4.06.2003
Josh,
It could be a coupling cap----though I haven't seen one that caused hum
to be injected into a system. If the bias is holding a steady settling,
then the diode and caps for the bias power supply are probably okay.
But, if the bias voltage (usually around negative 30 volts, for
6BQ5/EL84 tubes) is NOT steady---go after this part of the circuit.
I'd change out all electrolytic caps, use carbon comp replacement
resistors of the same value, a new diode (WATCH the polarity!!!----the
bias supply has the caps and diode facing the opposite direction, as the
main power supply). Eg: diode's "band" or positive end FACES the PT
and the 'lytics have the (+) ties to ground. Any Zener diodes in this
part of the circuitry should be replaced with like value new Zener.
Bad bias supplies not only knock out power tubes and associated
components, but they also can inject hum and false overtones into the
power section.
Some Dynaco amps put an 0.02 mfd cap (usually disc type) from the center
tap of the (6.3 VAC) heaters to ground. Test the two legs of the
heater. Use the VAC setting on your DVM---red lead to each 6.3 volt tap
the black to the point where that cap is grounded. Now, if the voltage
are more than 10-20% off each other (eg: one is +3.4 VAC and the other
is -3.0 VAC) a healthy 60Hz hum will be injected into the system (due to
the 0.4 volt difference in peak-to-trough heights, in this example).
60Hz hum is quite different than 120 Hz noise (which is usually injected
by a bad PS or bias supply). Kinda loud ruckus, which makes you think
the woofer is going to blow.
Make sure that cap is replaced and both legs give you close to the same
voltages. If not, I'd put in a "hum balance" pot which ties the two legs
together through a 100 ohm pot. Each leg to each end (to furthest tags)
on the pot. Center post or wiper to ground. What you are doing is
making a voltage divider which shunts the excess voltage of one leg to
ground----so that both legs make the same ABSOLUTE VAC. Twisting the
leads from PT to each tube should help cancel out EMF induced hum. A
small 1/2 watt linear pot is all you need.
Well, it's a thought. A lot of hum of a powerful nature usually can be
traced back to the heater taps (remember these boys pull amp levels of
VAC to the tubes) and/or bad can or mis-wiring around the can and/or PT.
Good luck. Keep me informed.
steve oda
windhund116@earthlink.net
It could be a coupling cap----though I haven't seen one that caused hum
to be injected into a system. If the bias is holding a steady settling,
then the diode and caps for the bias power supply are probably okay.
But, if the bias voltage (usually around negative 30 volts, for
6BQ5/EL84 tubes) is NOT steady---go after this part of the circuit.
I'd change out all electrolytic caps, use carbon comp replacement
resistors of the same value, a new diode (WATCH the polarity!!!----the
bias supply has the caps and diode facing the opposite direction, as the
main power supply). Eg: diode's "band" or positive end FACES the PT
and the 'lytics have the (+) ties to ground. Any Zener diodes in this
part of the circuitry should be replaced with like value new Zener.
Bad bias supplies not only knock out power tubes and associated
components, but they also can inject hum and false overtones into the
power section.
Some Dynaco amps put an 0.02 mfd cap (usually disc type) from the center
tap of the (6.3 VAC) heaters to ground. Test the two legs of the
heater. Use the VAC setting on your DVM---red lead to each 6.3 volt tap
the black to the point where that cap is grounded. Now, if the voltage
are more than 10-20% off each other (eg: one is +3.4 VAC and the other
is -3.0 VAC) a healthy 60Hz hum will be injected into the system (due to
the 0.4 volt difference in peak-to-trough heights, in this example).
60Hz hum is quite different than 120 Hz noise (which is usually injected
by a bad PS or bias supply). Kinda loud ruckus, which makes you think
the woofer is going to blow.
Make sure that cap is replaced and both legs give you close to the same
voltages. If not, I'd put in a "hum balance" pot which ties the two legs
together through a 100 ohm pot. Each leg to each end (to furthest tags)
on the pot. Center post or wiper to ground. What you are doing is
making a voltage divider which shunts the excess voltage of one leg to
ground----so that both legs make the same ABSOLUTE VAC. Twisting the
leads from PT to each tube should help cancel out EMF induced hum. A
small 1/2 watt linear pot is all you need.
Well, it's a thought. A lot of hum of a powerful nature usually can be
traced back to the heater taps (remember these boys pull amp levels of
VAC to the tubes) and/or bad can or mis-wiring around the can and/or PT.
Good luck. Keep me informed.
steve oda
windhund116@earthlink.net
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