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07-06-2023, 10:47 AM
Hi Guys
The limitations of the forum text formatting does not allow accurate depiction of the generic quantity names with respect to the use of subscripts. In general, when the subscript is an upper-case letter, the quantity is external to the tube or device; a lower-case subscript represents an internal quantity or parameter.
In ASCII (web text) an asterisk is often used to denote that the next number/letter is to be a superscript. For example, c*2 would represent the quantity 'c' raised to the second power (squared). I'm sure there is a corresponding standard for subscripts but I have not seen it. Below, I am going to use an apostrophe, as V'A. It might be more usual to use a hyphen, but then V-A looks like an equation and the reader has to understand by context that it is not.
In any case, specifically with voltage designations in tube circuits, TUT3 lists the preferred supply node designations. Other tube parameters are listed throughout the TUTs as they are needed. So, for an output stage, there is always a plate (anode) supply node called V'A. If the tube is a tetrode or pentode, there is usually a supply node for the screen, that we call V'S. The actual voltages at the tube terminals would be V'a and V's, respectively.
Often for convenience the internal / external aspect of the parameter may be set aside if there is an understood context for the presentation and the quickest form of the designation helps move things along.
When there is a voltage that uses a reference other than ground, the subscript will have two letters where the second designates the reference point, as in V'gk. This is the voltage between the grid (g) and the cathode (k), where the cathode is the reference.
In old texts, a "battery" supply designation is used, as would have been the case for simple experimenting, or for prior to the widespread availability of an AC mains distribution system:
A heater supply battery
B anode supply battery
C grid supply battery
D and beyond can be used randomly as required. Most other circuit voltages would be derived from B or C. A polarity designation is often added to B and C as B+ and C- to indicate how the battery is connected.
In an output stage where a choke or transformer load is used, the voltage at the plate will be more or less the same value as at the related filter cap. The screen voltage will be similar regardless of the screen resistor value.
In a preamp stage with a resistive plate load, there will be a significant voltage difference between the filter cap voltage and the tube anode voltage due to the drop across R'A. Preamp tubes have a deceptively low plate voltage rating as an inductive load is assumed, as was the case with very early circuits and applications. What percentage of circuits a tube is used in that have inductive loads is not known, but the rating is antiquated. Careful reading of the complete data sheet often indicates the true limit for the tube.
For example, when you look at 12AX7 ratings, you might see 300V listed for maximum plate voltage. Should that be what the filter cap voltage is, or should it be the voltage at the tube? Most people would assume the former just to be safe. The fact is that a 12AX7 has an arc voltage of 540V, which allows a supply voltage of 300V with an inductive load that can produce close to 540-300=240V peak. With a resistive load there are no flyback voltages, so B+ in this case can be close to 540V. This is a good thing since a cold start-up can potentially charge every supply filter cap to the highest unloaded voltage of the rectifier output.
The limitations of the forum text formatting does not allow accurate depiction of the generic quantity names with respect to the use of subscripts. In general, when the subscript is an upper-case letter, the quantity is external to the tube or device; a lower-case subscript represents an internal quantity or parameter.
In ASCII (web text) an asterisk is often used to denote that the next number/letter is to be a superscript. For example, c*2 would represent the quantity 'c' raised to the second power (squared). I'm sure there is a corresponding standard for subscripts but I have not seen it. Below, I am going to use an apostrophe, as V'A. It might be more usual to use a hyphen, but then V-A looks like an equation and the reader has to understand by context that it is not.
In any case, specifically with voltage designations in tube circuits, TUT3 lists the preferred supply node designations. Other tube parameters are listed throughout the TUTs as they are needed. So, for an output stage, there is always a plate (anode) supply node called V'A. If the tube is a tetrode or pentode, there is usually a supply node for the screen, that we call V'S. The actual voltages at the tube terminals would be V'a and V's, respectively.
Often for convenience the internal / external aspect of the parameter may be set aside if there is an understood context for the presentation and the quickest form of the designation helps move things along.
When there is a voltage that uses a reference other than ground, the subscript will have two letters where the second designates the reference point, as in V'gk. This is the voltage between the grid (g) and the cathode (k), where the cathode is the reference.
In old texts, a "battery" supply designation is used, as would have been the case for simple experimenting, or for prior to the widespread availability of an AC mains distribution system:
A heater supply battery
B anode supply battery
C grid supply battery
D and beyond can be used randomly as required. Most other circuit voltages would be derived from B or C. A polarity designation is often added to B and C as B+ and C- to indicate how the battery is connected.
In an output stage where a choke or transformer load is used, the voltage at the plate will be more or less the same value as at the related filter cap. The screen voltage will be similar regardless of the screen resistor value.
In a preamp stage with a resistive plate load, there will be a significant voltage difference between the filter cap voltage and the tube anode voltage due to the drop across R'A. Preamp tubes have a deceptively low plate voltage rating as an inductive load is assumed, as was the case with very early circuits and applications. What percentage of circuits a tube is used in that have inductive loads is not known, but the rating is antiquated. Careful reading of the complete data sheet often indicates the true limit for the tube.
For example, when you look at 12AX7 ratings, you might see 300V listed for maximum plate voltage. Should that be what the filter cap voltage is, or should it be the voltage at the tube? Most people would assume the former just to be safe. The fact is that a 12AX7 has an arc voltage of 540V, which allows a supply voltage of 300V with an inductive load that can produce close to 540-300=240V peak. With a resistive load there are no flyback voltages, so B+ in this case can be close to 540V. This is a good thing since a cold start-up can potentially charge every supply filter cap to the highest unloaded voltage of the rectifier output.
