Is Testing a Tube For Shorts with a Multimeter Conclusive (or Inconclusive)???

[1992guitars]

Short question without the long backstory: do professional grade, calibrated tube testers detect shorts that would not be detected by a simple continuity test with a multi-meter? Sorry if this has already been answered elsewhere.

Because I'm trying to learn this stuff and with exorbitantly long wait times and rising costs, once and for all, I'm learning to take my own repair and upkeep into my hands at last, and knowing how to detect a shorted tube would be useful to understand.

 

[Jon Snell]

The Thermionic Valve does not conduct electrons until the cathode is hot.
Flashover occurs when there is a high voltage between the anode and the two or three grids or between the grids and only happens when active, under high voltage and hot.

A simple test with a DVM that can utilise a mere 3 - 5 volts is not enough to show up a faulty valve.

 

[1992guitars]

This is actually what I was hoping to hear. Fantastic. Thanks

 

[Michael Smith]

A tube that was physically damaged on the inside would probably show a short on a multi-meter, but for that to happen the glass would probably already be broken.

 

[Pete Farrington]

A bad valve might have failed insulation between its heater and cathode.
But that’s just one failure mode amongst many.
A valve tester is kinda pointless though unless it can check valves under the harsh conditions close to those of a guitar amp. Which the vast majority don’t.
So why not just use a guitar amp?
A light bulb limiter can be used to protect it from being damaged by shorted valves.

 

[1992guitars]

So why not just use a guitar amp?
A light bulb limiter can be used to protect it from being damaged by shorted valves.

Thanks Pete. This is all helpful stuff. To be clear, there is no issue at the moment. If I don't learn ahead of time, I won't know what to do when I need the knowledge next time though. Question about that light bulb limiter....

I have one. I built it with my Dad. The more I research the more I hear about awful things like shorted tubes taking out an OT and general horror stories (which must be rare). When re-tubing (down the line), obviously powering up with the light bulb sounds like a good idea. I have no plans or investment in all of a sudden going NOS crazy but without a tube tester at my disposal, would one want to leave the amp on and plugged into the light bulb limiter for several hours (provided the bias is checked) with new tubes? What is the best course of action to realistically lower one's risk of getting a bad tube and all of sudden finding out the wrong way? Obviously I've shorted tubes before without causing any other major damage to the amp but I'm clear it's not desirable in any event!

 

[Jon Snell]

"The more I research the more I hear about awful things like shorted tubes taking out an OT and general horror stories (which must be rare)".

That is what the HT fuse is for, to protect agains short circuit conditions.

The light bulb limiter is good for faulty transformers, wiring and rectifier valves. Well worth the investment, if you can find a corner shop or fleabay that have some still.

 

[1992guitars]

Thanks Jon, glad to know I wasn't out to lunch quietly thinking this (about the HT fuse) the whole time. I suppose bad things could happen when people put a larger fuse in than should be there. I saw some thread where someone's NOS tubes took out his OPT on maybe a Vibrolux (I can't remember, tried to block it out!).

 

[Pete Farrington]

without a tube tester at my disposal, would one want to leave the amp on and plugged into the light bulb limiter for several hours (provided the bias is checked) with new tubes?

Again, although a regular valve tester will pick up many faults, plenty others will only manifest at high voltage, perhaps with the amp trying to work hard.

With a light bulb limiter, it’s best to have a selection of bulb wattages available.
A 25W bulb would be ok for a champ but not for a twin reverb. Whereas 250W may be necessary for an SVT.
But a 100W plus bulb won’t do much to protect a champ.
The bulb should be the lowest wattage that allows the amp to function in the test condition it’s in.

With a typical 30-60W amp and unknown / ‘new to me’ / otherwise suspect output / rectifier valves, I’d start with a 40 or 60W bulb and check idle was ok. Don’t attempt a final bias, just check anode cathode screen grid currents are reasonable.

Then try it with a 100W bulb and, after checking that the idle dissipation is ok with the increased voltage, with increasing signal output. The bulb will get brighter and will be reducing the amp’s voltages and power output. But if any of the valves start flashing / crackling / having internal lighting storms / shorting out, shut the amp down and mark up the bad valve. Don’t rely on remembering which it was.

HT fuses are great, but many amps don’t have them, and of those that do, their implementation often forces them to be a time delayed action / slow blow type. Which means they may take many seconds to respond to a problem valve.

 

[Wally]

Out of curiosity, I have used a multimeter to check for shorts in a tube that was known to be bad And which exhibited shorts on a simple tube tester. Yes, there are certain gross shorts that an ohmmeter will reveal.

 

[Lowerleftcoast]

knowing how to detect a shorted tube would be useful to understand

A short in a tube is not something that slowly comes on. It is a no warning event (or little warning). Your amp will tell you. Even if you had a tube tester, I doubt it would find the trouble. You would test the tube after the fact. Too late. Iow, there is little need for a tube tester. Having some good spare tubes on hand is a cheaper option.

Ask yourself: How often do I have my tubes checked on a tube tester? How often does it show a problem?

 

[Diverted]

Short question without the long backstory: do professional grade, calibrated tube testers detect shorts that would not be detected by a simple continuity test with a multi-meter? Sorry if this has already been answered elsewhere.

Because I'm trying to learn this stuff and with exorbitantly long wait times and rising costs, once and for all, I'm learning to take my own repair and upkeep into my hands at last, and knowing how to detect a shorted tube would be useful to understand.

Short question without the long backstory: do professional grade, calibrated tube testers detect shorts that would not be detected by a simple continuity test with a multi-meter? Sorry if this has already been answered elsewhere.

Because I'm trying to learn this stuff and with exorbitantly long wait times and rising costs, once and for all, I'm learning to take my own repair and upkeep into my hands at last, and knowing how to detect a shorted tube would be useful to understand.

I can answer from very recent personal experience: the only real tester is the circuit itself.
I have a very good mulitary tester, Hickok TV10D-U. Recently checking tubes from a 1932 Rickenbacher Type I (see recent post) the two 47 output tubes passed with flying colors. No shorts, no gas, strong readings. When I was ready to fire up the amp for the first time it went perfect, good tone right out of the gate. About five minutes in the sound went to crap. This continued to happen every time I powered it up until I checked voltages with tubes both hot and cold and found 200v on one of the grids. Turns out it had a plate to grid short that only showed up after the tube was quite warm.
So testers can be fallible. When testing be patient and let the tubes warm up fully!

 

[1992guitars]

With a typical 30-60W amp and unknown / ‘new to me’ / otherwise suspect output / rectifier valves, I’d start with a 40 or 60W bulb and check idle was ok. Don’t attempt a final bias, just check anode cathode screen grid currents are reasonable.

Pete, I'm making myself vulnerable here to expose my lack of knowing so openly, but where can I read more or any resource I should pursue to learn how to check cathode and screen grid currents? Anode/plate currents make sense to me just fine and I've checked the bias on two amps now so far without electrocuting myself (using the voltage drop from the center tap divided by the resistance of each half of the OPT). Just cathode and screen grid currents are the question and I will put in the work and time if I know what to study. Thanks as always for any suggestions you might have as I keep learning slowly but surely, one day (and night) at a time

 

[Pete Farrington]

I'm making myself vulnerable here to expose my lack of knowing so openly

Crikey, don’t worry about that
No one was born knowing this stuff.

how to check cathode and screen grid currents? Anode/plate currents make sense to me just fine

Sorry, instead of cathode current, I should have written anode / cathode current.
Cathode current is anode current plus screen grid current.
Screen grid current can be checked by measuring the voltage dropped across the screen grid resistor, and using Ohm’s law to derive the current, same as with your anode current measurements.
Personally I prefer to fit 1R cathode resistors and avoid probing output valve anodes unless absolutely necessary. As doing so causes the probe / lead / meter to act as a transmitting antenna for the anode signal, reducing the amp’s margin of stability, often resulting in oscillation.

 

[JIMMY JAZZMAN]

When growing up, we went to the drug store to test our TV tubes. Green (go) Red (buy a new tube). Is anything
out there that I can purchase to check just run of the mill amp tubes? Even an analog output would be great.

 

[Pete Farrington]

Is anything
out there that I can purchase to check just run of the mill amp tubes? Even an analog output would be great.

Absolutely.
It’s the valve amp that you use the valves in, and your ears.
Ideally, in the first instance with new / unknown / suspect valves, to reduce the risk of shorted valves damaging the amp, use a light bulb limiter between the amp and wall outlet.
If it doesn’t stay bright then plug the amp directly into the wall outlet.

My understanding is that none of the simpler valve testers puts the valve under test under the same conditions that your amp does. So the results from the tester may give a very different finding than just putting the darn thing in your amp and listening.

 

[1992guitars]

Screen grid current can be checked by measuring the voltage dropped across the screen grid resistor, and using Ohm’s law to derive the current, same as with your anode current measurements.

Thanks Pete, that makes sense now then. As far as Ohm's Law for the screen grid current calculations, when you say "same as with your anode measurements," for plate current, I was measuring the voltage drop from the center tap of the OPT to the anode, then dividing the voltage drop by the resistance the half of the OPT winding connected to that tube (and if I'm understanding, this is not the method you'd advise). But....

For screen current though, you're saying measure the drop across the SG resistor, and then divide that voltage by the resistance of that SG itself, correct? Just making sure I've understood correctly.

 

[Pete Farrington]

measure the drop across the SG resistor, and then divide that voltage by the resistance of that SG itself

Yes