06-12-2019, 01:51 PM
I must admit that my attention to this thread has been focused on the DMM aspect up to now and hadn't really taken on board how evil that nasty little adaptor is!
The first question that springs to mind is why would anybody want to use one?
It obviously inserts a 1Ω resistor between the anode and anode load and has SPDT switch which connects one meter lead to either the anode or the cathode whilst the other lead is permanently connected to the other side of the resistor.
The anode (or plate, if you prefer) voltage setting is a little ambiguous anyway because it is really measuring anode - cathode voltage.
So somebody measures the anode current and flicks the switch to measure the anode voltage, forgetting to change range on the DMM first ...
So, buy a new meter and start again.
Bob may say that he would never make such a mistake but, in my defence, I would refer him to his picture of the device and the setting of the DMM alongside it.
I don't know how many of these things have been sold and how many meters have been killed by people using them so i can only take Bob's device as an example - can I please make it clear that this is an attack on the device itself, it is not personally aimed at Bob!
I always insist on the negative meter lead being fitted with a crocodile clip (and have a second lead fitted with probe, if the probe end is not interchangeable). As an alternative I use probes with a spring hook so that the negative one can be clipped on a suitable earth - in this case, the earthy end of the cathode bias resistor, for example. My attention is then fully focused on the positive probe and where it goes (and it's a lot safer, too!)
Note too, that when making individual measurements, rather than just flicking a switch, the very act of removing the probe after each measurement should send an automatic prompt to the brain to consider the meter range selection.
Let's treat this as a fault finding exercise. Before applying power, measure the value of the cathode bias resistor - after all, if it has changed it could have some bearing on the fault!
With the unit powered, measure and note the anode voltage, then the cathode voltage. Using Ohms Law it will no be possible to calculate the combined anode and screen grid current, if required. However, the lion's share will be the anode current. If you really want only the anode current, measure the HT voltage and screen grid voltage to calculate the current through the grid's feed resistor and subtract it from the total.
What really niggles me about all this, though, is why?
Voltage readings in the published service data should be more than adequate for most purposes and should be available with any decent DIY design. The well known Mullard 5-10 design, for example, includes a table of voltages and also of relevance to an earlier part of this discussion, the design originally appeared in Practical Wireless only 3 years after the introduction of the AVO 8, by which time it was obviously already the instrument of choice for voltage readings.
The first question that springs to mind is why would anybody want to use one?
It obviously inserts a 1Ω resistor between the anode and anode load and has SPDT switch which connects one meter lead to either the anode or the cathode whilst the other lead is permanently connected to the other side of the resistor.
The anode (or plate, if you prefer) voltage setting is a little ambiguous anyway because it is really measuring anode - cathode voltage.
So somebody measures the anode current and flicks the switch to measure the anode voltage, forgetting to change range on the DMM first ...
So, buy a new meter and start again.
Bob may say that he would never make such a mistake but, in my defence, I would refer him to his picture of the device and the setting of the DMM alongside it.
I don't know how many of these things have been sold and how many meters have been killed by people using them so i can only take Bob's device as an example - can I please make it clear that this is an attack on the device itself, it is not personally aimed at Bob!
I always insist on the negative meter lead being fitted with a crocodile clip (and have a second lead fitted with probe, if the probe end is not interchangeable). As an alternative I use probes with a spring hook so that the negative one can be clipped on a suitable earth - in this case, the earthy end of the cathode bias resistor, for example. My attention is then fully focused on the positive probe and where it goes (and it's a lot safer, too!)
Note too, that when making individual measurements, rather than just flicking a switch, the very act of removing the probe after each measurement should send an automatic prompt to the brain to consider the meter range selection.
Let's treat this as a fault finding exercise. Before applying power, measure the value of the cathode bias resistor - after all, if it has changed it could have some bearing on the fault!
With the unit powered, measure and note the anode voltage, then the cathode voltage. Using Ohms Law it will no be possible to calculate the combined anode and screen grid current, if required. However, the lion's share will be the anode current. If you really want only the anode current, measure the HT voltage and screen grid voltage to calculate the current through the grid's feed resistor and subtract it from the total.
What really niggles me about all this, though, is why?
Voltage readings in the published service data should be more than adequate for most purposes and should be available with any decent DIY design. The well known Mullard 5-10 design, for example, includes a table of voltages and also of relevance to an earlier part of this discussion, the design originally appeared in Practical Wireless only 3 years after the introduction of the AVO 8, by which time it was obviously already the instrument of choice for voltage readings.
