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Old 21st January 2009, 05:15 PM   #1
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Default Question about Cathode bias in this Magnavox amp. . .

If you look at the output tubes , the cathode's are tied to the heater filament supply. I haven't seen this except on Magnavox amps. Every other schematic I look at has the cathode tied to ground via a resistor and possibly a capacitor (bypassing the resistor), but not to the heater filament supply.

Why is this ? Can I remove this link ? Seems as though it will just introduce more PS noise into the audio chain.

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......................Blake
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Old 21st January 2009, 05:45 PM   #2
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They're using the cathode voltage to bias the heater winding positive. This can reduce hum by turning off the parasitic diode that may exist between heater and cathode.

Power amps generally don't need much attention the heater circuit - grounding one side of the heater winding may be fine. A center-tap or hum pot MIGHT make a noticeable improvement, and positive bias might improve it further. A preamp might require these measures, or even DC heaters. An AC ground is required in all cases to avoid common-noise noise.
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Old 21st January 2009, 06:43 PM   #3
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So it is essentially used to reduce noise induced by the AC powering the heater filaments ?

You say it is typically used for preamps, less so in power amps. Strange that they would use it in the output tube section and not with the phase inverter/input tube, isn't it ?

I should be able to remove this without any harm , then ? Just check for any additional hum/low level AC voltage .

The linking of the heaters AC to the Cathode bias seems to be counter intuitive to what you are saying. I am not arguing with you, or trying to start trouble. It just seems that the connection of an AC source to the Cathode would introduce more of a problem than the introduction of a DC bias to the heaters would solve.

Should there be no AC component, due to it's "center feeding" by the two 100 ohm resistors ? I assume if that is the case, then it would be dependant on how well the 100 ohm resistors were matched, no ?

Thank you for your time..............................Blake
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Old 21st January 2009, 07:33 PM   #4
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No AC is being introduced at the output tube cathode (1 wire connection is not a complete circuit). The cathode voltage (~+16V to ground) is used a source of bias on the heater winding for the input tubes. A voltage divider from B+ is another way this is sometimes done. If the tuner / preamp is powered from this winding, it would reduce hum there too. If there is a hum pot, it may not have any audible affect on the power amp, but MIGHT help when there's a preamp connected.

On the other hand, some of these maggies have a separate winding for the tuner / preamp...
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