Can I test a valve with a Multimeter??????

The slick (but rather obvious) dude reply to this would be, ”Why am I going to take a second mortgage out on a valve tester, if I can do it with my three quid Maplin Multimeter?” You’d have to admit that even an accountant could have worked that one out.

So the answer is, clearly, “No you can’t.” But just a minute; this is a thinking blog! I made the choice between a Weetabix and a piece of toast this morning, so it has to be. The useful answer is “Up to a point”. So here we go, onwards and A&E wards.

At the top of this is a layout of a B9a valve base. It’s also the layout of the pins of the valve looking from the bottom, and what they connect to in the valve. This one is the good old ECC83, (or 82 or 81). The characteristics are different but the pin layout is the same. These are the things you can test with your multimeter on the ohms scale.

The heater integrity. The ohms reading across pins 4&5 will be very low (just an ohm or so) and the reading across either of those to pin 9 will be half that reading. If it’s open on any of those readings, the heater is broken and the valve is dud. It’s hard to tell if it’s shorted or not, but the usual reason for a shorted heater is that it’s shorted to the cathode, and the next test will tell you that.

Heater to cathode resistance. This should read an open circuit, pin 3 and pin 8 to pins 4,5 or 9 (or at least a lot of megohms if it’s a damp day). If not it has a heater- cathode short and is one for the bin. It will usually put out a very loud hum in that condition. Although on an ECC83 this is just a nuisance, on a power valve it can be a disaster, because it can destroy the bias and do a lot of damage.

Anode to anything resistance. You can test pin 1 and pin 6 to any other pin and it should read open circuit.

Same for the control grids (pins 2 and 7); open circuit to everything else.

So why bother with a valve tester? A valve tester tests a valve under dynamic conditions, which means it has voltages applied to it and various currents are measured to determine the characterists of the valve. A multimeter can only put a few millivolts across the various elements.

What it can do is tell you when a valve is definitely dud. A broken heater or shorted cathode to grid, grid to heater etc., and your valve will not do anything . But there are other faults that can look ok on a multi meter but the valve will not work.

So there we have it. Whatever it is.

Time for tea. Hmmm….maybe a macaroon??????