Trying mystery NOS tubes out - no tube tester nearby

[pjd3]

Would love to hear your feedback on this, please go easy on me, I'm tender - not really.

So time to see how the bias is doing in my main gig amp, a Sluckey 6V6 plexi. I've been using a pair of 6V6GT NOS Sylvanias from Mike at KCANOS tubes which per usual have been working great for a couple of years although they were less than a milli-amp in bias matching but have drifted a few milli=amps apart, or, one of them went from 20mA down to 17mA. It still sounds good.

But, I got curious about trying some mystery ANOS 6V6's that came from a selection of old hi-fi tube amps and receivers' to finally see how they faired in this amp. They looked good, and there were no filament to cathode shorts so I saw fit to give them a go.

It didn't go too well in my opinion. A couple of them were upwards of 30mA when I read voltage across the 1 ohm cathode bias resistor, while the original tube was at its usual 19-20mA. Another couple of them made loud popping sounds which was immediately worry-some. They were a mixture of RCA's, Westinghouse, and General Electric.

Well, you get what you get. And I probably shouldn't be using one of my amps as a "tube tester" for mystery NOS tubes. I probably won't ever again.

Does any of those symptoms tell you anything about the particular status of the tubes? The high bias current? And then the loud frequent popping in others? I know how a good tube behaves and how it sounds but, I'm not terribly versed on the symptoms that surround the developing problems of these vacuum tubes. Just to mention, the negative bias voltage was around - 38 to -40 I believe on the grids of the 6V6's,

Thanks for stopping by.
Best,
Phil D.

[Phil_S]

Someone with more experience and knowledge may disagree with my comments, so you should take them with a grain of salt.

I can't see the problem using the amp as a tube tester. Just use your common sense, which it seems you are doing. Noisy tubes -- no go. High current draw may be another story.

To some extent, with a pair of tubes in push-pull, fixed bias, they interact with each other because they are supplied from the same source and are generating signal for the same output transformer. So, your measurements may not be quite as conclusive as you'd prefer, but you will get some good information. In addition to checking current draw indirectly by the drop across the 1Ω sense resistor, I'd look to get a full set of voltages for A, G (negative bias), G2, and K. I'd choose the 20mA 6V6 as the "control" tube and pair all your oddball tubes the that one 6V6. With each tube swap, take a full set of reading on both tubes. If you have enough of them, you may discover that you have what appears to be a matched pair among those you are testing. When you find that pair, put both in the amp and see what the voltage readings tell you. I think you'll know right away if you have a matched pair.

If you are finding tubes drawing more current than you feel comfortable with, make the bias voltage more negative and do your testing at that setting.

[If you want to go further (not necessary) and If you've got enough swing in the bias pot, you can find 2 or 3 settings for bias voltage to work with, Get voltage readings (including what flows across the 1Ω sense resistor) at each fixed bias voltage. This will give you an idea of whether two tubes will function well as a pair. Obviously, you are looking for a pair that gives a reasonable match at all three bias voltages. Also, with 3 measurement points, you can calculate transconductance (gm) as a matching parameter. gm = (Ip test1 / Ip test2)/(Vg test 1/vg test 2) * 1000. When you have gm and static plate dissipation, then you can determine if there is a reasonable match.]

From there, you can choose a bias voltage that works for the pair and let your ears tell you if you like what you hear.

One more thought on tubes that draw more than 20mA. You can simply adjust the bias to cut it back to 20mA. In general, this simply indicates a strong testing tube. If your bias voltage won't adjust (needs to be more negative) it's pretty simple to modify the bias circuit so that it will be in the range needed.

[Stevem]

A higher current reading then the tubes you started with means that Tube was a stronger tube.
If the output stage was cathode biased you would also see a bigger voltage across that resistor / cap combo.

The popping sounds are not a good thing, and I would bet that in a dark room that those tubes doing that will be seen to have a very blue glow to them which is gas and a sign that the tube is on its way south!

A mismatch in current of 5 to 8 ma can allow different harmonic coloring out of the output stage that you might prefer, especially when driving the output stage into clipping.

It’s Really best to use a amp as a tube tester, because even most of the best old time and new testers do not test by applying anything more then 350 volts to the tube.
In regards to keeping your amp alive you should fuse the OT center tap ( even if your not using it as a test bed ) feed and have fly back diodes to ground off of each plate.
Also only hearing a tube in a amp will tell you if it’s making popping sounds.

Many of the Westinghouse 6V6s where black plate types just like the black plate RCA versions that are going for above $160 these days for a matched pair!

[TUBEDUDE]

That's good advice Phil and Stevem has supplied.
With mystery tubes I'd start with the bias maxed. Back it down until you get good conduction. Any popping noises and I'd shut it down immediately to protect your iron and pitch the tube (fusing the OT feed is wise).
In the dark you will likely see arcing in the offending tube.
I like my pairs of Westinghouse and Sylvania tubes as much as my RCA's.
I run unknowns thru the tube tester first to rule out dead shorts and gassy tubes.
With the unknowns, watch for tubes that have unstable bias. An unknown gassy tube may run into a death spiral after warming up sufficiently. As the bias voltage no longer exerts control over plate current. Monitor closely. I hope you get some gold in there!

[martin manning]

A higher current reading then the tubes you started with means that Tube was a stronger tube.

Possibly, but beware...

With the unknowns, watch for tubes that have unstable bias. An unknown gassy tube may run into a death spiral after warming up sufficiently. As the bias voltage no longer exerts control over plate current. Monitor closely.

[pjd3]

Thank you all the high quality input on how to go about inserting old mystery tubes into my main amp! It did get a little scary and I decided to take a break until I became more informed to what to look out for.

Normally, I would look at a higher bias current as an indication of a stronger tube, but in this case a couple of these tubes were showing 30mA with a bias setting that yielded 19-20mA from a NOS Sylvania that was sold as good/new to me from Mike at KCA. I wouldn't have expected a 6V6 to be 30% stronger or "better" than what is seemed to be a Sylvania in primo electrical condition as these NOS Sylvania I've been using that were in fact bought as a matched pair from KCA. That didn't seem right to me so I killed the standby right away when I measured that. 2 of the tubes did that.

I may go back and look at these tubes in the dark where I could see any "light/color shows" in the tubes as they began with the loud Popping sound. And chuck them right away.

Its interesting to me what would be the problem source of a tube that was manifesting unusually high bias current to the point where it would be indication a problem. A bad or worn grid perhaps? One that is no longer properly applying a negative potential to the electrons emitting off the cathode? I dunno. Just letting imagination roam.

Appreciate your feedback, I was sure hoping for a couple of decent tubes from this stash of 8 or 9 old 6V6;s

Best,
Phil D

[pdf64]

I’m perfectly happy to use my amps to check new / unknown valves, but ensure to power the amp via a light bulb limiter. Then if it shorts hard, goes into redplating death spiral etc, the amp will be protected from fault current.
Bulb wattage low for initial check, then higher for a high power output test. If it holds up, then switch the limiter out of circuit, check bias, and repeat full load test

[TUBEDUDE]

Phil D,
Regarding the seemingly high cathode current, remember cathode current contains the screen grid current in addition to the plate.current. But10mA is high.

"Its interesting to me what would be the problem source of a tube that was manifesting unusually high bias current to the"

Here I think maybe you mean "high bias voltage" or "high cathode current".
In class AB1 there isn't really any bias current.

With gassy tubes you can see the bright blue glow of the gas being ionozed by the high voltage.
If the ionized gas cloud becomes too great, it will suppress the bias voltages influence on the current stream, and max current flow will occur.

[pjd3]

Thanks Tubedude.

I'm using "Bias current" and "Cathode current" interchangeably - "bias current" meaning the current as calculated by ohms law by measuring voltage across a 1 ohm cathode resistor that I placed in series with the cathode for those purposes when I built the amp. I've often seen those terms used interchangeably and always assumed a reference to cathode current at no signal.
I've been left with the impression that while screen grids take up their own amount of current, that this current while not always negligable isn't going to cause a considerable offset in reasonable cathode current.

For actual bias voltage as an example, I believe I measure around -38 to -40 volts or so right at the grid of the "known normal/good" KCA Sylvanias that have been in the amp since it first got fired up a year or two ago.

I seem to be having a decent time with biasing up good tested new tubes so I assume I am spoiled by them, making life very easy for me during my amp builds. Its learning the idiosyncrasies of the questionable ones that I need to get versed with so, I thank you all for chipping in to a well needed education on my behalf.

Best,
Phil D.

[darefugee]

Phil, I built a curve tracer and will be delighted to test and match up your tubes for you if you're near Tampa Florida.

[pjd3]

Thanks very much darefugee, if I were near Tampa I'd take up your offer in a heartbeat.

I have other tubes also that I'm really curious about, old Mullard EL34's and others who's current condition I'd really be interested to know.

Thanks!
Best
Phil D.

[jjman]

That spread of idle results is not large to me. I allow 1 small pop, 1 time.