6V6 amplifier that uses a series heater resistor to
run the 12AX7 phono stage and the following 12AU7 multiple-input stage
at 4.73 volts when the rest of the amp is at 6.30 volts. What is the
purpose of this?

Brian

Some amplifiers used a selenium or similar (copper oxide?) rectifier to
supply filtered DC to the early stages, reducing hum. As these
rectifiers start to fail, their effective resistance increases and
output voltage decreases. It could be that's what's going on.

And other amplifiers with selenium rectifiers for the heaters were
designed to put only 5.0 volts or so on the heaters of the low level
stages when the rectifiers were new. Same thing with tuners that ran the
audio stages from a 5.0 volt winding on the power transformer, so I don't
think it is simply a matter of rectifier aging.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/