Multimeters

[TOA Bassman]

I've got a drawer full of cheap multimeters which all work perfectly well apart from the DC mA functions which I've fried whilst checking bias current by the transformer shunt method. I toasted another one last night checking a new set of valves - something happened momentarily within the valve and the fuse in the meter blew, but not quickly enough to save the meter. The valves and amp carried on as if nothing had happened but the meter is bound for the graveyard drawer.

Any recommendations for a better protected type of meter or additional protection for the £10 ones?

I know I could use a 1 Ohm resistor on the cathode but I like the simplicity of the transformer shunt method.

[Mick Bailey]

The Brymen DMMS are well protected. Sometimes sold under other brands - see here at 19:58;

https://www.youtube.com/watch?v=tXu0lsOjvDs

I have a older Wavetek (also sold as Beckman) that's overload protected to 3KV - they go for very little because of their basic functionality. Pretty much indestructible in tube amp use.

[Dave H]

I've got a drawer full of cheap multimeters which all work perfectly well apart from the DC mA functions which I've fried whilst checking bias current by the transformer shunt method.

If only the DC mA function is dead you could still use one to measure the bias current. Measure the resistance of the two halves of the OT then measure the voltage across each half and use ohm's law to calculate the bias current.

[J M Fahey]

I know I could use a 1 Ohm resistor on the cathode but I like the simplicity of the transformer shunt method.

You also like living dangerously and burning meters

That said, those which still measure DC means they are still alive, just the current meter fuse blew, usually 200mA fast blow, open case and replace them.

In any case, even using transformer shunt methods, you do NOT set them to "current" for God´s sake, but do what Dave H suggests above.
You **always** use a voltage measurement, just across winding DC resistanc instead of added 1 ohm cathode resistor, but the principle is exact same.

[Leo_Gnardo]

You also like living dangerously and burning meters

That was me too until a couple years ago. I got tired of replacing popped fuses in my trusty old Fluke DMM. Since then I've been using Dave H's method. Helps to have a calculator handy besides the multimeter, and you get a more accurate bias current reading.

[drewl]

Yeah, there's usually a fuse or fuseable link to protect the meter. My Flukes have them.

Years ago those rotary switches used to go bad often. I worked at a place that repaired test equipment, and we used to call it the "Beckman Switch"
HP had a crappy line of scopes which used a similar set up, so we used to joke around surmising that the guy who designed the Beckman Switch was fired, then went to HP and got canned for the same lousy design.

[g1]

Just use the 10A range.
The issue is not usually during the shunt measurement, it's the damage you do to the next piece of equipment you work on, because you forget the meter is set to current and is now essentially a straight piece of wire.

[Mick Bailey]

I rarely use a DMM on current ranges on for that very reason.

[TOA Bassman]

Thanks for the replies. I hadn't even considered using the OT primary resistance to calculate current. I tried it last night and was surprised at how mismatched the windings were - averaged to 60 ohms one side, 74 ohms on the other using three different meters - but got to very closely matched plate currents. I'll check the resistances again with the amp hot just out of interest.

As I said in my initial post, "...the fuse in the meter blew, but not quickly enough to save the meter..." They're actually F125mA, so it must have been a decent spike to kill the meter before the fuse went :-)

[Mick Bailey]

Mismatched resistance comes up frequently. It's entirely normal and the reason for the discrepancy is that the turns closer to the centre use less wire per turn and as the windings are layered you need progressively more wire per winding, therefore more resistance. Not all transformers are wound this way, but it's pretty common for run-of-the-mill guitar amps. It makes no difference to the current measurements

[Gnobuddy]

...using the OT primary resistance to calculate current. I tried it last night...

A few years ago, I tried this method on three different Fender guitar amps (a Blues Junior, a '65 Princeton Reverb reissue, and a Superchamp XD hybrid). In all three cases, the amp became unstable as soon as the meter leads touched the output transformer primary (i.e., one of the output valve anodes).

My hypothesis is that the meter lead acts as an antenna, radiating the signal at the output anode, and provides enough coupling back to the amp input to trigger outright oscillation.

-Gnobuddy

[Enzo]

In every case of that I have encountered, it only does that on one side, leaving me the remaining side for my useful readings.

[Gnobuddy]

In every case of that I have encountered, it only does that on one side, leaving me the remaining side for my useful readings.

That supports the hypothesis - positive feedback from one anode, negative feedback from the other.

The trouble is, I was hoping to verify that both output valves actually had (nearly) the same bias current. I failed at that because of the instability problem.

-Gnobuddy

[Enzo]

Once in a while reversing the meter probes helps.

[Gnobuddy]

Once in a while reversing the meter probes helps.

Thanks for the suggestion! I'll certainly try that next time the situation arises!

-Gnobuddy