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PostPosted: 23 Mar 2017, 12:06 
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The latest ones from a different batch are identical in DC resistance. <<

Just for giggles and if you have a meter than can do this, do an ACV reading on the B+ power-supply and let us know what you see.[/quote]

Many meters have a diode and thinking one is applying AC so will be just slightly more than double the measured v, as it was on mine. If it passes an AC signal through a diode it will chop off the bottom half of the sine wave then probably
average the resulting voltage over time, and multiply the average by $frac{sqrt{2}}{2} pi$ (about 2.2). So fed a DC signal, I would expect and get 2.2 times signal in one direction and 0 in the other.

What do/did you think we might see?



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PostPosted: 23 Mar 2017, 17:01 
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Juancho wrote:
The latest ones from a different batch are identical in DC resistance. <<

Just for giggles and if you have a meter than can do this, do an ACV reading on the B+ power-supply and let us know what you see.


Many meters have a diode and thinking one is applying AC so will be just slightly more than double the measured v, as it was on mine. If it passes an AC signal through a diode it will chop off the bottom half of the sine wave then probably
average the resulting voltage over time, and multiply the average by $frac{sqrt{2}}{2} pi$ (about 2.2). So fed a DC signal, I would expect and get 2.2 times signal in one direction and 0 in the other.

What do/did you think we might see?[/quote]

My Fluke will measure actual ACV over the DC as low as a few mv. Which can be very revealing. If you are getting a couple of volts, you have found your problem. Not enough to Hummmmmmmmmmm. But enough for parasitic Oscillations. Leading to red plating.


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PostPosted: 23 Mar 2017, 18:29 
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Quote:
In other words, it's adjacent to the screen grids. The redness is more common by Pins 6 and 7 than by 2/3, and in the 'best' tubes is normally by 6/7 and not 2/3.

What's the screen current?

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PostPosted: 24 Mar 2017, 04:16 
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blackdog wrote:
Quote:
In other words, it's adjacent to the screen grids. The redness is more common by Pins 6 and 7 than by 2/3, and in the 'best' tubes is normally by 6/7 and not 2/3.

What's the screen current?


With the 1k screen resistors now installed it's 7.1watts screen dissipation which makes me think that it would be above 9 watts without the screen resistor


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PostPosted: 24 Mar 2017, 07:39 
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Is there any way you could beg/borrow/steal a set of pre-blight Euro or US tubes from rectifier to output?

This may also be revealing.

By the way, the number of outright defective post-blight Tung-Sol branded tubes observed in the US is very nearly exceeding good ones lately. I expect that it will be no different in Europe, and possibly worse. :nuke:

Chinese tubes, while some are getting better, others are getting worse - these may be a mixed bag.

One other question: Do either of the owners of the problem children have a reliable, high-end tube tester of their own or immediately available to them? And can they do any level of matching other than 'what the meter says'?


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PostPosted: 24 Mar 2017, 10:26 
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I'm afraid neither of us has access to a tube tester. As a matter of interest, would this type of issue show up on a tube tester and how?

One of our suppliers claims to make such tests when matching the tubes and has checked a returned pair stating that they measure correctly.


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PostPosted: 24 Mar 2017, 12:20 
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Juancho wrote:
I'm afraid neither of us has access to a tube tester. As a matter of interest, would this type of issue show up on a tube tester and how?

One of our suppliers claims to make such tests when matching the tubes and has checked a returned pair stating that they measure correctly.


First, as you well know, the amount of current & voltage that a cathode can take is in part determined by clearances between it and the screen(s), the screens from each other, and the screen(s) from the plate. As well as the filament (heater) from the cathode itself. Similarly, the ability of the plate to dissipate heat is affected by these same clearances. Keep that in mind as I ramble on.

*IF* clearances are incorrect - the tube is poorly made or with poor materials that sag or move with heat - on a simple emissions-tester this may actually show up as a higher-than- expected 'quality' reading - for the short length of the typical test. WOW, a good thing!! Not so much. What it means in practice is that as the tube heats up, the ability for the plates to dissipate heat is compromised - even though the tube is operating well within its theoretical design limits.

If the tester gives the ability to measure plate current, filament current and voltage(s) on the grids and cathode, then this can be watched directly as the tube heats up and ultimately proves out or fails.

I keep a Hickok 539B (picture of a similar unit attached) that allows all measurement of all these things, and it is robust enough that I can leave a tube on the unit, loaded, for as long as I like and look for any issues.

Defects of this type are rare - better written, exceedingly rare in pre-blight tubes. But based on my direct observations these last few months, and hearing about more-and-more failure-in-service tube batches across entire brands, I am thinking that it will become amongst the most common failure modes of post-blight tubes, even ahead of wear.

Aside: I just tested twelve (12) "new" Tung-Sol KT88s for a friend. Only two (2) met the minimum reading on my Hickok. I have four OEM GE tubes that all tested 20% or more better than the minimum. They have more than 3,000 hours on them, about 1,500 of that in my hands.

I am skeptical of supplier tests. There are a vanishingly few US suppliers that actually DO meaningful tests and provide meaningful matches. The rest do not. I have never purchased from an off-shore tube supplier, so I will not write to what they do, or not. I will state for the record that there are some very good modern computer-based tube testers that measure many parameters and provide performance curves as well, all of them costing well into the US $ thousands. If I were ever to venture into the tube reselling business (please NOT!), I would invest in such a device and no tube would leave without a full pedigree attached. But that would be costly.

One last rant: I would state, categorically, that the entire Russian tube output from all sources is likely less than a single day's output from any one of Sylvania, RCA, Philco or Tung-Sol in 1960. And, brutally, they have no competition that would induce them to get any better than what appears to be a race-for-the-minimum. China? Yeah, right. Same issue, same quantity disconnect, same lack of real competition. And resellers absolutely know this. And build it into their prices. And you (thankfully not me - yet) are paying for it. Through your collective and several noses. I fully believe that if the RCA tube manual were not as common as it is, and were its specifications to be largely unknown to the tube using public, the various manufacturers extant today would RACE to revise every specification and requirement to be at least 50% lower than in 'the manual'. That way, they could still make crap and look good.

http://www.tuberadios.com/temp/hickok539b2.jpg


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PostPosted: 24 Mar 2017, 13:46 
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Thanks for the info-as I suspected unfortunately. Nice piece of kit that machine of yours! What is your reference to the 'blight'? Some event at the Factory /with New Sensor?


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PostPosted: 24 Mar 2017, 14:08 
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Juancho wrote:
Thanks for the info-as I suspected unfortunately. Nice piece of kit that machine of yours! What is your reference to the 'blight'? Some event at the Factory /with New Sensor?


Tubes made by the large legacy manufacturers when there was serious competition for quality and longevity at every level are "pre-blight". Anything made haphazardly since in repurposed factories with vintage and/or reconditioned equipment by poorly trained technicians using cheap materials and poor technique (pretty much everything today with a very, very few exceptions) are "post-blight".


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PostPosted: 24 Mar 2017, 14:13 
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And yet I have virtually no problems with signal tubes. Modern JJs are silent and on a par with the best NOS equivalent for example. In fact, I can say I have never returned an EH, JJ or Tung Sol signal tube despite getting through most of a hundred over the last few years.

But EH, Sovtek and Tung Sol power tubes, even aside from these 120/150s, have failed in huge numbers.


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