The 20 MFD 25 WVDC electrolytic that stabilizes the voltage across resistors 37 and 39 has its positive connected to the chassis, and its negative connected to the center tap of the transformer, because the center tap of the transformer is in fact more negative than the chassis. Studying conventional electron flow will reveal this fact. Resistors 37 and 39 serve to drop a small amount of voltage so that there will be negative bias points with respect to the audio tubes' cathodes.
Because the chassis is a common tie point does not necessarily mean that it is negative with respect to all other points within the radio, depending on the wiring, and when dealing with a common point such as earth, and when this earth is referenced to the alternating current in your home, (at least in the U.S.A.) the earth is neither always negative or positive with respect to the wiring in your home, but is constantly changing 60 times a second. It is merely 'cold' with reference to the wiring in your home only because it is connected to part of that wiring, and we consider all other points of that wiring to be 'hot' with respect to it. You could also correctly say that it is hot with respect to the wiring in your home, but we don't normally say that. That a common tie point is always negative or cold or anything of that nature is a misconception many people have, and in fact everything is all relative.
T.
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:The 20 MFD 25 WVDC electrolytic that stabilizes the voltage across resistors 37 and 39 has its positive connected to the chassis, and its negative connected to the center tap of the transformer, because the center tap of the transformer is in fact more negative than the chassis. Studying conventional electron flow will reveal this fact. Resistors 37 and 39 serve to drop a small amount of voltage so that there will be negative bias points with respect to the audio tubes' cathodes.
:
:Because the chassis is a common tie point does not necessarily mean that it is negative with respect to all other points within the radio, depending on the wiring, and when dealing with a common point such as earth, and when this earth is referenced to the alternating current in your home, (at least in the U.S.A.) the earth is neither always negative or positive with respect to the wiring in your home, but is constantly changing 60 times a second. It is merely 'cold' with reference to the wiring in your home only because it is connected to part of that wiring, and we consider all other points of that wiring to be 'hot' with respect to it. You could also correctly say that it is hot with respect to the wiring in your home, but we don't normally say that. That a common tie point is always negative or cold or anything of that nature is a misconception many people have, and in fact everything is all relative.
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:T.
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T.
Thank heaven for the wire color codes provided. I am placing a marked up thumbnail just below, in the ensuing context, so that you can refer to certain aspects of the units circuitry. Indeed this set is using a raising of the HV secondary center tap of the power transformer, up from its normally grounded connection, in order to be able to acquire two low level negative bias voltages for its circuitry.
Most common procedure is to have that tap going direct to ground. An alternate situation, exactly as we have here, is that they were wanting to acquire some negative voltage 1st grid biasing levels for the AF amp and the AF output amp. The way that they have done that, is to have the center tap lifted off from the ground by some low value resistors. Trace down the transformers HV secondary center tap via the emboldened [GREEN/BLACK] path and see that it initially encounters a 270 ohm and then a 68 ohm series pair of resistors. The pair is holding the center tap above ground referencing; therefore a negative voltage is being developed to the right side of that 270 ohm resistor. Now, since they were also needing a lower value of voltage for the AF amp, they had the second resistor to act as a voltage divider tap off. The selection of the resistors total values is relevant to the amount of bias voltage level being developed. A lower resistance value creates a lower voltage or a higher resistive value is used for establishing a higher negative voltage value. The purpose of two resistors in series was the establishing of a divisible ratio so that a lower level of bias could also be acquired at that tap off for the AF amp tubes bias level.
Additionally note that the negative leads of the 10 ufd @ 250 v and the 15/20 ufd @ 350 v filters are acquiring their DC
A quick check for seeing if this type of circuitry is being used in a sets design? Look at the treatment of the center tap of the high voltage winding, if going directly to ground, a common power supply is being utilized. HOWEVER, if you see that center tap being held above ground level by a resistor or series of resistors, they are using this floating type of supply and you need to watch a few of the power supply electrolytics, as one or more may SEEM to be . .or END UP getting re-wired back in with reversed polarity. Another tell tale sign is to check the AF and AF output tubes to see the treatment of their cathode connections. Their cathodes are connecting directly to ground.
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