I was looking at a SCHEMATIC for a Harman Kardon Trio. I dont understand how the bias circuit works.
Is an EL84 PP design with the cathodes of the EL-84 pairs strapped together. Each end of a pot is connected to the cathodes with a 330R resistor and also to the respective coupling cap load side with a 470R resistor. The wiper is grounded. This pot is supposed to balance the cathode bias between the two tubes.
I could see it if the cathodes were not coupled but I'm baffled by this setup. How can this work?
Can someone here explain it? Please.
Is an EL84 PP design with the cathodes of the EL-84 pairs strapped together. Each end of a pot is connected to the cathodes with a 330R resistor and also to the respective coupling cap load side with a 470R resistor. The wiper is grounded. This pot is supposed to balance the cathode bias between the two tubes.
I could see it if the cathodes were not coupled but I'm baffled by this setup. How can this work?
Can someone here explain it? Please.
It allows you to balance the current in the two output tubes. You can't actually change the total current flow but you can adjust it so that each tube gets one half of the total current. Similar circuit was used on the output tubes of the Heath W5. If you can get a copy of the Heath manual it will explain it. The original Williamson not only had a similar circuit but also had an adjustable resistor so that you could vary the total current as well.
OK, I get it now. The pot is used to divide DC between the two grids. The pot creates an adjustable voltage divider so that the pair of pentodes can be "balanced" so as to to have equal cathode current. Not a bad way to do it.
A couple of voltmeters to monitor voltage across the cathode resistors and a screwdriver should do it. (That and some new resistors - the vast majority are out of tolerance, especially the power resistors).
Thanks for the face time, professors.
A couple of voltmeters to monitor voltage across the cathode resistors and a screwdriver should do it. (That and some new resistors - the vast majority are out of tolerance, especially the power resistors).
Thanks for the face time, professors.
No, pair of volt meters won't help you. You need to measure currents. Most probably, some technological plugins between tubes and sockets were used by support technicians who used to replace tubes. I would cut cathode wires from sockets to insert 10 Ohm resistors there, to measure voltage drops across them.
Of course. The cathodes are strapped. Voltage drop across the 330 R resistors would be meaningless.
A round of 10 ohm resistors for all my pentodes! I can't see any other way to do it besides using a pair of those tube socket adaptors with the internal resistor and test leads, as you mentioned.
Thanks again. Great forum here.
A round of 10 ohm resistors for all my pentodes! I can't see any other way to do it besides using a pair of those tube socket adaptors with the internal resistor and test leads, as you mentioned.
Thanks again. Great forum here.
Balance without resistors
I think the balance circuit could be adjusted by using a scope and dummy loads on the speaker taps to monitor for noise that could be added to the cathodes using a signal generator or other source of low voltage AC. The signal would be applied through a cap to protect the test equipment from the DC.
The pentodes in push pull will cancel this noise when balanced and that should be evident on the scope.
I think the balance circuit could be adjusted by using a scope and dummy loads on the speaker taps to monitor for noise that could be added to the cathodes using a signal generator or other source of low voltage AC. The signal would be applied through a cap to protect the test equipment from the DC.
The pentodes in push pull will cancel this noise when balanced and that should be evident on the scope.
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