Bench-testing vintage power tubes?

[Mango Moon]

Greetings Everyone!

A quick, simple question from a complete newb:

I've got several vintage power tubes (6973s) that I'd like to have tested in terms of overall function and health . . . is that easily done? In other words, if I were to take them to a reliable (professional) tube testing service, would they be able to test them easily with equipment on-hand and tell me if one or more of the tubes was bad or about to fail?

Many thanks.

[Enzo]

Professional tube testing service? I like that. Down the street next to the professional coffee stirring service and the TV channel changing service.

If you go to an amp shop, you may find one who has a tube tester. I have one I don;t admit I have, hidden under my bench. People on the street have no idea what a tube tester tells them, so I don;t "test" tubes. But some local shop may be willing.

A tube tester can point out tubes that are bad, but they can't tell you they are good, really. Unless it is a darn fancy tube tester, it won;t tell you if the tube is microphonic or noisy, won;t tell you it has lost its tone. And tube testers do not usually put real world voltages and currents to the tubes under test. That is like testing a car engine but only at idle. A pair of 6L6s needs to put out 50 watts, not just sit there waiting for something to do.

The real test is sticking the tube in a circuit that uses it.

And no one is going to be able to tell you that a tube is about to fail.

[Mango Moon]

Enzo . . .

How fun is that? I swear, you never cease to amuse me my-friend, but yes, "professional" in the sense that they're not 'hobbyists' like me (i.e., they make their 'living' testing, selling, supplying vacuum tubes and other delicate electronic components), and yes, there is just such a service nearby to me. They are over 100 years old and we ain't talkin' no run-of-the-mill "amp shop". They have an extensive history in supplying a wide variety of vacuum tubes to industry, the military and the public. They also have what I (as an admitted novice) would characterize as an "above average" array of seemingly sophisticated diagnostic (load testing) equipment that is quantum leaps beyond a standard Hickok tube tester.

That said however, what you've shared here makes some degree of sense in that vacuum tubes of the type under discussion may indeed be no different than a standard incandescent light bulb - they're 'all or nothing' (i.e., they either work or they don't). But then, that was precisely the point of my question. So, thanks for answering it amigo! You da man!

I will say however that testing a single suspect tube in a functioning circuit (as you've suggested) when that circuit employs a full quad of the same variety of tube side-by-side, presents some rather obvious limitations, even to a novice like me. In other words, it would seem perfectly reasonable to me that one or more tubes in a quad of power tubes could indeed be inferior or compromised in some way and yet still present little to no readily detectable (visual or audible) symptoms when functioning in a field of four, but hey, I could be wrong, and it certainly wouldn't be the first time!

Thanks again.

[Enzo]

Well, I was of course trying to be funny. If you have a tube selling operation nearby, they probably have tube characterization equipment. But most folks do not have anything ike that nearby. And I would be wagering that that company doesn't call themselves tube testers, I suspect it is a sideline to their sales.

If ther is someone nearby who will test your tube, then by all means, go visit them.

Otherwise...

If you have an amp that uses a quad of something and you want to know if the tubes work, you have options. Pull half of them. We don;t really care if the impedance is off a notch, all we want to do is see if the tubes function OK. Try the remaining pair and then the pulled pair by themselves too. If the two pairs sound similar, then likely they all are OK. And things like noisy or microphonic will show up if you tap on them.

And for that matter, even in a push pull amp, even just one tube will operate. You will get some distortion when you drive it past the threshold of class AB, but even so, you can do static tests like current draw, microphonics, noise, etc. And once you have tested each tube individually just to see if it works, THEN you can go back and put in pairs to get a full sound out of the amp.

I often suggest this poor-man's tube tester approach to people. If you stick one powr tube at a time into the same socket on an amp, you can determine if any of the four tubes draws excess current or maybe way less current than the others under the same conditions there. The tone may suck like that, but the tone is not what we are testing. Likewise, a microphonic tube or a noisy tube doesn't need others to show of that fault. In other words, the crummy tube can;t hide among the others if it is all alone.

Yes, there are better ways, but if one doesn;t have those ways, this sort of approach will still help find bad tubes.

[Mango Moon]

Thanks Enzo! This is a great ('poor man's') approach and I may indeed try it at some point. Its certainly superior to springing for $300 for a new quad of NOS 6873's! However, I'm also learning from other sources that the particular amp in question here (a vintage Magnatone 280, rated at 50w) characteristically performed (by design) with lesser power and lesser overall volume than other comparable 50w amps, particularly Fender's.

Some brief background . . . the observation that prompted my initial question in this thread was the fact that this Magnatone 280 2x12 combo of mine (with 50w rating) does not produce anywhere near the expected output (max volume) of a typical (current day) 50w amp head or combo, and this concern led to several thoughts on my part about whether the original (vintage) speakers were worn-out or whether the filter caps were faulty or perhaps one or both power trannies were bad or whether the power tubes themselves were failing, etc. However, what I'm learning is that most Magnatones, because of fundamental design differences, simply never produced the higher volumes typically associated with present-day 50w amplifiers - differences like the primary impedance on both power transformers, a cathodyne (not differential) phase inverter, 10-15% less plate voltage (on average), etc. Thus, it would appear that there is no justifiable concern on my part about whether the current (6973) power tubes are inferior, faulty or failing, and that the amp itself is perfectly fine.

That said, I am most grateful for your suggested (methodical) approach to evaluating individual or lone power tubes in a field of multiple others. It will (no doubt) come in quite handy in future evaluations with other amps that I own.

[Steve Conner]

Well, given that Magnatone's most famous endorsee was Duane Eddy, I wouldn't expect the amps to be roaring monsters. It might be that they lack gain, not output power as such. You can't really tell the difference between 30W and 50W by ear.

If the speakers are Alnico, maybe the magnets have gone weak with age. Or maybe the speakers were inefficient to start with. Word on the intertubes seems to be that the Magnatone 260 came with poor speakers, and fitting more aggressive ones with ceramic magnets makes a big difference.

Try substituting another speaker cabinet and see if it gets louder.

Of course all of this assumes that the amp is in full working order. You should really get a measurement of the output power, which will settle that far better than trying to guess by ear. If it's only making 2 watts, you know you have a different problem.

[jrfrond]

There is nothing like testing a tube in an amp, but for used power tube testing and weeding, I use the MaxiMatcher. It really does a great job of rating power tubes by giving you numerical readings of plate current and transconductance, which you can compare to an onboard chart. It also reveals shorts. You can watch the digital readout for indications of a runaway, etc. Of course, true to it's moniker, it's great for matching tubes as well.

Power tube issues, however, often do NOT manifest themselves under static testing conditions. Many times, tube will not arc unless hit with the type of transient that balls-to-the-wall performance will supply. This is why we test ALL retubed guitar amps full-bore before they leave the shop. Of course, it can get loud, so the alternative test is guitar tracks recorded on CD with the amp plugged into a speaker-sim load.

My point is: tube-testing just gives you a baseline. You need to put the tubes into service to really find out the story.

[Mango Moon]

You can't really tell the difference between 30W and 50W by ear.

If the speakers are Alnico, maybe the magnets have gone weak with age. Or maybe the speakers were inefficient to start with. Word on the intertubes seems to be that the Magnatone 260 came with poor speakers, and fitting more aggressive ones with ceramic magnets makes a big difference. Try substituting another speaker cabinet and see if it gets louder.

Steve,

Thanks for your input, and yes, these are all very good points. The 260 was arguably one of Magnatone's most popular models, maybe even its most popular, but the improved volume in making the alnico to ceramic swap does not surprise me any. Suffice to say, I'll be trying a number of different cabinets (alnico and ceramic) with my 280 over the next few days, some super efficient and some not, just to see what goes on in terms of overall volume.

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jrfrond,

Thanks to you also for the added information including the details on the MaxiMatcher. I'm not yet familiar with this particular device and for all I know, it'll be outside my personal spending limits, but it sounds very-very practical, and I will certainly research it, in the event that it can help me with future diagnostics. Thanks again.

[km6xz]

What gives you the impression the amp is weak? Have you measured the gain? If the outputs are not being driven hard, due to the natural low gain of that amp, nothing from what you have written indicated a weak power section. What signal level, produces what output voltage across the load?
Use your signal generator to drive it into clipping onset and measure the power across a passive, non-reactive load. Square the RMS value and divide by the load resistance(since the load is none reactive). Remember also that amp was designed for lower mains voltages so everything should be hotter.

If you want to know the characteristics of the tubes, just call around for a calibration lab which might still have a Tektronix 570 tube curve tracer. I have one but the service call would be expensive, 5,000 miles one way. They can produce a full set of parameter measurements quickly. Don't be surprised if a cal lab charges more than the amp is worth however. It is not a hobby for them, they only deal with high cost test instruments normally, for companies needing precision and accuracy. I doubt any tube rebranders have test instruments or the knowledge to tell you much of anything except "buy our newest reissue NOS tubes" which are nothing related to NOS. Dealing with tubes you are primarily dealing with smoke and mirror snake oil sales people, or of the musician's version of esoteric hi-fi...long on hype and buzz words and short on facts or proof. Just in the last couple of weeks reading forums have filled me up to my ears with groundless hype and disinformation. It was not this bad even 5 years ago. Well there was always Aspen Pitman but he was not taken seriously. Now everyone is in on the shuck and jive. Seeing NOS being bandied about as if it meant something...it meant something specific at one time but now it means nothing except higher prices for the same Chinese and Eastern European tubes as being sold under other names for 1/4 the price. New Sensor seems to have cornered the fake vintage name market...Tung-Sol, Mullard, even producing fake low end "Svetlana" branded tubes. I guess the new rage will be Chinese NOS DeForest tubes or RCA. Dumont name is available.
Rant off.....