Re-tube em theory.. Why?

[nickb]

Thank you OTM. Between you, RDH3, and Dr. Google, I'm learning some things, I think, Dr Google found:
"Gm.....the symbol for mutual conductance -- mutual conductance....the measure of alternating plate current (measured in micromhos) in relation to the control grid signal voltage."

Seems to me, when power tube is in an amp, if voltages of all pins are in spec, bias current consistent with other tubes tested, seems to me measuring output current at the OT should be sufficient to determine mutual conductance. I'm saying any amp, with all pin voltages at power tube in spec, measured output power of a new tube could be the standard for other tubes tested in same amp.

Seems to me, IF there's an audiable difference with a failing tube, it should appear as a measurable loss in output power if checked, showing the loss of mutual conductance. My amp is my tube tester.

Yes, power output is my favorite way of testing power tubes and amps do make pretty good testers.

Bias and static plate current are a DC thing. Gm relates to changing things. It's like the slope of a line is gm and the yaxis offset is the bias current.

[Old Tele man]

Yep: gm = dIp/dVg

...although there actually *IS* is DC-relationship for gm, because 'idle' plate current (Ip.q) changes proportionally with changes in the DC bias grid voltage...so it is not exclusively an AC-function as most people think.

[cyclone]

I too have the trusty Hickok 533. I find the "life test" to be very useful in identifying tubes that still pass the mutual conductance test but are worn. You can do the same thing with other testers by dropping the filament voltage one step. If, during the transconductance test, you drop the filament voltage, you will see the needle drop as the filament cools. if it continues to drop out of the "good" range, tube is beat. It is pretty obvious, new tubes, the needle doesn't move much if at all.

I test my power tubes in my guitar and audio amps once a year or when they start to clip at a lower volume and sound raggedy. Sometimes, I change just one device :gasp!

Preamp tubes I don't touch once they are installed until they get microphonic. Tube testers can be hard on preamp tubes and handling tubes a lot is bad for them. A lot of people cause their own issues by "tube rolling" etc.

[mhuss]

A lot of people cause their own issues by "tube rolling" etc.

This ^^^^

[g1]

Tube testers can be hard on preamp tubes and handling tubes a lot is bad for them.

Are the tube testers hard on preamp tubes electronically, or physically because of the handling issues you mention?

[Enzo]

Plugged into a tube tester is about the easiest gig a tube can have. They rarely put any stress on a tube at all.

[g1]

That's what I figured, but I'm all ears if I'm unaware of some non-physical hazard.

[loudthud]

Plugged into a tube tester is about the easiest gig a tube can have. They rarely put any stress on a tube at all.

I don't know about that. I'll bet in the life test they knock the heater Voltage down, hook the plate and grid together and see how much current the tube will pass. Such a test may only be activated by a momentary switch.

[g1]

I'm not sure about other types, but for the Hickock 533 and Stark 9-66, the life test only reduces filament voltage, nothing else. They tell you how much of a drop in the meter indication is allowable. It is a regular toggle, not a momentary switch.

[The Dude]

I don't know about that. I'll bet in the life test they knock the heater Voltage down, hook the plate and grid together and see how much current the tube will pass. Such a test may only be activated by a momentary switch.

I've not heard of anything like this on any tester I've encountered. I'm with Enzo on this one. There's much more stress on a tube in an amp than on a tester. That is why testers are not a fully reliable way of testing tubes. If the tester says it's bad, it's bad. If the tester says it's good, it might still be bad.

[Justin Thomas]

And that's assuming the tester' s directions are accurate... I have an Eico 66-something, and according to lite, the settings charts are riddled with errors. The previous owner was obviously a guitarist, as he edited, fixed, and fudged the settings for just about every tube used in amps. Usually, as I understand the way the tester works, he Jack a up the plate voltages a lot... So, who do I trust more with my Telefunken ECC83?

Justin

[Chuck H]

As far as the suitability of tube testers for determining relative goodness... I can't even remember the model, but I use to get a lot of used tube pulls from Halted Electronics. I'd dig through the boxes and check them on the tester the store offered just for that purpose. The tester was really only good for recognizing grossly bad tubes. About half of the tubes I brought home would have noise issues, microphonics, would drift around or otherwise fail put into a guitar amp. And the life test? For a guitar amp I think you want a tube that is reasonably strong. Like over 80%. With the higher voltages and extreme dynamic conditions in guitar amps as compared to table radios or T.V.s there is a lot of emission demanded of the cathode. It's pretty easy to spot a tired tube in a guitar amp if you're familiar with the tone, like your personal amp, or by simply swapping suspect tubes with younger tubes. Leo wrapped it up pretty well in the first response. For some amps and players a tube that "still has some life left" is fine. For other players that want/need the most dynamics and drive the amp hard you need to keep strong, young tubes in the amp.

My practical, blue collar experience on what's already been covered

[Mick Bailey]

A tube tester doesn't replicate the dynamics of a guitar amp and so even the best offer only a static test. A combo especially subjects a tube to conditions that probably wouldn't exist in any other piece of commercial equipment. Apart from vibration, there's the effects of the speaker magnet (noted in some amps for causing overheating - some Laneys, for example), additional heat from adjacent power tubes, and overall susceptibility to noise from stray EM fields.

If I had a reason to discard a working tube, noise and microphonics would be at the top of the list. Sometimes a tube that isn't obviously microphonic will feed back at high volume.

A tube tester that measured noise at guitar amp gain and voltage, plus vibrated the tube at different frequencies to get a reading for microphonics would be a better bet.

This unit gives a more practical evaluation of tubes for audio use; https://www.youtube.com/watch?v=205__gxWJc8

[mhuss]

I've often thought that making a dedicated noise-testing fixture using realistic voltages and loads would be a great idea. I'd like to use a vibrating motor (like in those in cell phones) or something else more consistent than the 'pencil tapping' technique, though.

[nickb]

Agreed that something consistent is needed. I don't think a vibrator will do it and it needs to be impulsive in nature viz. have a broad frequency content. A vibrator is going to more a low freq sine wave. I was thinking more of like an automatic center punch kind of mechanism or perhaps using a weight and electromagnet.