Quote:
Originally Posted by ct92404
go lower or short out (or at least it's extremely rare). But I'm wondering if one of the resistors went bad and could cause a short giving a direct pathway for the 275 plate voltage to other parts of the tube? I don't have the diagram with me at the moment, but I remember that a 275 v pin is connected to another pin THROUGH a resistor.
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Chris,
A lot can be worked out from the plate voltage on a valve(tube) signal amplifier stage.(I'll use the word tube since its an American set)
The first thing to do is look at the the plate load. If it is a transformer (RF or audio type) the DC resistance is relatively low, especially RF coils. So in this case, over a very large range of the tube's plate currents, the tube's plate voltage will appear with the meter to be about the same as the HT voltage or the voltage supplying the other terminal of the load.
In these cases the only way to get a handle on the tube's plate current, is to look at the voltage across the tube's cathode resistor and divide the resistance value into that to find the current. For example you could have a signal stage in the radio frequency section where the tube was biased off, or lost its emission and had little or no plate current, the plate voltage would not be very informative and the cathode voltage would be a better test.
When resistors are used as plate loads, for audio amplifier circuits, normally the voltage you see on the plate is very roughly 50% to 60% of the HT supply. This is so the voltage there can swing around without clipping to the HT voltage or bottom out when the tube is fully conducting.
In RF circuits or IF amplifier circuits, even though the voltage drops across the RF & IF coils themselves are very low, there are often series resistors in the HT supply of a few k ohms or more and filtered to ground with bypass capacitors. The voltage drops across these resistors gives an idea of the plate current to the tubes they supply. So that is a way of getting a handle on the plate currents.
In tube TV circuits where are sync separators, or circuits that handle pulses, usually the plate loads are resistors, the DC voltage on the tube plates could be quite high or even in some cases low in the absence of signal, depending on the design of the stages. In this case the DC levels on the plates with a meter can be less informative. Also using a digital meter, set on DC, might show all sorts of fluctuations say when being fed with a DC voltage with a large AC signal amplitude pulse on it, like for example separated horizontal sync pulses or separated vertical sync pulses.
It is possible therefore that some fluctuations you are seeing are measurement artifacts. Analog meters tend to ignore the pulse component and average out the values. It could be though that when you are noticing the fluctuations when the picture sync is lost that this represents the sync pulse signal coming and going due to a fault further upstream and not in the actual stage you are testing.
This is really why, especially in parts of the circuit where pulses are present, you really need to put a scope on it to see what is going on there, the meter has very limited utility in these sub circuits.
Also, make sure to keep your tests well away from the anode of the horizontal output tube and the efficiency diode area, the peak voltages there can be very high and just as, or more destructive, to test instruments as the actual EHT.