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In Reply to: RE: 300B Tube-life expectancy... SURVEY!!! posted by Randy Thatcher on December 01, 2007 at 09:22:08
You mentioned "...the Pi filter of the 300B supply..." I don't know the circuit, but I'll take this to mean that it is a fullwave bridge rectifier followed by a CRC filter. Please ignore the rest of this post if that is incorrect ...
Again, I don't know the circuit but if there are separate DC filament supplies for each 300B, check to see if both are the same voltage. (I keep some old beat-up Sovtek 300Bs around just for such checks...)
Normally the R in the CRC filter will drop the voltage to achieve the right DC output. Check the resistor for proper value first, in case it has drifted over the years. If all the values are correct, then increasing this resistor will reduce the voltage. As a first approximation, Ohm's law says the added resistance would be 1.0v divided by 1.2 amps, or 0.83 ohms - nowhere near 50 ohms! The actual value will be a little different due to the ripple at that point in the circuit, but not a lot different. You could as you suggest put in a variable resistor, but it must be able to handle the current which will be the 1.2A plus the rms value of the ripple at that point. I would look for at least 2 amps. A 5 ohm pot rated for 20 watts will handle 2 amps, or a 1 ohm resistor rated for 4 watts. (A 50 ohm pot would have to be rated 200 watts...)
Are the bridge rectifiers original? If they have been replaced with Schottky diodes - a good idea from a noise viewpoint - that would increase the output voltage.
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